## 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.

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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