Aufbau und Funktionsweise

Ein MOSFET ist ein aktives Bauelement mit mindestens drei Anschlüssen (Elektroden): G (gate, dt. Steuerelektrode), D (drain, dt. Abfluss), S (source, dt. Quelle). Bei einigen Bauformen wird ein zusätzlicher Anschluss B (bulk, Substrat) nach außen geführt. Meistens ist das Bulk jedoch intern mit der Source verbunden.

Wie andere Feldeffektransistoren wirkt der MOSFET wie ein spannungsgesteuerter Widerstand, das heißt, über die Gate-Source-Spannung UGS kann der Widerstand zwischen Drain und Source RDS und somit der Strom IDS (vereinfacht ID) durch RDS um mehrere Größenordnungen geändert werden. Der Schlüssel zum Verständnis dieser Widerstandsänderung in einer MOS-Struktur liegt in der Entstehung (Anreicherungstypen) bzw. Zerstörung (Verarmungstypen) eines leitenden Kanals unter dem Gate (Details siehe unten).

Grundtypen [Bearbeiten]

Ähnlich wie der Bipolartransistor kann auch der MOSFET in die zwei grundlegenden Varianten p-Typ (auch p-leitend oder PMOS) und n-Typ (auch n-leitend oder NMOS) eingeteilt werden. Werden, beispielsweise in integrierten Digitalschaltungen, beide Typen gemeinsam verwendet, spricht man von CMOS (engl.: Complementary MOS). Zusätzlich gibt es von beiden Varianten jeweils zwei Formen, die sich im inneren Aufbau und in den elektrischen Eigenschaften unterscheiden:

Verarmungstyp (engl.: depletion) – auch selbstleitend, normal-an, normal leitend

Anreicherungstyp (engl.: enhancement) – auch selbstsperrend, normal-aus, normal sperrend

In der Praxis werden mit großer Mehrheit Anreicherungstypen eingesetzt.

Grundsätzlicher Aufbau und physikalische Funktion [Bearbeiten]

Als Beispiel sei der selbstsperrende n-Kanal-MOSFET gegeben.

Als Grundmaterial dient ein schwach p-dotierter Siliziumeinkristall (Substrat). In dieses Substrat sind zwei stark n-dotierte Gebiete eingelassen, die den Source- bzw. Drain-Anschluss erzeugen. Zwischen den beiden Gebieten befindet sich weiterhin das Substrat, wodurch eine npn-Struktur entsteht, die vorerst keinen Stromfluss zulässt (vgl. npn-Transistor: Ohne Basisstrom ist der Transistor gesperrt). Genau über diesem verbleibenden Zwischenraum wird nun eine sehr dünne, widerstandsfähige Isolierschicht (Dielektrikum, meist Siliziumdioxid) aufgebracht. Das Dielektrikum trennt die darüberliegende Gate-Elektrode vom Silizium (genauer vom Kanalgebiet). Als Gate-Material wurde bi

s Mitte der 1980er Aluminium verwendet, das von n+ bzw. p+ dotiertem (entartetes) Polysilizium (Abkürzung für polykristallines Silizium) abgelöst wurde.

Durch diesen Aufbau bilden Gate-Anschluss, Dielektrikum und Bulk-Anschluss einen Kondensator, der beim Anlegen einer Spannung zwischen Gate und Bulk aufgeladen wird. Durch das elektrische Feld wandern im Substrat Minoritätsträger (bei p-Silizium Elektronen) an die Grenzschicht und rekombinieren mit den Majoritätsträgern (bei p-Silizium Defektelektronen). Das wirkt sich wie eine Verdrängung der Majoritätsträger aus und wird „Verarmung" genannt. Ab einer bestimmten Spannung Uth (engl. threshold voltage, Schwellspannung) ist die Verdrängung der Majoritätsladungsträger so groß, dass sie nicht mehr für die Rekombination zur Verfügung stehen. Es kommt zu einer Ansammlung von Minoritätsträgern, wodurch das eigentlich p-dotierte Substrat nahe an der Isolierschicht n-leitend wird. Dieser Zustand wird starke „Inversion" genannt. Der entstandene dünne n-leitende Kanal verbindet nun die beiden n-Gebiete Source und Drain, wodurch Ladungsträger (beinahe) ungehindert von Source nach Drain fließen können.

Operationsbereiche eines n-Kanal-MOSFET

Prinzipiell sind Source- und Drain-Anschluss zunächst gleichwertig, meist ist der Aufbau aber nicht symmetrisch, um ein besseres Verhalten zu erzielen. Außerdem wird bei den meisten Bauformen Bulk intern mit Source verbunden, da ein Potentialunterschied zwischen Source und Bulk die Eigenschaften des Transistors (vor allem die Schwellenspannung) negativ beeinflusst (body effect). Auf die grundlegende Funktion hat die Verbindung keinen Einfluss. Allerdings entsteht nun zusätzlich eine Diode zwischen Source- und Drain-Anschluss, die parallel zum eigentlichen Transistor liegt (Bulk mit dem p-dotierten Substrat und Drain mit dem n-Gebiet bilden den p-n-Übergang). Diese sogenannte Body-Diode ist als Pfeil im Schaltsymbol des MOSFETs dargestellt und zeigt beim n-Kanal-MOSFET vom Bulk-Anschluss zum Kanal. Bei der Anwendung ist die Body-Diode in der Regel in Sperrrichtung gepolt, bei manchen Schaltanwendungen kann sie jedoch genutzt werden, um Inversbetrieb zu verhindern. Sie kann den vollen Drainstrom des MOSFETs führen, schaltet aber relativ langsam, so dass für schnelle Schalter externe Dioden benutzt werden. Siehe auch: Feldeffekttransistor

Varianten im Aufbau [Bearbeiten]

Neben den konventionellen MOSFET-Varianten existieren noch diverse Spezialvarianten mit verändertem Aufbau. Ziel ist ein besseres Schaltungverhalten, was jedoch mit zum Teil deutlich erhöhtem Herstellungsaufwand verbunden ist. Beispiele sind der VMOS-FET oder der FinFET, wobei letzterer den Vorteil eines vergrößerten Kanalbereichs bietet; aufgrund der Kanalbereiche oft auch als Dual- (Tetrode) oder Tri-Gate bezeichnet.[2][3] Diese werden zum Beispiel in HF-Schaltungen eingesetzt (HF-Verstärker, multiplikativer Mischer).

                                                      

Luis Fernando Cantor B.

Electronica de Estados Solidos

Seccion 2

Building Mixers

A much more flexible option is to use a mixer, a device which allows the outputs of multiple players to be summed together. With this technique, no input switching is required and the advanced crossfading features of the Sony CD changers can be taken advantage of. I am placing to sample mixer designs on this page which you can build. The first, a passive mixer, can be made with only resistors and RCA jacks and give reasonable performance (depending on you degree of audiophility). The second circuit is a active mixer which gives very good performance and is very low noise and distortion for an active circuit. If you are capable of building it, this is a far superior circuit. Mixers are also commercially available. Most have features which are unnecessary in this application (e.g. adjustable gain on each input).

simple passive mixer which is inexpensive and easy to make. It is easily expanded to any number of players at the expense of lost gain. Only 1/2 of the circuit is shown (i.e. one for the left, one for the right). This circuit is relatively low noise and distortion, however it has the following disadvantages over an active circuit:

*The volume of each CD player drops to 1/3 it's original value (-9.5dB) with 3 players.

*You'll begin to see high frequency roll off if you have more than a few nF of cable capacitance.

The 2k resistors should be replaced with larger values if your CD player outputs are not rated for such a low impedance (each player sees 3k ohms each when all 3 players are connected). Increasing the resistance may reduce distortion from you CD player output stage, but you will begin to pay the penalty of increased noise and high frequency roll-off.

simple active mixer which is inexpensive and easy to make. It is easily expanded to any number of players. Only 1/2 of the circuit is shown (i.e. one for the left, one for the right). This circuit is very low noise and distortion. No gain is lost when connecting multiple players. Overall gain can be adjusted by changing R4. The 2k resistors should be replaced with larger values if your CD player outputs are not rated for such a low impedance (each player sees 2k ohms). Increasing the resistance may reduce distortion from you CD player output stage, but you will begin to pay the penalty of increased noise.

Luis Fernando Cantor B.

Electronica de Estados Solidos

Seccion 2

Using your Op-Amp Circuit in the real world

Almost all active (semi-conductor or tube based) audio units add some degree of DC voltage bias. What this means is that the audio signal can have positive or negative voltage bias - this issomething that can cause large distortion errors, or other problems as you stack modules together. The easiest way to fix this is to assume that you will encounter the  problem and use a blocking capacitor at the input and output to eliminate the DC offset.

A capacitor is used in this context to pass only the AC (Complex Waveform) component of the signal. As many of you might know, capacitors are also used in Cross-Over networks, as well as voltage storage in a power supply (to even out the voltage levels). For our use - which are circuits for audio signals at levels that are used in Microphones, Guitars, Keyboards and other electronic musical gear - there are some values that tend to work quite well. All the designs in this series will use those values for the types of input and output stages that you would use in this environment. Yes, it is possible to optimize this area, however, for our purposes, the values that I use will do an excellent job.

One question that comes up frequently is how to pick the correct voltage part when choosing a capacitor - its quite simple - choose a part that is as high or higherrated than the power supply. If you are using 2 9 volt batteries to drive the circuit - thats 18 volts - you'll probably use 25 volt or higher capacitors. Never use a capacitor rated at less than the maximum voltage of the power supply. I frequently use 50 volt or 100 volt capacitors - they are often the same price and will give the same results. Don't skimp here. One problem that you will encounter with the input stages that are Non-Inverting is that the input stage needs a ground reference. Since the capacitor won't pass the DC equivalent of ground thru the capacitor, we need to add a fairly high resistance resistor that provides the ground ref

erence for us. We don't need this on Inverting input stages because the Non-Inverting input is tied to ground and the Inverting input accomplishes the same thing by use of negative feedback. All the circuits shown here have these components in place along with the values.

In Figure 2, you see a standard Non-Inverting Amplifier. The circuit looks pretty similar, except that there is an extra component in the Input DC Blocking section - its the ground reference resistor. You'll also note that the values of Cin differ depending on whether you are building an Inverting orNon-Inverting input stage. These values have to do with the input impedance differences. The output DC Blocking components are the same in either type of amplifier.

You'll note one additional component - that is the shorting input jack that I show f

or the Signal In connection. This type of j

ack is wired such that when no 1/4 inch phone jack is plugged in, the input is set specifically to signal ground. Doing this eliminates a lot of potential noise problems.

In Figure 3 we take the 4 channel Mixer from Part 1 and add the components needed to interface this with the outside world. The inputs are designed for high impedance devices, such as Guitars, Keyboards, High Impedance Dynamic Microphones and the like, that use single sided (unbalanced) inputs. You'll also notice that I've added a level control on each input to the mixer, otherwise, theDC blocking section is pretty much the same as the Figure 1 above.

                                                                                

Luis Fernando Cantor B.

