Active-mode NPN transistors in circuits
The diagram opposite is a schematic representation of an NPN transistor connected to two voltage sources. To make the transistor conduct appreciable current (on the order of 1 mA) from C to E, VBE must be above a minimum value sometimes referred to as the cut-in voltage. The cut-in voltage is usually about 600 mV for silicon BJTs at room temperature but can be different depending on the type of transistor and its biasing. This applied voltage causes the lower P-N junction to 'turn-on' allowing a flow of electrons from the emitter into the base. In active mode, the electric field existing between base and collector (caused by VCE) will cause the majority of these electrons to cross the upper P-N junction into the collector to form the collector current IC. The remainder of the electrons recombine with holes, the majority carriers in the base, making a current through the base connection to form the base current, IB. As shown in the diagram, the emitter current, IE, is the total transistor current, which is the sum of the other terminal currents (i.e., ).
In the diagram, the arrows representing current point in the direction of conventional current - the flow of electrons is in the opposite direction of the arrows because electrons carry negative electric charge. In active mode, the ratio of the collector current to the base current is called the DC current gain. This gain is usually 100 or more, but robust circuit designs do not depend on the exact value (for example see op-amp). The value of this gain for DC signals is referred to as hFE, and the value of this gain for AC signals is referred to as hfe. However, when there is no particular frequency range of interest, the symbol β is used[citation needed].
It should also be noted that the emitter current is related to VBE exponentially. At room temperature, an increase in VBE by approximately 60 mV increases the emitter current by a factor of 10. Because the base current is approximately proportional to the collector and emitter currents, they vary in the same way.
Active-mode PNP transistors in circuits
The diagram opposite is a schematic representation of a PNP transistor connected to two voltage sources. To make the transistor conduct appreciable current (on the order of 1 mA) from E to C, VEB must be above a minimum value sometimes referred to as the cut-in voltage. The cut-in voltage is usually about 600 mV for silicon BJTs at room temperature but can be different depending on the type of transistor and its biasing. This applied voltage causes the upper P-N junction to 'turn-on' allowing a flow of holes from the emitter into the base. In active mode, the electric field existing between the emitter and the collector (caused by VCE) causes the majority of these holes to cross the lower P-N junction into the collector to form the collector current IC. The remainder of the holes recombine with electrons, the majority carriers in the base, making a current through the base connection to form the base current, IB. As shown in the diagram, the emitter current, IE, is the total transistor current, which is the sum of the other terminal currents (i.e., ).
In the diagram, the arrows representing current point in the direction of conventional current - the flow of holes is in the same direction of the arrows because holes carry positive electric charge. In active mode, the ratio of the collector current to the base current is called the DC current gain. This gain is usually 100 or more, but robust circuit designs do not depend on the exact value. The value of this gain for DC signals is referred to as hFE, and the value of this gain for AC signals is referred to as hfe. However, when there is no particular frequency range of interest, the symbol β is used[citation needed].
It should also be noted that the emitter current is related to VEB exponentially. At room temperature, an increase in VEB by approximately 60 mV increases the emitter current by a factor of 10. Because the base current is approximately proportional to the collector and emitter currents, they vary in the same way.
The bipolar point-contact transistor was invented in December 1947 at the Bell Telephone Laboratories by John Bardeen and Walter Brattain under the direction of William Shockley. The junction version known as the bipolar junction transistor, invented by Shockley in 1948, enjoyed three decades as the device of choice in the design of discrete and integrated circuits. Nowadays, the use of the BJT has declined in favour of CMOS technology in the design of digital integrated circuits.
Regions of operation
Bipolar transistors have five distinct regions of operation, defined mostly by applied bias:
Forward-active (or simply, active): The base-emitter junction is forward biased and the base-collector junction is reverse biased. Most bipolar transistors are designed to afford the greatest common-emitter current gain, βF, in forward-active mode. If this is the case, the collector-emitter current is approximately proportional to the base current, but many times larger, for small base current variations.
Reverse-active (or inverse-active or inverted): By reversing the biasing conditions of the forward-active region, a bipolar transistor goes into reverse-active mode. In this mode, the emitter and collector regions switch roles. Because most BJTs are designed to maximize current gain in forward-active mode, the βF in inverted mode is several (2-3 for the ordinary germanium transistor) times smaller. This transistor mode is seldom used, usually being considered only for failsafe conditions and some types of bipolar logic. The reverse bias breakdown voltage to the base may be an order of magnitude lower in this region.
