CE Transistor Amplifier Lab Manual
To Study Variation of Gain of CE Transistor Amplifier with Load
1. Aim

To study the variation of voltage gain of a Common Emitter (CE) transistor amplifier with different load resistances and to analyze the relationship between gain and load resistance.

2. Apparatus Used
  • NPN silicon transistor (BC107 or 2N2222)
  • Resistors: 10kΩ, 1kΩ, 470Ω, and variable resistors (100kΩ, 10kΩ)
  • Capacitors: 10μF, 100μF (electrolytic)
  • DC power supply (0-15V)
  • Function generator
  • Dual trace oscilloscope
  • Digital multimeter
  • Breadboard and connecting wires
  • Different load resistors (1kΩ, 2kΩ, 5kΩ, 10kΩ, 20kΩ)
3. Circuit Diagram
ce Amplifier with load

Fig. 1: Common Emitter Transistor Amplifier Circuit with Variable Load Resistance

4. Theory

A common emitter (CE) amplifier is a type of transistor amplifier configuration where the emitter of the transistor is common to both input and output circuits. The CE configuration is widely used because it provides good voltage, current, and power gain.

In this experiment, we focus on how the voltage gain of a CE amplifier varies with changes in the load resistance (RL). The voltage gain of a CE amplifier is defined as the ratio of the output voltage to the input voltage:

Av = Vout / Vin

The theoretical voltage gain of a CE amplifier depends on the transistor's transconductance (gm), the output resistance of the transistor (ro), and the load resistance (RL). For an AC signal, the voltage gain can be expressed as:

Av = -gm(RL || ro)

Where:

  • gm is the transconductance of the transistor = IC/VT (VT ≈ 26mV at room temperature)
  • ro is the output resistance of the transistor = VA/IC (VA is the Early voltage)
  • RL is the load resistance
  • (RL || ro) represents the parallel combination of RL and ro

For practical purposes, when RL is much smaller than ro, the gain becomes approximately:

Av ≈ -gmRL = -ICRL/VT

This shows that the voltage gain is directly proportional to the load resistance RL when other factors remain constant. As RL increases, the voltage gain increases linearly until RL becomes comparable to ro. At this point, the gain begins to saturate and approaches a maximum value of approximately -gmro.

The relationship between voltage gain and load resistance can be observed experimentally by varying RL and measuring the corresponding changes in voltage gain.

5. Formula

Voltage Gain (Av) = Vout / Vin

Theoretical Voltage Gain = -gm(RL || ro)

For small RL: Av ≈ -gmRL = -ICRL/VT

Where: VT ≈ 26mV at room temperature

Current Gain (β) = IC / IB

Transconductance (gm) = IC / VT

Parallel combination: RL || ro = (RL × ro) / (RL + ro)

6. Procedure
  1. Set up the circuit as shown in the circuit diagram on the breadboard.
  2. Initially set the collector load resistance (RL) to 1kΩ.
  3. Apply a DC voltage of +10V to the collector through the collector resistor.
  4. Connect the function generator to the input of the amplifier circuit through the coupling capacitor C1.
  5. Set the function generator to produce a sinusoidal signal of frequency 1kHz with an amplitude of 20mV peak-to-peak.
  6. Connect channel 1 of the oscilloscope to the input (base) of the transistor and channel 2 to the output (collector).
  7. Measure and record the peak-to-peak voltage of both input (Vin) and output (Vout) signals.
  8. Calculate the voltage gain using the formula: Av = Vout / Vin
  9. Replace the load resistor with different values (2kΩ, 5kΩ, 10kΩ, 20kΩ) and repeat steps 7 and 8 for each value.
  10. Draw a graph of voltage gain (Av) versus load resistance (RL).
  11. Observe how the gain varies with the load resistance and verify the theoretical relationship.
7. Observation Table
S.No. Load Resistance RL (kΩ) Input Voltage Vin (mVpp) Output Voltage Vout (mVpp) Voltage Gain Av = Vout/Vin
1 1
2 2
3 5
4 10
5 20
8. Calculations

For each load resistance value, calculate the voltage gain using the formula:

Av = Vout / Vin

For example, if with RL = 1kΩ:

Vin = 20mVpp and Vout = 240mVpp

Then, Av = 240mV/20mV = 12

To determine the relationship between gain and load resistance, plot a graph of Av versus RL. The graph should show an increasing trend initially, which may start to level off at higher load resistance values.

Additionally, we can calculate the theoretical voltage gain for each RL value using:

Av(theoretical) = -gm(RL || ro)

To calculate gm, measure the collector current IC:

gm = IC/VT = IC/26mV

Compare the experimental and theoretical values to analyze any discrepancies.

