Question

Draw transfer characteristics of common emitter n-p-n transistor. Point out the region in which the transistor operates as an amplifier. Define the following terms used in transistor amplifiers: 

(i) Input resistance 

(ii) Output resistance 

(iii) Current amplification factor

Answer 1

In active region the transistor is used as an amplifier. 

(i) Input Resistance: It is the ratio of change in emitter base voltage (ΔV_EB)  to the corresponding change in emitter current(ΔI_E) at constant collector-base voltage (V_CB) i. e.

 

Physically input resistance is the hindrance offered to the signal current. The input resistance is very small, of the order of a few ohms, because a small change in V_EB causes a large change in I_E. 

(ii) Output Resistance: It is the ratio of change in collector-base voltage to the corresponding change in collector current at constant emitter current I_E

The output resistance is very high, of the order of several-tens kilo ohm because a large change in collector-base voltage causes a very small change in collector current. 

(iii) Current amplification factors of a transistor (α and β): The current gain a is defined as the ratio of change in collector current to the change in emitter current for constant value of collector voltage in common base configuration, i.e.,

Practical value of a ranges from 0.9 to 0.99 for junction transistor. The current gain b is defined as the ratio of change in collector current to the change in base current for constant value of collector voltage in common emitter configuration i. e.,