GATE Electrical – Analog Electronics
Page 18 : Miller Effect in Amplifiers
The Miller Effect explains how a small capacitance between the input and output terminals of an amplifier appears as a much larger capacitance at the input.
This effect significantly reduces the high-frequency performance of amplifiers.
Origin of Miller Effect
In transistor amplifiers, there is a small internal capacitance between:
- Base and Collector in BJT
- Gate and Drain in MOSFET
This capacitance is called the feedback capacitance.
Miller Capacitance Formula
If a capacitance C exists between input and output, the effective input capacitance becomes:
Cin = C (1 − Av)
Where:
- C = feedback capacitance
- Av = voltage gain of amplifier
Example
If:
- Feedback capacitance = 2 pF
- Voltage gain Av = −100
Then:
Cin = 2 (1 − (-100)) Cin = 202 pF
Thus a very small capacitor appears very large at the input.
Effect on Amplifier
- Reduces bandwidth
- Limits high frequency response
- Introduces unwanted feedback
How to Reduce Miller Effect
- Use Cascade Amplifier
- Use Common Base configuration
- Use Cascode amplifier
Important GATE Points
- Miller effect increases effective input capacitance.
- It reduces amplifier bandwidth.
- Occurs due to feedback capacitance.
- Cascode amplifier reduces Miller effect.
Next Page → Differential Amplifier
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