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Explain how Transistor Circuit is used as an Amplifier

Explain how Transistor Circuit is used as an Amplifier

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- A transistor raises the strength of a weak signal and thus acts as an amplifier. Fig. 4.7 shows the basic circuit of a transistor amplifier. The weak signal is applied between emitter-base junction and output is taken across the load R_C connected in the collector circuit. In order to achieve faithful amplification, the input circuit should always remain forward biased. To do so, a d.c. voltage V_EE is applied in the input circuit in addition to the signal as shown. This d.c. voltage is known as bias voltage and its magnitude is such that it always keeps the input circuit forward biased regardless of the polarity of the signal.

- As the input circuit has low resistance, therefore, a small change in signal voltage causes an appreciable change in emitter current. This causes almost the *same change in collector current due to transistor action. The collector current flowing through a high load resistance RC produces a large voltage across it. Thus, a weak signal applied in the input circuit appears in the amplified form in the collector circuit. It is in this way that a transistor acts as an amplifier.


Illustration: The action of a transistor as an amplifier can be made more illustrative if we consider typical circuit values. Suppose collector load resistance RC = 5 kilo-ohms. Let us further assume that a change of 0.1V in signal voltage produces a change of 1 mA in emitter current. Obviously, the change in collector current would also be approximately 1 mA. This collector current flowing through collector load RC would produce a voltage = 5 kilo-ohms × 1 mA = 5 V. Thus, a change of 0.1 V in the signal has caused a change of 5 V in the output circuit. In other words, the transistor has been able to raise the voltage level of the signal from 0.1 V to 5 V i.e. voltage amplification is 50.

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Wilfykil answered the question on August 14, 2019 at 12:48

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