Explain the working process of the amplifying action of an n-p-n transistor in common emitter configuration by making a circuit diagram and obtain the formula for voltage amplification.
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In a common emitter amplifier, the input signal modulates the base current, which causes a larger change in the collector current, leading to voltage amplification.
Step 1: Basic Structure of an n-p-n Transistor.
An n-p-n transistor consists of three layers of semiconductor material: the emitter (n-type), base (p-type), and collector (n-type). In a common emitter configuration, the emitter is common to both the input and output circuits.
Step 2: Circuit Diagram.
Here’s the circuit diagram for the common emitter amplifier:
Step 3: Working of the Amplifying Action.
In the common emitter configuration, the input signal is applied to the base-emitter junction, and the output is taken across the collector-emitter junction. When a small input voltage is applied to the base, it causes a small change in the base current \( I_B \). This small change causes a much larger change in the collector current \( I_C \), because the current gain \( \beta \) of the transistor is typically large. The output voltage is therefore amplified in proportion to the input voltage.
Step 4: Voltage Amplification Formula.
The voltage gain \( A_V \) of the amplifier is the ratio of the change in the output voltage to the change in the input voltage:
\[
A_V = \frac{\Delta V_{\text{out}}}{\Delta V_{\text{in}}}
\]
For a common emitter amplifier, the voltage gain is approximately:
\[
A_V = -\frac{R_C}{r_e}
\]
where:
- \( R_C \) is the collector resistance,
- \( r_e \) is the small-signal emitter resistance.
Step 5: Conclusion.
Thus, the n-p-n transistor in common emitter configuration amplifies the input voltage by a factor of \( A_V \), and the amplification depends on the ratio of the collector resistance to the small-signal emitter resistance.