Electronica de Estados Solidos

seccion 2

Simple Mixer Schematics

A final  word on the capacitor

There are capacitors in two main circuit functions on the schematics above. the first is an electrolytic blocking capacitor. The idea is that a DC voltage can't got through a capacitor in series. What this means is that any DC offset voltage emanating from a preceding stage or source will be knocked on the head. only the AC voltage (The audio signal) will get through. The reason for this is simple. Suppose you had 10 sources each with a +1volt DC offset. This would add up in the mixer stage to be +10 volts. Not exactly desirable. It is therefore usual to use a blocking capacitor to stop this happening. This may not be so in all cases but is a rule of thumb for most audio circuits. The blocking capacitor is placed on the input near to where an unknown source is to enter the circuit. It is also usual to have one on the output stage which blocks any DC from leaving your circuit and propagating into any following equipment. The reason you need them on both input and output is simply that you never know what you might connect your circuit up to and there is no convention. If you are unsure of the polarity required for the blocking capacitor you can use a bi-polar electrolytic. Which is essentially two normal electrolytic capacitors back to back in the one package. The value of these capacitors are not important as long as it has no effect on the audio signal (IE accidently creates a lowpass filter) and the voltage rating is sufficient enough that it won't burn out. Usually 16 volt rating is sufficient. 25 volts to be on the safe side. 50 volts is called "over-engineering". The value of the capacitor can be anywhere between 0.1uF to 47uF but usually between 1.0uF and 10uF.

The other two capacitors, 27pF and 47pF are optional and for stability of the op-amps. Truth be known these were left in the schematic by accident because I simply modified the circuit from one I was working on at the time of writing. The original circuit was designed to closely approximate another commercial mixer as I was extending it's capabilities.

Out of interest these two capacitors cause the op-amps to behave as slight intergrator-filters limiting the top end response slightly above the audio bandwitdth. This is some times necessary where the op-amps used have such a high gain-bandwitch product that they tend to saturate with RF or at least HF signals. Thus becoming unstable in certain situations. Generally speaking these are largely irrelevant to the design.

Luis Fernando Cantor B.

Electronica de Estados Solidos

Seccion 2

Simple Mixer Schematics

Two more variations

The third circuit shows a Mixer with input attenuation. This is a fairly simple concept. A potentiometer is placed in the signal's path between the source and the summing resistor. When the wiper of the POT is at the top it simply represents a 10K load to the source. 10K is a pretty high value and most line level devices can easily drive this load. With the wiper at the other end of the pot it still represents a 10K load to the source but the input is effectively at ground (Shorted out) so no signal gets through. With the wiper in mid way position the input loading is still 10K, however the signal has to flow through a 5K resistance and is also dumped to ground by 5K. Halving the potential reaching the input resistor/summing node.

The fourth and final circuit shows a full on stereo mixer. Two new types of input networks are shown. the first is a stereo-in with balance. Similar to your stereo amplifier etc. A dual gain pot is use for volume whilst balance is single. Note that following the volume pot is a 10K resistor connected to one end of the balance pot. With the wiper in the centre position and connected to ground as it is, means that the incoming audio is virtually running through a 22.5K resistor to ground. That is 10K +(1/2 of 25K) = 22.5K Because of this attenuation the feedback resistor around the virtual earth op-amp is increased to 33K to compensate. This is not exactly unity gain but it comes awfully close. A very slight and probably un-noticeable gain.

The other input is MONO in but is pannable between left and right. The same deal as above applies here except that the first two 10K resistors are joined together so that the signal is split across two paths. Strictly speaking the first two 10K resistors in the stereo input are not necessary but are needed for the mono circuit so that the pan pot does not short out the signal when at either extremes of travel. They are included in the stereo input simply to compensate for unity gain over all. This input scheme is the basis for 99% of all large mixing consoles.

Epilogue 

Well hopefully I've provided enough information so you could go out and roll your own designs. And hopefully I've been able to work it in such a way that it's relatively understandable. If there are any mistakes, errors or omissions, please feel free to point them out. But Please no nit-picking. I'm only doing this because of the number of questions asked on this subject and the relative interest for people to design their own.

No responsibility is taken for any damages or any other shortcomings if you actually use this information. If you start out building one of my designs and end up wiring yourself to the national grid, it's you're problem.

Luis Fernando Cantor B.

Electronica de Estados Solidos

seccion 2

Simple Mixer Schematics

Virtual Earth 

Circuit 2 shows a basic active mixer. It uses 2 virtual earth preamps. One for the summing node and 1 to re-invert the phase of the signal.

The summing node (The point at which all the resistors meet) enteres into the inverting input of the op-amp. A feedback resistor is connected between the output of the op-amp and the inverting input. The function off this feedback loop is essentially to limit the open-loop gain of the op-amp.

Any signal entering the inverting input of the op-amp will appear at the output but it will be upside down. That is to say 180 degrees out of phase. In other words if you put 2 volts in you'd expect -2 volts out. To achieve unity gain (that is no gain or amplification at all) the feedback resistor must be the same as the summing resistor. In this case 10K. All the summing resistors are 10K and the feedback resistor is 10K. Because the feedback resistor feeds the output signal back to the inverting input of the op-amp @ 180 degrees out of phases it cancels out any gain. It also means that the inverting input of the op-amp is held pretty close (If not exactly) at zero volts. or earth potential. Thus the term "virtual earth".

Any signal coming in through the summing resistor is like dumping it to ground via 10K. It theoretically has the same loss. However the feedback resistor of 10K gives the exact opposite in gain. So if you feed 2 volts in you will get 2 volts out only it will be upside down.

Because the summing node (The inverting input of the op-amp) is at virtually earth potential, there is little chance that this signal will bleed it's way out to any of the other inputs. Essentially speaking all the audio sources are isolated from each other.

However we're still left with the problem of the phase being wrong. If the output of the first op-amp were recombined with one of the other signals at a later stage it would cancel out rather than mix. So we have to re-invert the phase with yet another op-amp. This is a unity gain amplifier just like the first except that there is no summing node as such. (Except for the feedback resistor of course) The output of these two stages will now be the summ of all the inputs with the correct phase. Because of the inherent compensation of the feedback/op-amp/summing node, there is virtually no limit to the number of inputs you can put on this. Most modern op-amps have enough drive capability that 128 inputs would be just peanuts.

However it must be remembered that you are summing the inputs so if you had a powersupply of say +/- 15 volts, and 4 inputs of +5 volts each, The result would be 20 volts mathematically speaking. But the op-amp can only produce +15 volts so you would be clipping by 25%. Distortion occurs. Most op-amps can't swing exactly to the supply rails so clipping and distortion would be even worse. In practice however most audio signal wouldn't exceed a few hundred milivolts. A 2 volt peak to peak signal is considered to be a very high level.

