Name: Common Base Configuration
Uploaded: 2019-07-19
Duration: 14 min 41 s
Description: Let's connect the battery to all three terminals of a transistor and understand the relation between the emitter current, base current and collector current.

Question

Explain with a circuit diagram the action of a transistor as a switch.

Accepted Answer

A switch is a device which can turn ON and OFF current is an electrical circuit. The circuit for use of a transistor as a switch is shown below:

$V_{BB} and V_{CC}$ are two dc supplies which bias base-emitter and emitter collector junctions respectively. Let $V_{BB}$ be the input supply voltage. This is also input dc voltage $V_{C}$ The dc output voltage is taken across collector-emitter terminals, $R_{L}$ is the load resistance in the output circuit.

The output characteristic curve is shown below:

When the transistor is non-conducting $(I_{C} = 0)$ , it is said to be ‘switched off’ but when it is conducting $(I_{C} \neq 0)$ ; it is said to be ‘switched ON. As long as the input voltage $V_{i}$ is low and unable to overcome the barrier voltage of the emitter base junction, $V_{o}$ is high ( $I_{C} = 0$ and $V_{o} = V_{CC}$ ), so the transistor is ‘switched OFF’ and if it is high enough to derive the transistor into saturation $I_{C}$ ( is high and so $(V_{0} = V_{CC} - I_{C} R_{L})$ is low, very near to zero, so the transistor is ‘switched ON’. Thus we can say low input switches the transistor is OFF state and high input switches it ON. The switching circuits are designed in such a way that the transistor does not remain in active state.

Explain with a circuit diagram the action of a transistor as a switch.

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