Electronic Circuit Design Lab Experiments

Study of BJT as a Switch

Sep 24 2023

Aim

Apparatus Required:

Breadboard ,Workstation, Function Generator, Trainer Kit, Patch cords, DC Power supply

Theory:

Transistor is a three terminal, bipolar, current controlled device. It works in three different regions: Active, Cutoff and Saturation according to applied biasing condition. In this circuit, a square wave input is applied. When input is high, the transistor is turned on and works in saturation region. So maximum current IC flows through transistor as well as LED. Hence LED emits the light. When input is low (low means not enough to turn on the transistor), the transistor remains in cutoff. So current IC is zero thus LED does not emit the light. As the input is square wave, the LED will turn on and off alternately. If output is observed on CRO from the collector then it will be also a square wave but out of phase by 180° with input. Thus transistor is working as a switch which can be made on or off by an external input.

Circuit Diagram:

Procedure:

Connect the circuit as shown in figure.
Apply 1 Hz square wave signal to the base and ground from function generator.
Apply 5V VCC to collector and ground.
Observe the indication of LED and see the output waveforms on CRO.

Result:Result are verified through oscilloscope at the output terminal

Viva Questions:

Q.-1. How BJT work as a switch?

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Q.-2. In which region BJT work as a switch ?

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Q.-3. Why BJT called as Bipolar device?

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Q.-4. Explain the Base width modulation?

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Q.-5. Is it possible to make a BJT as a small signal amplifier

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