BJT Amplifier Experiment

Introduction

In this experiment, we will design and test a basic BJT (Bipolar Junction Transistor) amplifier. BJT amplifiers are widely used in analog circuits for signal amplification. This experiment introduces amplifier design concepts, including setting the operating point and measuring gain.

Objective

The objective of this experiment is to understand how a BJT can be used to amplify a small AC signal, measure its voltage gain, and observe its response characteristics.

Components Required

• NPN Transistor (e.g., 2N2222 or BC547)
• Resistors (1kΩ, 10kΩ, and 100kΩ)
• Capacitors (10µF and 100nF)
• DC Power Supply (e.g., 9V battery)
• Signal Generator
• Oscilloscope or Multimeter
• Connecting Wires
• Breadboard

Circuit Diagram

Procedure

1. Set Up the Circuit: Connect the circuit as shown in the circuit diagram. The transistor's base is biased using a resistor (100kΩ), the emitter is connected to ground through a small resistor (1kΩ), and the collector is connected to the power supply through a load resistor (10kΩ).
2. Connect Input Signal: Connect a small AC signal (e.g., 1 kHz sine wave at 50mV peak-to-peak) from the signal generator to the base of the transistor through a coupling capacitor (10µF).
3. Power the Circuit: Turn on the DC power supply and set it to 9V.
4. Observe Output Signal: Connect the oscilloscope to the collector of the transistor. Observe the amplified output signal on the oscilloscope screen.
5. Measure Gain: Calculate the voltage gain by measuring the peak-to-peak voltage of the output signal and dividing it by the input signal’s peak-to-peak voltage.

Calculations

The voltage gain (A_V) of the amplifier is calculated as:

A_V = V_out / V_in

Using the oscilloscope, measure the input voltage (V_in) and output voltage (V_out) to determine the gain.

Observations and Analysis

• The output signal should be an amplified version of the input signal.
• The output signal will be inverted in phase compared to the input, as typical for a common-emitter amplifier.
• Record the measured gain and compare it with the theoretical gain calculated from component values.

Conclusion

This experiment demonstrates how a BJT can amplify a small AC signal. By properly biasing the transistor, we achieved signal amplification and measured the voltage gain. This setup highlights the fundamental principles of analog amplification and BJT operation in amplifier circuits.