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

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