9. Result
  1. The variation of voltage gain (Av) with load resistance (RL) was studied experimentally.
  2. It was observed that the voltage gain increases with increasing load resistance initially, following approximately a linear relationship.
  3. At higher load resistance values, the rate of increase in gain diminishes, showing a tendency to saturate.
  4. The experimental results [were/were not] in close agreement with the theoretical predictions, with a deviation of approximately [x]%.
  5. The maximum voltage gain achieved was [value] at a load resistance of [value] kΩ.

Note: Complete this section after performing the experiment with actual measured values.

10. Precautions
  1. Ensure proper biasing of the transistor to avoid distortion of the output signal.
  2. Maintain the input signal amplitude small enough to prevent the amplifier from entering saturation or cutoff regions.
  3. Use coupling capacitors of appropriate values to ensure proper signal transfer at the frequency of operation.
  4. Ensure all connections are secure and correct before powering on the circuit.
  5. Avoid touching the circuit components when the power is on to prevent electric shock and circuit disturbance.
  6. Check the polarity of electrolytic capacitors and ensure they are connected correctly.
  7. Use the oscilloscope probes with proper ground connections to avoid measurement errors.
  8. Ensure the transistor does not overheat during the experiment by limiting the collector current.
  9. Verify the functionality of all components before assembling the circuit.
  10. Keep the input frequency within the bandwidth of the amplifier for accurate measurements.
11. Viva Voice Questions
What is the common emitter configuration and why is it widely used?
The common emitter configuration is a transistor amplifier arrangement where the emitter terminal is common to both input and output circuits. It is widely used because it provides high voltage gain, moderate current gain, and moderate input and output impedances, making it versatile for various applications.
How does the voltage gain of a CE amplifier vary with load resistance?
The voltage gain of a CE amplifier increases approximately linearly with load resistance for small values of RL. As RL becomes comparable to the output resistance of the transistor (ro), the rate of increase diminishes, and the gain eventually saturates, approaching a maximum value of approximately gmro.
Why do we use coupling capacitors in the CE amplifier circuit?
Coupling capacitors (C1 and C2 in the circuit) are used to block DC components while allowing AC signals to pass through. They isolate the DC biasing of different stages and prevent DC offsets from affecting subsequent stages, ensuring only the AC signal is amplified.
What is the significance of the bypass capacitor across the emitter resistor?
The bypass capacitor across the emitter resistor provides a low-impedance path for AC signals, effectively shorting the emitter resistor for AC components. This prevents AC degeneration and ensures maximum voltage gain for the amplifier. Without this capacitor, the gain would be significantly reduced due to negative feedback.
What happens to the gain of a CE amplifier if the load resistance is extremely high?
When the load resistance becomes extremely high (approaching infinity), the voltage gain approaches a maximum value determined by the transistor's transconductance (gm) and output resistance (ro): Av(max) ≈ -gmro. Beyond this point, further increases in load resistance do not significantly increase the gain.
How does temperature affect the voltage gain of a CE amplifier?
Temperature affects the voltage gain primarily by changing the transistor's parameters. As temperature increases, the collector current (IC) tends to increase, which increases the transconductance (gm). However, the thermal voltage (VT) also increases with temperature. Since gm = IC/VT, these effects partially offset each other. Overall, temperature increases tend to cause a slight increase in gain and can also affect the DC operating point.
What is the role of the biasing network in a CE amplifier?
The biasing network (typically consisting of resistors connected to the base, collector, and emitter) establishes a stable DC operating point for the transistor. Proper biasing ensures the transistor operates in the active region, allowing it to amplify AC signals without distortion. The biasing network also helps maintain stability across temperature variations and component tolerances.
Why is the voltage gain of a CE amplifier negative?
The voltage gain of a CE amplifier is negative because there is a 180-degree phase shift between the input and output signals. When the base voltage increases, the collector current increases, causing a larger voltage drop across the collector resistor. This results in a decrease in the collector voltage, creating an inverted (phase-shifted) output signal relative to the input.
What would happen to the voltage gain if the emitter bypass capacitor is removed?
If the emitter bypass capacitor is removed, the emitter resistor would not be bypassed for AC signals, leading to negative feedback (degeneration). This would significantly reduce the voltage gain of the amplifier. The gain would then be approximately Av ≈ -RC/RE, where RC is the collector resistance and RE is the emitter resistance.
How does the input impedance of a CE amplifier affect its performance?
The input impedance of a CE amplifier affects how much it loads the preceding stage or signal source. A higher input impedance is generally desirable as it draws less current from the source and causes less signal attenuation. In a CE configuration, the input impedance is moderate (typically a few kilohms) and is primarily determined by the base-emitter resistance and the biasing resistors in parallel with it.

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