Luis Fernando Cantor B.

Electronica de Estados Solidos

Seccion 2

Simple Mixer Schematics

Virtual Earth 

Circuit 2 shows a basic active mixer. It uses 2 virtual earth preamps. One for the summing node and 1 to re-invert the phase of the signal.

The summing node (The point at which all the resistors meet) enteres into the inverting input of the op-amp. A feedback resistor is connected between the output of the op-amp and the inverting input. The function off this feedback loop is essentially to limit the open-loop gain of the op-amp.

Any signal entering the inverting input of the op-amp will appear at the output but it will be upside down. That is to say 180 degrees out of phase. In other words if you put 2 volts in you'd expect -2 volts out. To achieve unity gain (that is no gain or amplification at all) the feedback resistor must be the same as the summing resistor. In this case 10K. All the summing resistors are 10K and the feedback resistor is 10K. Because the feedback resistor feeds the output signal back to the inverting input of the op-amp @ 180 degrees out of phases it cancels out any gain. It also means that the inverting input of the op-amp is held pretty close (If not exactly) at zero volts. or earth potential. Thus the term "virtual earth".

Any signal coming in through the summing resistor is like dumping it to ground via 10K. It theoretically has the same loss. However the feedback resistor of 10K gives the exact opposite in gain. So if you feed 2 volts in you will get 2 volts out only it will be upside down.

Because the summing node (The inverting input of the op-amp) is at virtually earth potential, there is little chance that this signal will bleed it's way out to any of the other inputs. Essentially speaking all the audio sources are isolated from each other.

However we're still left with the problem of the phase being wrong. If the output of the first op-amp were recombined with one of the other signals at a later stage it would cancel out rather than mix. So we have to re-invert the phase with yet another op-amp. This is a unity gain amplifier just like the first except that there is no summing node as such. (Except for the feedback resistor of course) The output of these two stages will now be the summ of all the inputs with the correct phase. Because of the inherent compensation of the feedback/op-amp/summing node, there is virtually no limit to the number of inputs you can put on this. Most modern op-amps have enough drive capability that 128 inputs would be just peanuts.

However it must be remembered that you are summing the inputs so if you had a powersupply of say +/- 15 volts, and 4 inputs of +5 volts each, The result would be 20 volts mathematically speaking. But the op-amp can only produce +15 volts so you would be clipping by 25%. Distortion occurs. Most op-amps can't swing exactly to the supply rails so clipping and distortion would be even worse. In practice however most audio signal wouldn't exceed a few hundred milivolts. A 2 volt peak to peak signal is considered to be a very high level.

Simple Mixer Schematics

Active state

Active Mixer stages that use Op-Amps are generally known as virtual earth pre-amps. These are inverting in nature. 180 degrees out of phase. IE: The signal coming out of the mixer is upside down as compared to that which is entering it. You then need to use another inverting pre-amp to recover the phase.

This would seem silly at first until you realize that virtual earth means that the inverting node of the op-amp is held virtually at ground (zero volts) potential. Any signal entering the stage via one resistor cannot find it's way back out of any other resistor. This prevents the audio from *say* one synth, polluting the audio from another. Particularly useful in a Mixer with many busses and sends.

Generally speaking the pre-amps stage does not provide any gain. IE: is 1:1 unity gain. A signal passing through a resistor with no load also presents no loss. Even with values beyond 1 meg. Although you may drop the effective current at the other end of the resistor. In this case the current loss is largely irrelevant. Especially at line-level. And is compensated by the op-amp's drive current in an active system.

It is better to have a mixer stage with no gain (or unity gain) because this will not amplify the noise. If good quality op-amps are used, they will not add significantly to the over all noise performance. So the RMS voltage coming out of the mixer should be the same as the sum of all it's inputs. If gain is necessary for a microphone or phono etc, the gain should be a special stage at the top of the chain. IE: the first preamp in the mixer channel. This is then mixed with everything else once the microphone is amplified to line level. This gain stage only adds noise to the microphone and not to the sum of the signals passing through the mixer.

It is interesting to note that resistors themselves add noise to a circuit. This is known as thermal noise. Generally speaking the rule of thumb is: The larger the value the resistor, the greater the thermal noise. This may not be significant in mixer stages at line level but where large gains are required it is desirable to use smaller value resistors. (as small as possible within reason.) Of course sometimes this cannot be achieved but is worth remembering as a rule of thumb. Metal film resistors have less thermal noise than carbon film resistors and are more temperature stable over all. So now there's two reasons to use Metal films in audio circuits.

Driving the busses

Note here that Mixers are more repetitious than complex. The circuits are relatively simple it's just that there's a lot of them. Especially in large recording consoles.

Usually these desks are seen in two halves. The input half and the output half. No matter how complex the input half may become, the output half is essentially just a virtual earth pre-amp as described in the circuits above. Often it is required to have many such busses for things like effects sends, subgroups, monitor bus and so on.

One of the beauties of the virtual earth mixer is that there is also virtually no limit to the number of additional busses as well as the main bus. One could arrange an effects send buss that derives it's signal from the same channel as the main bus. Except that each has it's own volume, pan and assignment independent of each other.

Luis Fernando Cantor B.

Electronica de Estados Solidos

seccion 2

Simple Mixer Schematics

PREAMBLE 

I've been cooking audio circuits for so long now I no-longer need a recipe. A lot of the theory I have forgotten over the years because I've just gotten to know the circuits by instinct. But this should serve as something of a guide to designing mixers from scratch.

The idea:
Most people reading this would be well aware of what a mixer is used for but I'll reiterate here. The job of an Audio mixer is to combine various audio signals into a single audio signal. It is better known in electronic terms as a summing circuit. That is to say that the output is the sum of all of the inputs. A summing note is often represented as a circle with a PLUS (+) symbol in it.

