13 Jul 2026

Transistor as a Switch and NOT Gate

practical ug-iv electronics transistor switch not-gate

Experimental arrangement

Transistor-switch test circuit showing the controlled bias, collector load, and measured output state.

Aim

To study the switching action of an NPN transistor and verify its use as a NOT gate.

Apparatus

NPN transistor, DC supply, collector resistor, LED with current-limiting resistor, signal switch, breadboard, and multimeter.

Theory

In the cutoff region the base current is zero and the transistor behaves as an open switch. In saturation, both junctions are forward biased and the transistor behaves approximately as a closed switch. With the collector connected to the output node, a low input produces a high output and a high input produces a low output.

Observations

Input $V_{in}$	Transistor state	Output $V_{out}$	LED state
$0\,\text{V}$	Cutoff	$4.82\,\text{V}$	OFF
$5\,\text{V}$	Saturation	$0.18\,\text{V}$	ON

Truth table

Input $A$	Output $Y=\overline{A}$
0	1
1	0

Result

The transistor operates as a switch and gives the NOT-gate relation $Y=\overline{A}$.

Precautions

1. Use a base resistor to limit base current.
2. Observe the transistor pin configuration before wiring.
3. Use a current-limiting resistor with the LED.
4. Switch off the supply before changing connections.

Viva Questions

1. What are the switching regions? Cutoff and saturation.
2. Why is the collector output inverted? An increase in collector current produces a larger voltage drop across the collector resistor.
3. What is saturation voltage? It is the small collector-emitter voltage when the transistor is fully ON.
4. Why is a resistor connected to the base? It prevents excessive base current.