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Chapter: Electronic Circuits - BJT Amplifiers

Common Emitter Amplifier Circuit

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

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 RE 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 CE:

 

CE is connected in parallel with emitter resistance RE 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 CE:

 

The impedance of the capacitor is given by,

XC = 1/ (2∏fc)

 

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 RC.

VC = VCC - ICRC

 

Increase in IC results in a drop in collector voltage VC, as VCC is constant. Vi increases in a positive direction, Vo 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 RC 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.

 

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