Audio is of course an AC (Alternating current) signal but if we look at the incoming signals as a frozen moment in time we can represent it as 2 or more DC voltages. This is only useful to illustrate the point.

If we had two signals to be mixed. The first was 2 volts and the second was 3, the output should be the sum of these two voltages. 2+3=5. If on the other hand the two voltages were 2 volts and -3 volts then the output would be -1 volt. We are now subtracting 3 volts from the +2 volts leaving -3. It is important to recognise that we are dealing with what is known as a bipolar signal. That is one that can be positive or negative around a zero base-line.

When you get to the stage of adding many signals together, the complexity grows. In1 + In2 + In3 + In4 + .... and so on.

Because each incoming signal has it's own load impedance it is impractical just to wire all of them together and hope for the best. Especially when the following device you are trying to mix into also represents it's own load impedance. Sometimes you may be able to get away with it because the combined impedance is quite high. However most of the time it drags the whole network down and causes one or more devices to fail or distort or what ever. Usually no damage is done but it just won't work.

What is required is a little load isolation. (See Circuit 1: Passive mixer) The trade off is that you can't use terribly high value resistors because of the losses that they may cause. Especially if the load impedance of the following device is a little low. This will give the effect of severely attenuating some or all of the signals. A practical trade off has to be reached and this is as much trial and error as anything because the conditions change with each new device added or changed.

The device used at the summing node, IE: an amplifier or tape deck should be able to provide enough gain to compensate for the combined losses through the resistors and the combined loading of the system. But the loading will change depending on the combination of devices you have hooked into it.

This approach also creates another side effect. That is that a signal flowing into the summing node via one source can pollute the audio signals of other devices. Say you had two cassette decks that you wished to mix. However you also wanted to send the audio from cassette deck #1 to an effects processor. The audio from cassette deck #2, although attenuated slightly, will find it's way back to the audio from cassette deck 1 and also go to the effects processor.

Luis Fernando Cantor B. 

Electronica de estados solidos 

seccion 2

Simple Mixer Schematics

Active state

Active Mixer stages that use Op-Amps are generally known as virtual earth pre-amps. These are inverting in nature. 180 degrees out of phase. IE: The signal coming out of the mixer is upside down as compared to that which is entering it. You then need to use another inverting pre-amp to recover the phase.

This would seem silly at first until you realize that virtual earth means that the inverting node of the op-amp is held virtually at ground (zero volts) potential. Any signal entering the stage via one resistor cannot find it's way back out of any other resistor. This prevents the audio from *say* one synth, polluting the audio from another. Particularly useful in a Mixer with many busses and sends.

Generally speaking the pre-amps stage does not provide any gain. IE: is 1:1 unity gain. A signal passing through a resistor with no load also presents no loss. Even with values beyond 1 meg. Although you may drop the effective current at the other end of the resistor. In this case the current loss is largely irrelevant. Especially at line-level. And is compensated by the op-amp's drive current in an active system.

It is better to have a mixer stage with no gain (or unity gain) because this will not amplify the noise. If good quality op-amps are used, they will not add significantly to the over all noise performance. So the RMS voltage coming out of the mixer should be the same as the sum of all it's inputs. If gain is necessary for a microphone or phono etc, the gain should be a special stage at the top of the chain. IE: the first preamp in the mixer channel. This is then mixed with everything else once the microphone is amplified to line level. This gain stage only adds noise to the microphone and not to the sum of the signals passing through the mixer.

It is interesting to note that resistors themselves add noise to a circuit. This is known as thermal noise. Generally speaking the rule of thumb is: The larger the value the resistor, the greater the thermal noise. This may not be significant in mixer stages at line level but where large gains are required it is desirable to use smaller value resistors. (as small as possible within reason.) Of course sometimes this cannot be achieved but is worth remembering as a rule of thumb. Metal film resistors have less thermal noise than carbon film resistors and are more temperature stable over all. So now there's two reasons to use Metal films in audio circuits.

Driving the busses

Note here that Mixers are more repetitious than complex. The circuits are relatively simple it's just that there's a lot of them. Especially in large recording consoles.

Usually these desks are seen in two halves. The input half and the output half. No matter how complex the input half may become, the output half is essentially just a virtual earth pre-amp as described in the circuits above. Often it is required to have many such busses for things like effects sends, subgroups, monitor bus and so on.

One of the beauties of the virtual earth mixer is that there is also virtually no limit to the number of additional busses as well as the main bus. One could arrange an effects send buss that derives it's signal from the same channel as the main bus. Except that each has it's own volume, pan and assignment independent of each other.

Simple Mixer Schematics

PREAMBLE 

I've been cooking audio circuits for so long now I no-longer need a recipe. A lot of the theory I have forgotten over the years because I've just gotten to know the circuits by instinct. But this should serve as something of a guide to designing mixers from scratch.

The idea: 
Most people reading this would be well aware of what a mixer is used for but I'll reiterate here. The job of an Audio mixer is to combine various audio signals into a single audio signal. It is better known in electronic terms as a summing circuit. That is to say that the output is the sum of all of the inputs. A summing note is often represented as a circle with a PLUS (+) symbol in it.

Audio is of course an AC (Alternating current) signal but if we look at the incoming signals as a frozen moment in time we can represent it as 2 or more DC voltages. This is only useful to illustrate the point.

If we had two signals to be mixed. The first was 2 volts and the second was 3, the output should be the sum of these two voltages. 2+3=5. If on the other hand the two voltages were 2 volts and -3 volts then the output would be -1 volt. We are now subtracting 3 volts from the +2 volts leaving -3. It is important to recognise that we are dealing with what is known as a bipolar signal. That is one that can be positive or negative around a zero base-line.

When you get to the stage of adding many signals together, the complexity grows. In1 + In2 + In3 + In4 + .... and so on.

Because each incoming signal has it's own load impedance it is impractical just to wire all of them together and hope for the best. Especially when the following device you are trying to mix into also represents it's own load impedance. Sometimes you may be able to get away with it because the combined impedance is quite high. However most of the time it drags the whole network down and causes one or more devices to fail or distort or what ever. Usually no damage is done but it just won't work.

What is required is a little load isolation. (See Circuit 1: Passive mixer) The trade off is that you can't use terribly high value resistors because of the losses that they may cause. Especially if the load impedance of the following device is a little low. This will give the effect of severely attenuating some or all of the signals. A practical trade off has to be reached and this is as much trial and error as anything because the conditions change with each new device added or changed.

The device used at the summing node, IE: an amplifier or tape deck should be able to provide enough gain to compensate for the combined losses through the resistors and the combined loading of the system. But the loading will change depending on the combination of devices you have hooked into it.

This approach also creates another side effect. That is that a signal flowing into the summing node via one source can pollute the audio signals of other devices. Say you had two cassette decks that you wished to mix. However you also wanted to send the audio from cassette deck #1 to an effects processor. The audio from cassette deck #2, although attenuated slightly, will find it's way back to the audio from cassette deck 1 and also go to the effects processor.

Active Versus Passive Devices

Electronic components are classed into either being Passive devices or Active devices. Active devices are different from passive devices. These devices are capable of changing their operational performance, may deliver power to the circuit, and can perform interesting mathematical functions. While a device that does not require a source of energy for its operation.

What are Active Devices?

An active device is any type of circuit component with the ability to electrically control electron flow (electricity controlling electricity). In order for a circuit to be properly called electronic, it must contain at least one active device. Active devices include, but are not limited to, vacuum tubes, transistors, silicon-controlled rectifiers (SCRs), and TRIACs.

All active devices control the flow of electrons through them. Some active devices allow a voltage to control this current while other active devices allow another current to do the job. Devices utilizing a static voltage as the controlling signal are, not surprisingly, called voltage-controlled devices. Devices working on the principle of one current controlling another current are known as current-controlled devices. For the record, vacuum tubes are voltage-controlled devices while transistors are made as either voltage-controlled or current controlled types. The first type of transistor successfully demonstrated was a current-controlled device.

What are Passive Devices?

Components incapable of controlling current by means of another electrical signal are called passive devices. Resistors, capacitors, inductors, transformers, and even diodes are all considered passive devices.

Passive devices are the resistors, capacitors, and inductors required to build electronic hardware. They always have a gain less than one, thus they can not oscillate or amplify a signal. A combination of passive components can multiply a signal by values less than one, they can shift the phase of a signal, they can reject a signal because it is not made up of the correct frequencies, they can control complex circuits, but they can not multiply by more than one because they lack gain.

Diodes

Diodes are basically a one-way valve for electrical current. They let it flow in one direction (from positive to negative) and not in the other direction. Most diodes are similar in appearance to a resistor and will have a painted line on one end showing the direction or flow (white side is negative). If the negative side is on the negative end of the circuit, current will flow. If the negative is on the positive side of the circuit no current will flow. More on diodes in later sections.

Integrated Circuits

Integrated Circuits, or ICs, are complex circuits inside one simple package. Silicon and metals are used to simulate resistors, capacitors, transistors, etc. It is a space saving miracle. These components come in a wide variety of packages and sizes. You can tell them by their "monolithic shape" that has a ton of "pins" coming out of them. Their applications are as varied as their packages. It can be a simple timer, to a complex logic circuit, or even a microcontroller (microprocessor with a few added functions) with erasable memory built inside.

Transistors

A transistor is a semiconductor device, commonly used as an amplifier or an electrically controlled switch. The transistor is the fundamental building block of the circuitry in computers, cellular phones, and all other modern electronic devices.

Because of its fast response and accuracy, the transistor is used in a wide variety of digital and analog functions, including amplification, switching, voltage regulation, signal modulation, and oscillators. Transistors may be packaged individually or as part of an integrated circuit, some with over a billion transistors in a very small area - part of a trend of increasing transistor density known as Moore's Law.

Transistor stands for transit resistor, the temporary name, now permanent, that the inventors gave it. These semidconductors control the electrical current flowing between two terminals by applying voltage to a third terminal. You now have a minature switch, presenting either a freeway to electrons or a brick wall to them, depending on whether a signal voltage exists. Bulky mechanical relays that used to switch calls, like the crossbar shown above, could now be replaced with transistors. There's more.

Transistors amplify when built into a proper circuit. A weak signal can be boosted tremendously. Let's say you have ten watts flowing into one side of the transistor. Your current stops because silicon normally isn't a good conductor. You now introduce a signal into the middle of the transistor, say, at one watt. That changes the transistor's internal crystalline structure, causing the silicon to go from an insulator to a conductor. It now allows the larger current to go through, picking up your weak signal along the way, impressing it on the larger voltage. Your one watt signal is now a ten watt signal.

Transistors use the properties of semi-conductors, seemingly innocuous materials like geranium and now mostly silicon. Materials like silver and copper conduct electricity well. Rubber and porcelain conduct electricity poorly. The difference between electrical conductors and insulators is their molecular structure, the stuff that makes them up. Weight, size, or shape doesn't matter, it's how tightly the material holds on to its electrons, preventing them from freely flowing through its atoms.

First Transistor

Active Versus Passive Devices in the News

Seven Steps to Successful Analog-to-Digital Signal Conversion

Understand how to balance gain blocks and noise, and perform noise calculations for proper signal conditioning Signal processing - Business - Technology - Electronics - Integrated circuit

Class D Speaker Amplifier offers automatic level control.

Designed for battery-operated portable devices, 2.2 W MAX98500 integrates boost converter to provide constant output power. Class D amplifier is equipped with battery-tracking Automatic Level Control circuit that limits max output swing as supply voltage drops. ALC helps to avoid clipping and prevents battery voltage from collapsing. Housed in 2.1 x 2.1 mm WLP, MAX98500 accepts 2.5-5.5 V supply ...

