Miller Effect
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Transcript of Miller Effect
Miller Effect Capacitance
Definition
The Miller effect accounts for the increase in the equivalent input capacitance of an inverting voltage amplifier due to amplification of the effect of capacitance between the input and output terminals. Although the term Miller effect normally refers to capacitance, any impedance connected between the input and another node exhibiting gain can modify the amplifier input impedance via this effect.
Inverting Amplifiers
For any inverting amplifier, the input capacitance will be increased by a Miller effect capacitance sensitive to the gain of the amplifier and the interelectrode (parasitic) capacitance between the input and output terminals of the active device.
Applying Kirchhoff’s current law gives
Using Ohm’s law yields
and
Network employed in the derivation of an equation for the Miller input capacitance.
Substituting, we obtain
and
but
and
In General,
Non-Inverting Amplifiers
For non-inverting amplifiers such as the common-base and emitter-follower configurations, the Miller effect capacitance is not a contributing concern for high-frequency applications.
Network employed in the derivation of an equation for the Miller output capacitance.
Applying Kirchhoff’s current law results in
with
Assuming that,
Substituting Vi = Vo/Av from Av = Vo/Vi results in
and
or
resulting in
Where Av >> 1,
Example:
Example for High-Frequency Response – BJT Amplifier:
Equivalent Circuit:
Example for High-Frequency Response – FET Amplifier:
Equivalent Circuit: