Class 12 > Unit # 22:Solid State of Electronics > Transistor as a Switch

Working of Transistor as a Switch - Talha's Physics Academy

Talha's Physics Academy

Working of Transistor as a Switch

Video Lecture

Watch the complete video lecture below to understand how a bipolar junction transistor (BJT) operates as a solid-state switch.

Transistor as a Switch

"The Bipolar Junction Transistor (BJT) can be used as a switch by operating it in the saturated and cut-off regions of its characteristics curves, serving as a solid-state switching element to control DC power to a load."

Principle of Operation

In the off state, the NPN transistor $Q_1$ allows no current to flow through the power circuit unless a forward voltage is applied to its base-emitter circuit. When switch $S_1$ is closed, battery $B_1$ forward-biases the base-emitter junction by applying the necessary positive forward voltage $V_{BE}$, with the emitter serving as the common connection for both the input control and power circuits.

The magnitude of this forward voltage determines the base current within the input control circuit, which in turn directly controls the amount of output collector current flowing through the power circuit. Meanwhile, battery $B_2$ provides a reverse voltage to the collector, establishing a reverse polarity where the collector remains more positive than the base.

Fig: Circuit diagram showing a transistor operating as a switch to control a lamp load.

Working States of the Transistor Switch

1. When Switch $S_1$ is Open (OFF State)

  • No current flows in the base-emitter or control circuit because no forward voltage is applied.
  • The resistance from the emitter to the collector of the transistor is extremely high.
  • No current flows in the power circuit, and the connected lamp remains unlit.

2. When Switch $S_1$ is Closed (ON State)

  • A small control current flows through the base circuit, limited by resistor $R_1$.
  • The resistance from the emitter to the collector of $Q_1$ decreases significantly.
  • A large collector current flows through the power circuit, illuminating the lamp.

Opening switch $S_1$ in the control circuit subsequently turns off the lamp in the power circuit because the resistance from the emitter to the collector of $Q_1$ increases again, almost to infinity.

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