Rohde & Schwarz find scope for expansion

Rohde & Schwarz is today launching its first dedicated oscilloscopes. It has developed its own ASICs and A to D converters to support the new design which is described as 20 times faster than existing devices. Oscilloscope - Technology - Theory of Measurements - Electronics - Integrated circuit

Lenny Z Perez M

CRF

http://eesfrequencyresponseofamplifiers.blogspot.com/2010/06/active-versus-passive-devices.html

TRANSISTOR SPECIFICATIONS

Transistors are available in a large variety of shapes and sizes, each with its own unique characteristics. The characteristics for each of these transistors are usually presented on SPECIFICATION SHEETS or they may be included in transistor manuals. Although many properties of a transistor could be specified on these sheets, manufacturers list only some of them. The specifications listed vary with different manufacturers, the type of transistor, and the application of the transistor. The specifications usually cover the following items.

A general description of the transistor that includes the following information:

The kind of transistor. This covers the material used, such as germanium or silicon; the type of transistor(NPN or PNP); and the construction of the transistor(whether alloy-junction, grown, or diffused junction, etc.).

Some of the common applications for the transistor, such as audio amplifier, oscillator, rf amplifier, etc.

General sales features, such as size and packaging(mechanical data).

The "Absolute Maximum Ratings" of the transistor are the direct voltage and current values that if exceeded in operation may result in transistor failure. Maximum ratings usually include collector-to-base voltage, emitter-to-base voltage, collector current, emitter current, and collector power dissipation. The typical operating values of the transistor. These values are presented only as a guide. The values vary widely, are dependent upon operating voltages, and also upon which element is common in the circuit. The values listed may include collector-emitter voltage, collector current, input resistance, load resistance, current-transfer ratio(another name for alpha or beta), and collector cutoff current, which is leakage current from collector to base when no emitter current is applied. Transistor characteristic curves may also be included in this section. A transistor characteristic curve is a graph plotting the relationship between currents and voltages in a circuit. More than one curve on a graph is called a "family of curves." Additional information for engineering-design purposes.

TRANSISTOR IDENTIFICATION

Transistors can be identified by a Joint Army-Navy (JAN) designation printed directly on the case of the transistor. The marking scheme explained earlier for diodes is also used for transistor identification. The first number indicates the number of junctions. The letter "N" following the first number tells us that the component is a semiconductor. And, the 2- or 3-digit number following the N is the manufacturer's identification number. If the last number is followed by a letter, it indicates a later, improved version of the device. For example, a semiconductor designated as type 2N130A signifies a three-element transistor of semiconductor material that is an improved version of type 130:

You may also find other markings on transistors that do not relate to the JAN marking system. These markings are manufacturers' identifications and may not conform to a standardized system. If in doubt, always replace a transistor with one having identical markings. To ensure that an identical replacement or a correct substitute is used, consult an equipment or transistor manual for specifications on the transistor.

TRANSISTOR MAINTENANCE

Transistors are very rugged and are expected to be relatively trouble free. Encapsulation and conformal coating techniques now in use promise extremely long life expectancies. In theory, a transistor should last indefinitely. However, if transistors are subjected to current overloads, the junctions will be damaged or even destroyed. In addition, the application of excessively high operating voltages can damage or destroy the junctions through arc-over or excessive reverse currents. One of the greatest dangers to the transistor is heat, which will cause excessive current flow and eventual destruction of the transistor.

To determine if a transistor is good or bad, you can check it with an ohmmeter or a transistor tester. In many cases, you can substitute a transistor known to be good for one that is questionable and thus determine the condition of a suspected transistor. This method of testing is highly accurate and sometimes the quickest, but it should be used only after you make certain that there are no circuit defects that might damage the replacement transistor. If more than one defective transistor is present in the equipment where the trouble has been localized, this testing method becomes cumbersome, as several transistors may have to be replaced before the trouble is corrected. To determine which stages failed and which transistors are not defective, all the removed transistors must be tested. This test can be made by using a standard Navy ohmmeter, transistor tester, or by observing whether the equipment operates correctly as each of the removed transistors is reinserted into the equipment. A word of caution-indiscriminate substitution of transistors in critical circuits should be avoided.

Lenny Z Perez M

CRF

http://eesfrequencyresponseofamplifiers.blogspot.com/2010/06/transistor-specifications.html

The History of the Integrated Circuit

Our world is full of integrated circuits. You find several of them in computers. For example, most people have probably heard about the microprocessor. The microprocessor is an integrated circuit that processes all information in the computer. It keeps track of what keys are pressed and if the mouse has been moved. It counts numbers and runs programs, games and the operating system. Integrated circuits are also found in almost every modern electrical device such as cars, television sets, CD players, cellular phones, etc. But what is an integrated circuit and what is the history behind it?

Electric Circuits

The integrated circuit is nothing more than a very advanced electric circuit. An electric circuit is made from different electrical components such as transistors, resistors, capacitors and diodes, that are connected to each other in different ways. These components have different behaviors.

The transistor acts like a switch. It can turn electricity on or off, or it can amplify current. It is used for example in computers to store information, or in stereo amplifiers to make the sound signal stronger.

The resistor limits the flow of electricity and gives us the possibility to control the amount of current that is allowed to pass. Resistors are used, among other things, to control the volume in television sets or radios.

The capacitor collects electricity and releases it all in one quick burst; like for instance in cameras where a tiny battery can provide enough energy to fire the flashbulb.

The diode stops electricity under some conditions and allows it to pass only when these conditions change. This is used in, for example, photocells where a light beam that is broken triggers the diode to stop electricity from flowing through it.

These components are like the building blocks in an electrical construction kit. Depending on how the components are put together when building the circuit, everything from a burglar alarm to a computer microprocessor can be constructed.

The Transistor vs. the Vacuum Tube

Of the components mentioned above, the transistor is the most important one for the development of modern computers. Before the transistor, engineers had to use vacuum tubes. Just as the transistor, the vacuum tube can switch electricity on or off, or amplify a current. So why was the vacuum tube replaced by the transistor? There are several reasons.

