Practical Amplifier Circuit

 

The practical amplifier circuit for faithful amplification is shown in the figure. The function of the each circuit element is described as below.

Biasing circuit

• The resistance R1, R2 and emitter resistance R_E provides biasing and stabilization circuit. 
• The function of the biasing circuit is to established operating point in the middle of the load line otherwise negative cycle of the signal is cut off.

Input capacitor

• The input capacitor C_in is connected between source and base of the transistor. 
• The input capacitor offers infinite reactance to DC therefore block it and allows only AC signal pass through it. 
• If the input capacitor C_in is not used, the signal source resistance will come parallel to resistance R2 and change the bias.

Emitter bypass Capacitor

• The emitter bypass capacitor allows only AC signal pass through it. 
• It provides low reactance path to the amplified AC signal. 
• If it is not used, the AC signal passes through emitter resistance RE and causes voltage drop across it. 
• This will result in reduction of the output voltage.

Coupling Capacitor

• The function of the coupling capacitor is to couple output stage of first amplifier to the input stage of the second amplifier. 
• If the coupling capacitor is not used, the collector resistance directly comes in parallel with the upper resistance R1 therefore change the biasing condition of the amplifier. 
• The coupling capacitor blocks DC of the first stage and allows passage of AC signal for next stage.

Current Passing through Various Circuit Elements

Base current

• When AC signal is applied, the base current i_b flows through base of the transistor. 
• The base current I_B flows through transistor base when only DC voltage is applied therefore total base current

        i_B = i_b + I_B

       Where

       i_b = Base current when AC signal applied

      I_B = Base current when No signal is applied ( only DC voltage )

      i_B = Total base current

Collector current

• When AC signal is applied, the collector current i_c flows through collector of the transistor. 
• The collector current I_C flows through transistor base when only DC voltage is applied therefore total collector current

      i_C = i_c + I_C

      Where

      i_c = Collector current when AC signal applied

      I_C = Collector current when No signal is applied ( only DC voltage )

       i_C = Total collector current 

Emitter current

• When AC signal is applied, the emitter current i_e flows through emitter of the transistor. 
• The emitter current I_E flows through transistor emitter when only DC voltage is applied therefore total emitter current

        i_E = i_e + I_E

       Where

       i_e = Emitter current when AC signal applied

      I_E = Emitter current when No signal is applied ( only DC voltage )

      i_E = Total emitter current

Finally it should be remember that

     i_E = i_e + I_E

    i_C = i_c + I_C

    i_B = i_b + I_B and

Sum of current

     i_E = i_C + i_B

    i_e = i_c + i_b ( when AC signal is applied )

    I_E = I_C + I_B ( when only DC voltage is applied )

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