18/11/2020

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 RE 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.

practical-amplifier-circuit


Input capacitor

  • The input capacitor Cin 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 Cin 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 ib flows through base of the transistor. 
  • The base current IB flows through transistor base when only DC voltage is applied therefore total base current

        iB = ib + IB

       Where

       ib = Base current when AC signal applied

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

      iB = Total base current

Collector current

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

      iC = ic + IC

      Where

      ic = Collector current when AC signal applied

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

       iC = Total collector current 

Emitter current

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

        iE = ie + IE

       Where

       ie = Emitter current when AC signal applied

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

      iE = Total emitter current

Finally it should be remember that

     iE = ie + IE

    iC = ic + IC

    iB = ib + IB and

Sum of current

     iE = iC + iB

    ie = ic + ib ( when AC signal is applied )

    IE = IC + IB ( when only DC voltage is applied )

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