Common Emitter Amplifier Circuit

Common Emitter Amplifier Circuit

Fig. Practical common-emitter amplifier circuit

From above circuit, it consists of different circuit components. The functions of these components are as follows:

1.  Biasing Circuit:

Resistors R₁, R₂ and R_E forms the voltage divider biasing circuit for CE amplifier and it sets the proper operating point for CE amplifier.

2.  Input Capacitor C₁:

C₁ couples the signal to base of the transistor. It blocks any D.C. component present in the signal and passes only A.C. signal for amplification.

3.  Emitter Bypass Capacitor C_E:

C_E is connected in parallel with emitter resistance R_E to provide a low reactance path to the amplified A.C. This will reduce the output voltage and reducing the gain value.

4.  Output Coupling Capacitor C₂:

C₂ couples the output of the amplifier to the load or to the next stage of the amplifier. It blocks D.C. and passes only A.C. part of the amplified signal.

Need for C₁, C₂, and C_E:

The impedance of the capacitor is given by,

X_C = 1/ (2∏f_c)

Phase reversal:

The phase relationship between the input and output voltages can be determined by considering the effect of positive and negative half cycle separately. The collector current is β times the base current, so the collector current will also increases. This increases the voltage drop across R_C.

V_C = V_CC - I_CR_C

Increase in I_C results in a drop in collector voltage V_C, as V_CC is constant. V_i increases in a positive direction, V_o goes in negative direction and negative half cycle of output voltage can be obtained for positive half cycle at the input.

In negative half cycle of input, A.C. and D.C. voltage will oppose each other. This will reduce the base current. Accordingly collector current and drop across R_C both will reduce and it increases the output voltage. So positive half cycle at the output for negative half cycle at the input can be obtained. So there is a phase shift of 180º between input and output voltages for a common emitter amplifier.