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Bipolar Junction Transistor [BJT] - Transistor as a switch | 12th Physics : UNIT 10a : Semiconductor Electronics

Chapter: 12th Physics : UNIT 10a : Semiconductor Electronics

Transistor as a switch

The transistor in saturation and cut-off regions functions like an electronic switch that helps to turn ON or OFF a given circuit by a small control signal.

Transistor as a switch

The transistor in saturation and cut-off regions functions like an electronic switch that helps to turn ON or OFF a given circuit by a small control signal. The circuit is shown in Figure 9.34.


• Presence of dc source at the input (saturation region):

When a high input voltage (Vin = +5V ) is applied, the base current (IB) increases and in turn increases the collector current. The transistor will move into the saturation region (turned ON). The increase in collector current (IC) increases the voltage drop across RC , thereby lowering the output voltage, close to zero. The transistor acts like a closed switch and is equivalent to ON condition.

• Absence of dc source at the input (cut-o region):

A low input voltage (Vin = 0V ), decreases the base current (IB) and in turn decreases the collector current (IC). The transistor will move into the cut-off region (turned OFF). The decrease in collector current (IC) decreases the drop across RC , thereby increasing the output voltage, close to +5 V. The transistor acts as an open switch which is considered as the OFF condition.

It is manifested that, a high input gives a low output and a low input gives a high output. In addition, we can say that the output voltage is opposite to the applied input voltage. Therefore, a transistor can be used as an inverter (NOT gate) in computer logic circuitry.

 

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