Analog Electronics – Page 30
Common Emitter Amplifier Gain using h-Parameters
The hybrid h-parameter model is widely used for analyzing transistor amplifiers. It simplifies the transistor into a linear two-port network.
CE h-Parameter Model
The equations for h-parameter model are:
V1 = h11 I1 + h12 V2
I2 = h21 I1 + h22 V2
For Common Emitter configuration:
- h11 = hie (Input resistance)
- h21 = hfe (Current gain)
- h12 = hre (Reverse voltage gain)
- h22 = hoe (Output admittance)
Voltage Gain Derivation
Output voltage:
Vo = − Ic RL
Collector current:
Ic = hfe Ib
Input voltage:
Vi = Ib hie
Therefore voltage gain:
Av = Vo / Vi
Av = − (hfe RL) / hie
Current Gain
Current gain of CE amplifier:
Ai = Ic / Ib = hfe
Power Gain
Power gain is:
Ap = Av × Ai
Example Problem
Given:
- hfe = 120
- hie = 1.2 kΩ
- RL = 3 kΩ
Find Voltage Gain
Av = − (hfe RL) / hie
Av = − (120 × 3000) / 1200
Av = −300
Key Observations
- Voltage gain increases with higher hfe
- Voltage gain increases with larger load resistance
- Higher hie reduces voltage gain
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