Analog Electronics – Page 35
High Frequency Analysis of Amplifiers
At high frequencies, the gain of an amplifier decreases due to internal capacitances of the transistor.
The main capacitances affecting high frequency response are:
- Base–Emitter Capacitance (Cπ)
- Collector–Base Capacitance (Cμ)
Transistor Internal Capacitances
These capacitances form RC networks which limit amplifier bandwidth.
- Cπ → between base and emitter
- Cμ → between collector and base
Miller Effect
The collector-base capacitance appears multiplied at the input due to voltage gain.
This phenomenon is called Miller Effect.
CM = Cμ (1 − Av)
Where:
- CM = Miller capacitance
- Cμ = Collector-base capacitance
- Av = Voltage gain
Effect of Miller Capacitance
- Increases input capacitance
- Reduces bandwidth
- Limits high frequency operation
Upper Cutoff Frequency
The upper cutoff frequency occurs when gain drops to 0.707 of midband gain.
fH = 1 / (2πRC)
Example Problem
If:
- Cμ = 2 pF
- Voltage gain Av = −100
Find Miller capacitance.
CM = Cμ (1 − Av)
CM = 2 pF (1 − (−100))
CM = 2 × 101 = 202 pF
Important GATE Points
- Miller effect increases input capacitance
- High voltage gain increases Miller capacitance
- Bandwidth decreases due to Miller effect
- Common base amplifier avoids Miller effect
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