Question

(i) Draw a circuit diagram to study the input and output characteristics of a n-p-n transistor in its common emitter configuration. Draw the typical input and output characteristics.

(ii) Explain, with the help of a circuit diagram, the working of n-p-n transistor as a common emitter amplifier.

Answer 1

(i) Common emitter characteristics: The circuit diagram for common emitter n-p-n transistor is shown in Fig. (a). The input is applied across the emitter and base and output is taken across collector and emitter. Here emitter is common to both input and output circuits. The most important characteristics of common base connections are input characteristics and output characteristics.

(a) Input characteristics

A graph showing the relationship between base-emitter voltage and base-current and different constant collector-emitter is called input characteristics of the transistor.

To obtain input characteristic, the emitter is forward biased by base emitter voltage V_be and the collector is reverse biased by collector emitter voltage V_ce. The collector voltage V_ce is kept constant at a suitable value. The base voltage V_p is varied in small steps and the base current I_b is noted corresponding to each value of the base-emitter voltage. A curve is then plotted between V_be and I_b for a given value of V_ce. This is one characteristic. Similar characteristic curves can be drawn for different fixed values of V_ce (say 2V, 3V, 4V etc.,) as shown in Fig. (b)

(b) Output Characteristics

A graph showing the relationship between the collector emitter voltage and collector current at constant base curent is called output characteristics of the transistor.

To obtain output characteristics the base current is kept constant at a suitable value the collector-emitter voltage V_ce is varied in small steps and the collector current I_c is noted corresponding to each value of the base current I_b. A curve is then plotted between V_ce and I_c for a given constant value of I_b. This is one characteristic. Similar characteristic curves can be drawn for different fixed values of I_b (say 10µA, 15 µA, 20 µA etc. as shown in Fig.

(ii)

Common emitter Transistor amplifier

The circuit diagram for common emitter n-p-n amplifier is as shown in Fig. In this circuit, the emitter lead is common to both the input (emitter-base) and output (collector-emitter) circuit and is grounded. The emitter is forward biased by emitter base battery V_eb and collector is reversed biased by collector-emitter battery V_ec, so output resistance is more than input resistance i.e. R₀ >> R_i.

Let I_e, I_b and I_c be emitter, base and collector current respectively.

So I_e = I_b + I_c ...........(1)

If R_L is load resistance, then the potential drop across R_L is I_cR_L

Hence, collector voltage

V_c = V_ec - I_cR_L ..........(2)

When a.c. is fed to the input signal let the first half cycle of input voltage is positive. It will make base more positive and hence emitter current will increase and correspondingly collector current also increases. Hence I_cR_L factor increases from eq. (2) we find that collector voltage will decrease. Thus corresponding to positive half cycle of a.c. input signal, negative output half cycle will be obtained.

During second half cycle of a.c. input, voltage will be negative and it will make base more negative which decreases the emitter current and hence collector current lc also decreases. So I_cR_L factor also decreases. From eq. (2) we find that collector voltage will decrease. Thus corresponding to negative half cycle of a.c. input signal, positive output half cycle will be obtained.

Thus in common emitter amplifier the output voltage signal are out of phase with each other as shown in Fig. since input resistance is moderately low (1-2 kΩ) and the output resistance is moderately high (≅ 50 kΩ), so the collector current (output current is very large as compared to base current (input current).

1. Ac current gain (ß_a.c.)

The ratio of change in collector current to the change in base current at constant collector voltage. Hence, 

ß_ac = \((\frac{\bigtriangleup I_c}{\bigtriangleup I_b})_{Ve}\) ...........(3)

2. d.c. current gain (ß_d.c)

The ratio of collector current to the base current at constant collector voltage is called dc current gain. Hence,

ß_d.c. = \((\frac{\bigtriangleup I_c}{\bigtriangleup I_b})_{Ve}\) ...........(4)

ß_d.c. is very nearly equal to ß_a.c., so we generally denote the current gain in CE mode as ß. Unless otherwise specified ß means ac current gain. The value of ß varies from 20 to 50.

3. Voltage gain (A_v)

The ratio of change in output voltage to the change in input voltage is called voltage gain.

i.e. A_v = \(\frac{\bigtriangleup V_0}{\bigtriangleup V_b} = \frac{\bigtriangleup I_c}{\bigtriangleup I_b} . \frac{R_L}{R_i}\)

or A_v = ß. \(\frac{R_L}{R_i}\) ..........(5)

or A_v = ß (resistance gain)

Which is higher than that obtained in case of common-base transisor amplifier.

4. Power gain (P)

The ratio of change in output power to the change in input power is called power gain. Hence,

Power gain = voltage gain x current gain

 = ß \((\frac{R_L}{R_i})\) x ß

So, power gain P = ß² (Resistance gain)

= very high power gain ...........(6)