The vacuum tube looks and behaves very much like a light bulb; it generates a lot of heat and has a tendency to burn out. Also, compared to the transistor it is slow, big and bulky

When engineers tried to build complex circuits using the vacuum tube, they quickly became aware of its limitations. The first digital computer ENIAC, for example, was a huge monster that weighed over thirty tons, and consumed 200 kilowatts of electrical power. It had around 18,000 vacuum tubes that constantly burned out, making it very unreliable.

When the transistor was invented in 1947 it was considered a revolution. Small, fast, reliable and effective, it quickly replaced the vacuum tube. Freed from the limitations of the vacuum tube, engineers finally could begin to realize the electrical constructions of their dreams, or could they?

The Tyranny of Numbers

With the small and effective transistor at their hands, electrical engineers of the 50s saw the possibilities of constructing far more advanced circuits than before. However, as the complexity of the circuits grew, problems started arising.

When building a circuit, it is very important that all connections are intact. If not, the electrical current will be stopped on its way through the circuit, making the circuit fail. Before the integrated circuit, assembly workers had to construct circuits by hand, soldering each component in place and connecting them with metal wires. Engineers soon realized that manually assembling the vast number of tiny components needed in, for example, a computer would be impossible, especially without generating a single faulty connection.

Another problem was the size of the circuits. A complex circuit, like a computer, was dependent on speed. If the components of the computer were too large or the wires interconnecting them too long, the electric signals couldn't travel fast enough through the circuit, thus making the computer too slow to be effective.

So there was a problem of numbers. Advanced circuits contained so many components and connections that they were virtually impossible to build. This problem was known as the tyranny of numbers.

Jack Kilby's Chip - the Monolithic Idea

In the summer of 1958 Jack Kilby at Texas Instruments found a solution to this problem. He was newly employed and had been set to work on a project to build smaller electrical circuits. However, the path that Texas Instruments had chosen for its miniaturization project didn't seem to be the right one to Kilby.

Because he was newly employed, Kilby had no vacation like the rest of the staff. Working alone in the lab, he saw an opportunity to find a solution of his own to the miniaturization problem. Kilby's idea was to make all the components and the chip out of the same block (monolith) of semiconductor material. When the rest of the workers returned from vacation, Kilby presented his new idea to his superiors. He was allowed to build a test version of his circuit. In September 1958, he had his first integrated circuit ready. It was tested and it worked perfectly!

Although the first integrated circuit was pretty crude and had some problems, the idea was groundbreaking. By making all the parts out of the same block of material and adding the metal needed to connect them as a layer on top of it, there was no more need for individual discrete components. No more wires and components had to be assembled manually. The circuits could be made smaller and the manufacturing process could be automated.

Jack Kilby is probably most famous for his invention of the integrated circuit, for which he received the Nobel Prize in Physics in the year 2000. After his success with the integrated circuit Kilby stayed with Texas Instruments and, among other things, he led the team that invented the hand-held calculator.

Robert Noyce

Robert Noyce came up with his own idea for the integrated circuit. He did it half a year later than Jack Kilby. Noyce's circuit solved several practical problems that Kilby's circuit had, mainly the problem of interconnecting all the components on the chip. This was done by adding the metal as a final layer and then removing some of it so that the wires needed to connect the components were formed. This made the integrated circuit more suitable for mass production. Besides being one of the early pioneers of the integrated circuit, Robert Noyce also was one of the co-founders of Intel. Intel is one of the largest manufacturers of integrated circuits in the world.

Chip Production Today - in Short

Chip production today is based on photolithography. In photolithography a high energy UV-light is shone through a mask onto a slice of silicon covered with a photosensitive film. The mask describes the parts of the chip and the UV-light will only hit the areas not covered by the mask. When the film is developed, the areas hit by light are removed. Now the chip has unprotected and protected areas forming a pattern that is the first step to the final components of the chip.

Next, the unprotected areas are processed so their electrical properties change. A new layer of material is added, and the entire process is then repeated to build the circuit, layer by layer. When all the components have been made and the circuit is complete a layer of metal is added. Just as before, a layer of photosensitive film is applied and exposed through a mask. However, this time the mask used describes the layout of the wires connecting all the parts of the chip. The film is developed and the unexposed parts are removed. Next, the metal not protected with film is removed to form the wires. Finally, the chip is tested and packaged.

When making chips today, a process called "stepping" is often used. On a big wafer of silicon the chips are made one next to the other. The silicon wafer is moved in steps under the mask and the UV-light to expose the wafer. In this way, chip after chip can be made using the same mask each time.

Below is a more sequential description of the process of making a modern integrated circuit. But let us first take a look at the special place where integrated circuits are produced - the clean room.

The Clean Room

The sizes of the components on chips produced in a modern chip fabrication plant are extremely small. For a better understanding of how small they are, pick a hair from your head and cut it in half. Now look at the cross section. On this tiny area, hard to see with the bare eye, you can fit thousands of modern transistors.

With sizes this small, the production of a chip demands precision at an atomic level. Tiny particles like a hair, a speck of dust, a dead skin cell, bacteria or even the single particles in tobacco smoke become huge objects that are big enough to ruin a chip.

Therefore, chip production takes place in a clean room. This is a specially designed room, where furniture is built from special materials that don't give off particles, and where extremely effective air filters and air circulation systems change the air completely up to ten times a minute.

To further prevent contamination, workers wear special suits called "bunny suits." These protective outfits are made of ultra clean material and sometimes have their own air filtering systems.

Chip Production Today - in Detail

Building an integrated circuit like a computer chip is a very complex process. It is divided into two major parts, front end and back end. In the front end, you make the components of the circuit. In the back end, you add metal to connect the components and then you test and package the chip. Below is a simplified description of the steps.

Lenny Z Perez M

CRF

http://eesfrequencyresponseofamplifiers.blogspot.com/2010/06/history-of-integrated-circuit.html