Transistor as an Amplifier

Transistor is a semiconductor device with three terminals viz., Emitter (E), Base (B) and Collector (C) and thus has two junctions viz., Base-Emitter (BE) junction and Base-Collector (BC) junction as shown by Figure 1a. Such a device can operate in three different regions viz., cutoff, active and saturation. Transistors are fully-off in cut-off region while fully-on when operating in saturation region.

However, while they operate in active region, they act as amplifiers i.e. they can be used to increase the strength of the input signal without altering it significantly. The reason behind such a behavior can be understood by analyzing the working of transistor in terms of charge carriers. For this, let us consider a npn bipolar junction transistor (BJT) biased to operate in active region (BE junction is forward biased while BC junction is reverse biased) as shown by Figure 1b.

Here, in general, the emitter will be heavily doped, the base will be lightly doped and the collector will be moderately doped. Further the base will be narrow, the emitter will be broader and the collector will be much broader.
bipolar junction transistor (bjt) showing regions and junctions

The forward bias applied between the base and the emitter terminals of the transistor causes the flow of base current, IB into the base region. However its magnitude is less (usually in terms of μA as VBE is just around 0.6 V, in general).

This can be considered as the movement of electrons out of the base region or the injection of holes into the base region, in equivalent sense. Further, these injected holes attract the electrons in the emitter region towards them, resulting in the recombination of holes and electrons.

However due to the less doping of base in comparison with the emitter, there will be more number of electrons when compared to holes. Thus even after recombination effect, much more electrons will be left free. These electrons now cross the narrow base region and move towards the collector terminal influenced by the bias applied between the collector and the base regions.

This constitutes nothing but the collector current IC moving into the collector. From this it can be noticed that by varying the current flowing into the base region (IB), one can obtain a very large variation in collector current, IC. This is nothing but the current amplification, which leads to the conclusion that the npn transistor operating in its active region acts as a current amplifier. The associated current gain can be mathematically expressed as-

Now consider the npn transistor with the input signal applied between its base and emitter terminals, while the output being collected across the load resistor RC, connected across the collector and the base terminals, as shown by Figure 2.

Now consider the npn transistor with the input signal applied between its base and emitter terminals, while the output being collected across the load resistor RC, connected across the collector and the base terminals, as shown by Figure 2.
transistor as an amplifier
Further note that the transistor is always ensured to operate in its active region by using appropriate voltage supplies, VEE and VBC. Here a small change in the input voltage Vin is seen to change the emitter current IE appreciably as the resistance of the input circuit is low (due to the forward bias condition).

This in turn changes the collector current almost in the same range due to the fact that the magnitude of the base current is quite less for the case under consideration. This large change in IC causes a large voltage drop across the load resistor RC which is nothing but the output voltage.

Hence one gets the amplified version of the input voltage across the output terminals of the device which leads to the conclusion that the circuit acts like a voltage amplifier. Mathematical expression for the voltage gain associated with this phenomenon is given by

Although the explanation provided is for the npn BJT, similar analogy holds good for even pnp BJTs. Following on the same grounds, one can explain the amplifying action of other kind of transistor viz., Field Effect Transistor (FET). Further it is to be noted that there exist many variations to the amplifier circuit of transistors like

  1. First Set: Common Base/Gate Configuration, Common Emitter/Source Configuration, Common Collector/Drain Configuration
  2. Second Set: Class A amplifiers, Class B amplifiers, Class C Amplifiers, Class AB amplifiers
  3. Third Set: Single Stage Amplifiers, Muti-Stage Amplifiers, and so on. However the basic working principle remains the same.
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