Question

Discuss common transistor biasing in the case of p-n-p transistor. Discuss its characteristics curve and current amplification factor ?

Answer 1

The variation of base current I_b (input) with base emitter voltage (V_BE) at constant collector emitter voltage (V_CE) is called input characteristic.

Circuit diagram to study the common-emitter characteristics of a transistor is shown.

Conclusion—

(i) The input characteristics are similar to forward bias characteristics of a junction diode.

(ii) For a given value of emitter-base voltage (V_BE), the base current decrease with the increase in collector emitter voltage.

Output Characteristics of Common Emitter Transistor

The variation of the collector current I_C (output) with collector-emitter voltage (V_CE) at constant base current (I_b ) is called output characteristic. 

(i) Keep the base current (I_b) fixed (say, 10 μA).

(ii)  Now change the collector-emitter voltage (V_CE) using variable resistance R₂ and note the corresponding values of collector current. (I_C) 

(iii) The graph between the various values of V_CE and I_C is plotted which is the output characteristic of the transistor.

(iv) A set of such curves can be plotted at different fixed values of base current ((say 20μA, 30 μA, 40 μA).

Conclusion—

(i) For a given value of base current, collector current increases rapidly with the collector-emitter voltage in the begining but at high value of V_CE , collector current becomes constant.

(ii) For a given value of V_CE , the collector current (I_C) is higher for high value of base current (I_b).

Output resistance (r_o) is defined as the ratio of small change in V_CE (i.e. ΔV_CE) to the small change in collector current (ΔI_C) at constant base current.

This is also know as a.c. resistance (i.e. effective resistance in the output for an a.c. input signal).

D.C.resistance = V_CE/I_C

Output characteristics of a transistor in common-emitter configuration are divided into three regions :

(i) active region,

(ii) cut off region and

(iii) saturation region

Active region -  Active region lies above I_B = 0 as shown in figure. In this region, collector junction in reverse biased and emitter junction in forward biased for a given value of IB, collector current increases as / VCE / increases.

A transistor is operated in active region if it is used as an amplifier.

Cut off region—Cut off region lies below IB = 0. The collecotor current has finite value under this condition. In order to cut off the transistor, the emitter junction has to made slightly reverse biased in addition to IB = 0.

Saturation region—Saturation region lies close to zero voltage axis where all the curves coincide. In this region, collector current is independent of the base current.

Current Amplification factor (β) :

(i) d.c. current gain (βdc)—

It is defined as the ratio of the collector current (I_e ) to the base current (I_B). That is βdc = I_C/I_B

Since I_C > I_B , so βdc is greater than 1. The value of βdc lies between 20 and 200.

(ii) Small signal current gain (βac)—

It is defined as the ratio of small change in collector current (ΔI_C) to the small change in base-current (ΔI_B) at constant collector-emitter voltage (V_CE).

That is,