GATE Electrical – Analog Electronics
Page 14: Common Emitter (CE) Amplifier
The Common Emitter amplifier is the most widely used transistor amplifier configuration because it provides high voltage gain and moderate input and output resistance.
CE Amplifier Circuit
In the CE amplifier configuration:
- Input signal is applied between base and emitter
- Output is taken between collector and emitter
- The emitter terminal is common for both input and output
Working Principle
A small change in base current produces a large change in collector current. This results in amplification of the input signal.
Collector current relationship:
IC = β IB
Since the collector resistor converts current variation into voltage variation, a large output voltage appears across the load.
Voltage Gain
Using the small signal model, voltage gain of CE amplifier is:
Av = -gm RC
Where:
- gm = transconductance
- RC = collector resistance
Negative sign indicates phase inversion.
Input Resistance
Input resistance of CE amplifier is approximately:
Rin = rπ
Typical value ranges from:
1 kΩ – 10 kΩ
Output Resistance
Output resistance of CE amplifier is approximately:
Rout ≈ RC
Typical value ranges from:
10 kΩ – 50 kΩ
Phase Inversion
One important feature of the CE amplifier is that the output signal is 180° out of phase with the input signal.
This means when the input signal increases, the output voltage decreases.
Important GATE Points
- CE amplifier provides high voltage gain.
- Output signal is inverted (180° phase shift).
- Moderate input resistance.
- Moderate output resistance.
- Most widely used transistor amplifier.
Next Page → Emitter Follower (Common Collector Amplifier)
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