Transistor as an amplifier

Consider a npn transistor in CE configuration as shown above along with its input characteristics.

A transistor raises the strength of a weak input signal and thus acts as an amplifier. The weak signal to be amplified is applied between emitter and base and the output is taken across the load resistor R_C connected in the collector circuit.

In order to use a transistor as an amplifier it should be operated in active region i.e. emitter junction should be always FB and collector junction should be RB. Therefore in addition to the a.c. input source V_i two d.c. voltages V_EB and V_CE are applied as shown. This d.c. voltage is called bias voltage.

As the input circuit has low resistance, a small change in te signal voltage V_i causes a large change in the base current thereby causing the same change in collector current (because I_C = βI_B).

The collector current flowing through a high load resistance R_C produces a large voltage across it. Thus a weak signal applied at the input circuit appears in the amplified form at the output. In this way transistor acts as an amplifier.

Example: Let R_C = 5KΩ, V_in =1V, I_C =1mA then output V=I_CR_C =5V

Bias stabilization

The process of making operating point independent of temperature changes or variation in transistor parameters is called the stabilization.

We know that for transistor to operate it should be properly biased so that we can have a fixed operating point. To avoid any distortions, the Q-point should be at the center of the load line.

But in practice this Q-point may shift to any operating region (saturation or cur-off region) making the transistor unstable. Therefore in order to avoid this, biasing stability should be maintained.

Causes for Bias instability

Bias instability occurs mainly due to two reasons.

1. Temperature
2. Current gain

1. Temperature (T)

The temperature at the junctions of a transistor depends on the amount of current flowing through it. Due to increase in temperature following parameters of a  transistor will change.

(a)base-emitter voltage (V_BE)

V_BE increases at a rate of 2.4mV/⁰C. With increase in temperature the base current I_B will increase and since I_C= βI_B, I_C is also increased hence, changing the Q-point.

(b) Reverse saturation current ( I_CBO)

We know that I_C = βI_B + (1+β) I_CBO where I_CBO is the reverse saturation current. As the temperature increases I_CBO increases there by increase in I_C and hence changing the Q-point.

2. Current gain (β)

In the process of manufacturing the transistors different transistors of same type may have different parameters ( i.e. if we take two transistor units of same type and use them in the circuit there is a change in the β value in actual practice ). The biasing circuit will be designed according to the required β value but due to the change in β from unit to unit the operating point may shift.