Question

Explain the operation of transistor as a switch and as an amplifier, with neat circuit diagrams. 

Answer 1

Explanation of transistor as a switch:

The transistor can be used for two types of application viz. amplification and switching. For the amplification, transistor is biased in its active region.

For switching applications it is biased to operate in the cut off (full OFF) region or saturation (full ON) region.

a.Transistor in cut- off region:

In the cut –off region both the junctions of the transistor are reverse biased and very small reverse current flows through the transistors.

The voltage drop across the transistor (VCE) is high nearly equal to supply voltage VCC. . So, in the cut off region the transistor is equivalent to an open switch as shown in figure.

b. Transistor in the saturation region:

When Vin is positive a large base current flows and transistor saturates.

In the saturation region both the junctions of a transistor are forward biased. The voltage drop across the transistor (VCE) is very small of the order of 0.2V to 1 V, depending on the type of transistor and collector current is very large.

In saturation the transistor is equivalent to a closed switch.

Explanation of transistor as an amplifier:

For a transistor to act as an amplifier, it should be properly biased. The Emitter Base junction is forward biased and collector Emitter junction is reverse biased.

The low resistance in input circuit lets any small change in input signal to result in an appreciable change in the output. The emitter current caused by the input signal contributes the collector current, which when flows through the load resistor RL, results in a large voltage drop across it. Thus a small input voltage results in a large output voltage, which shows that the transistor works as an amplifier.