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Show and explain a Practical Circuit of Transistor Amplifier

      

Show and explain a Practical Circuit of Transistor Amplifier

  

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Wilfred
- It is important to note that a transistor can accomplish faithful amplification only if proper associated circuitry is used with it. Fig. 5.3 shows a practical single stage transistor amplifier. The various circuit elements and their functions are described below:

(i) Biasing circuit: The resistances R1, R2 and RE form the biasing and stabilisation circuit.
- The biasing circuit must establish a proper operating point otherwise a part of the negative half-cycle of the signal may be cut off in the output.

(ii) Input capacitor Cin: An electrolytic capacitor Cin (˜ 10 µF ) is used to couple the signal to the base of the transistor. If it is not used, the signal source resistance will come across R2 and thus change the bias. The capacitor Cin allows only a.c. signal to flow but isolates the signal source from R5.
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i. Emitter bypass capacitor CE. An emitter bypass capacitor CE ( ˜ 100µF ) is used in parallel with RE to provide a low reactance path to the amplified a.c. signal. If it is not used, then amplified a.c. signal flowing through RE will cause a voltage drop across it, thereby reducing the output voltage.
ii. Coupling capacitor CC. The coupling capacitor CC (˜10µF) couples one stage of amplification to the next stage. If it is not used, the bias conditions of the next stage will be drastically changed due to the shunting effect of RC. This is because RC will come in parallel with the upper resistance R1 of the biasing network of the next stage, thereby altering the biasing conditions of the latter. In short, the coupling capacitor CC isolates the d.c. of one stage from the next stage, but allows the passage of a.c. signal.

- It may be noted that a capacitor offers infinite reactance to d.c. and blocks it completely whereas it allows a.c. to pass through it.

- Various circuit currents: It is useful to mention the various currents in the complete amplifier circuit. These are shown in the circuit of Fig. 5.3
(i) Base current. When no signal is applied in the base circuit, d.c. base current IB flows due to biasing circuit. When a.c. signal is applied, a.c. base current ib also flows. Therefore, with the application of signal, total base current iB is given by:
iB = IB + ib

(ii) Collector current. When no signal is applied, a d.c. collector current IC flows due to biasing circuit. When a.c. signal is applied, a.c. collector current ic also flows. Therefore, the total collector current iC is given by:
iC = IC + ic where IC = ß IB = zero signal collector current ic = ß ib = collector current due to signal.

(iii) Emitter current. When no signal is applied, a d.c. emitter current IE flows. With the application of signal, total emitter current iE is given by:
iE = IE + ie
It is useful to keep in mind that:
IE = IB + IC
ie = ib + ic
Now base current is usually very small, therefore, as a reasonable approximation,
IE ˜ IC and ie ˜ ic
Wilfykil answered the question on August 15, 2019 at 08:32


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