Saturation: With both junctions forward-biased, a BJT is in saturation mode and facilitates high current conduction from the emitter to the collector. This mode corresponds to a logical "on", or a closed switch.
Cutoff: In cutoff, biasing conditions opposite of saturation (both junctions reverse biased) are present. There is very little current flow, which corresponds to a logical "off", or an open switch.
Avalanche breakdown region
Although these regions are well defined for sufficiently large applied voltage, they overlap somewhat for small (less than a few hundred millivolts) biases. For example, in the typical grounded-emitter configuration of an NPN BJT used as a pulldown switch in digital logic, the "off" state never involves a reverse-biased junction because the base voltage never goes below ground; nevertheless the forward bias is close enough to zero that essentially no current flows, so this end of the forward active region can be regarded as the cutoff region.
To understand the three regions of operation of the transistor, consider the circuit below:
The first region is called "cutoff". This is the case where the transistor is essentially inactive.
In cutoff, the following behavior is noted:
* Ib = 0 (no base current)
* Ic = 0 (no collector current)
* Vbe <>
Whenever we observe the terminals of a BJT and see that the emitter-base junction is not at
least 0.6-0.7 volts, the transistor is in the cutoff region. In cutoff, the transistor appears as an
open circuit between the collector and emitter terminals. In the circuit above, this implies Vout
is equal to 10 volts.
The second region is called "saturation". This is where the base current has increased well
beyond the point that the emitter-base junction is forward biased. In fact, the base current has
increased beyond the point where it can cause the collector current flow to increase. In saturation,
the transistor appears as a near short circuit between the collector and emitter terminals.
In the circuit above, this implies Vout is almost 0 volts, but actually about 0.2 volts.
In saturation, the following behavior is noted:
* Vce <= 0.2V. This is known as the saturation voltage, or Vce(sat)
* Ib > 0, and Ic > 0
* Vbe >= 0.7V
Using the two states of cutoff and saturation, the transistor may be used as a switch. The collector
and emitter form the switch terminals and the base is the switch handle. In other words,
the small base current can be made to control a much larger current between the collector and
emitter. For example, the circuit above can be modified to control an electric motor. The motor
would replace the collector resistor and transistor would act as a switch. See the drawing
below.
When high current motors are switched on and off, mechanical switch contacts can eventually
wear out causing the switch to fail., The BJT can operate as a switch however that has no mechanism that causes it wear out. When it is saturated, the bottom terminal of the motor is
essentially connected to ground. When cutoff, the bottom end of the motor is seemingly not
connected to anything. Used in this manner, the switch only has to handle 1/100 of the motor
current, greatly increasing its life.
The final region of operation of the BJT is the "forward active" region. It is in this region that
the transistor can act as a fairly linear amplifier. In this region, we see that:
* 0.2 <>
* Ib > 0 and Ic > 0
* Vbe >= 0.7V
Thus the transistor is on and the collector to emitter voltage is somewhere between the cutoff
and saturated states. In this state, the transistor is able to amplify small variations in the voltage
present on the base. The output is extracted at the collector. In the forward active state, the
collector current is proportional to the base current by a constant multiplier called "beta",
denoted by the symbol b. Thus in the forward active region we will also observe that:
* Ic = b*Ib
When high current motors are switched on and off, mechanical switch contacts can eventually
wear out causing the switch to fail., The BJT can operate as a switch however that has no
mechanism that causes it wear out. When it is saturated, the bottom terminal of the motor is
essentially connected to ground. When cutoff, the bottom end of the motor is seemingly not
connected to anything. Used in this manner, the switch only has to handle 1/100 of the motor
current, greatly increasing its life.
The final region of operation of the BJT is the "forward active" region. It is in this region that
the transistor can act as a fairly linear amplifier. In this region, we see that:
* 0.2 <>
* Ib > 0 and Ic > 0
* Vbe >= 0.7V
Thus the transistor is on and the collector to emitter voltage is somewhere between the cutoff
and saturated states. In this state, the transistor is able to amplify small variations in the voltage
present on the base. The output is extracted at the collector. In the forward active state, the
collector current is proportional to the base current by a constant multiplier called "beta",
denoted by the symbol b. Thus in the forward active region we will also observe that:
* Ic = b*Ib
CRF
Lenny Z. Perez M.
19.877.181
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