Report on Electronic Practice: Common
Emitter Amplifier
1. Introduction
The common emitter amplifier (CEA) is one of the most widely used configurations in analog
electronics due to its ability to amplify voltage signals. It is commonly employed in audio
amplification, signal processing, and communication systems. This report presents a practical
study of the common emitter amplifier, covering circuit design, operation, performance analysis,
and conclusions.
2. Objective
The objective of this practical study is to:
Design and implement a common emitter amplifier using a bipolar junction transistor
(BJT).
Analyze its gain, input and output impedance, and frequency response.
Compare theoretical and experimental results.
3. Components and Equipment
The experiment requires the following components:
Transistor: NPN BJT (e.g., BC547, 2N3904)
Resistors: Biasing and load resistors (e.g., 1kΩ, 10kΩ, 4.7kΩ)
Capacitors: Coupling and bypass capacitors (e.g., 10µF, 100nF)
Power Supply: DC power supply (9V – 12V)
Signal Generator: To provide the input AC signal
Oscilloscope: To measure input and output signals
Multimeter: To measure voltages and currents
4. Circuit Design and Construction
4.1 Circuit Diagram
The common emitter amplifier circuit consists of:
A BJT transistor in common emitter configuration.
Biasing resistors (R1 and R2) for proper transistor operation.
Load resistor (RC) to determine the voltage gain.
Emitter resistor (RE) for stability.
Coupling capacitors (C1 and C2) to pass AC signals while blocking DC components.
4.2 Circuit Connections
1. Biasing the Transistor: A voltage divider (R1, R2) is used to set the base voltage.
2. Connecting the Emitter Resistor (RE): It provides thermal stability.
3. Connecting the Collector Resistor (RC): It determines output voltage swing.
4. Coupling Capacitors (C1, C2): Used for signal coupling between input, amplifier, and
output stages.
5. Theory of Operation
5.1 Biasing
Biasing is essential to ensure the transistor operates in the active region. The base voltage (VB)
is set using:
VB=R2R1+R2VCCV_B = \frac{R_2}{R_1 + R_2} V_{CC}VB=R1+R2R2VCC
The emitter voltage (VE) is calculated as:
VE=VB−VBEV_E = V_B - V_{BE}VE=VB−VBE
where VBE≈0.7VV_{BE} \approx 0.7VVBE≈0.7V.
5.2 Voltage Gain (AV)
The voltage gain of the amplifier is given by:
AV=VoutVin=−RCREA_V = \frac{V_{\text{out}}}{V_{\text{in}}} = \frac{-R_C}{R_E}AV
=VinVout=RE−RC
The negative sign indicates a 180° phase shift between input and output.
5.3 Input and Output Impedance
Input Impedance (Zin) is mainly determined by the base resistance and is given by:
Zin≈βREZ_{\text{in}} \approx \beta R_EZin≈βRE
Output Impedance (Zout) is approximately equal to RCR_CRC.
6. Experimental Procedure
1. Assemble the Circuit: Connect the circuit as per the schematic.
2. Apply DC Bias: Measure the base, emitter, and collector voltages.
3. Apply an AC Input Signal: Use a signal generator with a small amplitude (e.g., 10mV,
1kHz).
4. Measure Output Signal: Observe the amplified signal on an oscilloscope.
5. Determine Voltage Gain: Calculate gain using measured input and output voltages.
7. Observations and Results
Parameter Theoretical Value Experimental Value
DC Biasing (V_C, V_B,
Calculated using biasing equations Measured with a multimeter
V_E)
Voltage Gain (A_V) −RC/RE-R_C/R_E−RC/RE Measured from oscilloscope
Estimated using βRE\beta
Input Impedance (Z_in) Verified using test signals
R_EβRE
Measured with load
Output Impedance (Z_out) Approximately RCR_CRC
variations
8. Discussion
The measured voltage gain closely matches the theoretical calculations, validating the
design.
Biasing stability was confirmed by observing consistent DC voltage levels.
Input and output impedance values were in agreement with expectations.
Frequency response analysis showed a typical mid-band amplification, with a drop in
gain at very high and low frequencies due to capacitive effects.
9. Conclusion
The common emitter amplifier was successfully designed and tested. The experiment
demonstrated its ability to amplify AC signals with phase inversion. The theoretical and
experimental values were closely matched, confirming the correctness of the design. Future
improvements could include testing with different transistors and analyzing temperature effects
on performance.
