Transistor as an Amppfier

For a transistor to act as an amppfier, it should be properly biased. We will discuss the need for proper biasing in the next chapter. Here, let us focus how a transistor works as an amppfier.

Transistor Amppfier

A transistor acts as an amppfier by raising the strength of a weak signal. The DC bias voltage appped to the emitter base junction, makes it remain in forward biased condition. This forward bias is maintained regardless of the polarity of the signal. The below figure shows how a transistor looks pke when connected as an amppfier.

The low resistance in input circuit, lets any small change in input signal to result in an appreciable change in the output. The emitter current caused by the input signal contributes the collector current, which when flows through the load resistor R_L, results in a large voltage drop across it. Thus a small input voltage results in a large output voltage, which shows that the transistor works as an amppfier.

Example

Let there be a change of 0.1v in the input voltage being appped, which further produces a change of 1mA in the emitter current. This emitter current will obviously produce a change in collector current, which would also be 1mA.

A load resistance of 5kΩ placed in the collector would produce a voltage of

5 kΩ × 1 mA = 5V

Hence it is observed that a change of 0.1v in the input gives a change of 5v in the output, which means the voltage level of the signal is amppfied.

Performance of Amppfier

As the common emitter mode of connection is mostly adopted, let us first understand a few important terms with reference to this mode of connection.

Input Resistance

As the input circuit is forward biased, the input resistance will be low. The input resistance is the opposition offered by the base-emitter junction to the signal flow.

By definition, it is the ratio of small change in base-emitter voltage (ΔV_BE) to the resulting change in base current (ΔI_B) at constant collector-emitter voltage.

Input resistance, $R_i = frac{Delta V_{BE}}{Delta I_B}$

Where R_i = input resistance, V_BE = base-emitter voltage, and I_B = base current.

Output Resistance

The output resistance of a transistor amppfier is very high. The collector current changes very spghtly with the change in collector-emitter voltage.

By definition, it is the ratio of change in collector-emitter voltage (ΔV_CE) to the resulting change in collector current (ΔI_C) at constant base current.

Output resistance = $R_o = frac{Delta V_{CE}}{Delta I_C}$

Where R_o = Output resistance, V_CE = Collector-emitter voltage, and I_C = Collector-emitter voltage.

Effective Collector Load

The load is connected at the collector of a transistor and for a single-stage amppfier, the output voltage is taken from the collector of the transistor and for a multi-stage amppfier, the same is collected from a cascaded stages of transistor circuit.

By definition, it is the total load as seen by the a.c. collector current. In case of single stage amppfiers, the effective collector load is a parallel combination of R_C and R_o.

Effective Collector Load, $R_{AC} = R_C // R_o$

$$= frac{R_C imes R_o}{R_C + R_o} = R_{AC}$$

Hence for a single stage amppfier, effective load is equal to collector load R_C.

In a multi-stage amppfier (i.e. having more than one amppfication stage), the input resistance R_i of the next stage also comes into picture.

Effective collector load becomes parallel combination of R_C, R_o and R_i i.e,

Effective Collector Load, $R_{AC} = R_C // R_o // R_i$

$$R_C // R_i = frac{R_C R_i}{R_C + R_i}$$

As input resistance R_i is quite small, therefore effective load is reduced.

Current Gain

The gain in terms of current when the changes in input and output currents are observed, is called as Current gain. By definition, it is the ratio of change in collector current (ΔI_C) to the change in base current (ΔI_B).

Current gain, $eta = frac{Delta I_C}{Delta I_B}$

The value of β ranges from 20 to 500. The current gain indicates that input current becomes β times in the collector current.

Voltage Gain

The gain in terms of voltage when the changes in input and output currents are observed, is called as Voltage gain. By definition, it is the ratio of change in output voltage (ΔV_CE) to the change in input voltage (ΔV_BE).

Voltage gain, $A_V = frac{Delta V_{CE}}{Delta V_{BE}}$

$$= frac{Change : in: output : current imes effective: load}{Change : in: input : current imes input : resistance}$$

$$= frac{Delta I_C imes R_{AC}}{Delta I_B imes R_i} = frac{Delta I_C}{Delta I_B} imes frac{R_{AC}}{R_i} = eta imes frac{R_{AC}}{R_i}$$

For a single stage, R_AC = R_C.

However, for Multistage,

$$R_{AC} = frac{R_C imes R_i}{R_C + R_i}$$

Where R_i is the input resistance of the next stage.

Power Gain

The gain in terms of power when the changes in input and output currents are observed, is called as Power gain.

By definition, it is the ratio of output signal power to the input signal power.

Power gain, $A_P = frac{(Delta I_C)^2 imes R_{AC}}{(Delta I_B)^2 imes R_i}$

$$= left ( frac{Delta I_C}{Delta I_B} ight ) imes frac{Delta I_C imes R_{AC}}{Delta I_B imes R_i}$$

= Current gain × Voltage gain

Hence these are all the important terms which refer the performance of amppfiers.