Miller effect

[sagh]

Can anyone simply explain the Miller Effect? I am so confused.

 

[Diver300]

https://en.wikipedia.org/wiki/Miller_effect

A capacitor between the input and ground will absorb a certain amount of charge when the input voltage changes.

A capacitor beteen the input and output will experience a much larger voltage change, because when the input goes up, the output goes down. The voltage change for a capacitor between the input and output will be larger by approximately the gain of the amplifier. The larger voltage change across the capacitor means that it absorbs more charge, all of which has to come from the input, so the capacitance appears to be a much larger value.

 

[dr pepper]

The capacitance is also in series with effectively a zener diode, if you applied 10v to the gate of a fet via a high value resistor the voltage will quickly rise the vth (threshold voltage, the voltage at which the fet starts to turn on), then it will level out till the capacitance is charged, then finally reach close to the 10v i/p.
Fet gate driver ic's are designed to charge and discharge this capacitance as quickly as possible for switching applications, as while the fet is bewteen on and off off states its effectively a resistor and will dissipate energy as heat.

 

[Tony Stewart]

Anything not acting as a conductor is acting as a dielectric whether it is a pure dielectric or a semiconductor reverse biased.

Thus the coupling capacitance of the dielectric is dependant on the dielectric constant and the geometry of the conductors.

In essence, this is what the Miller Effect defines between the semiconductor inverting output and input for both BJT's and FET's.

 

[sagh]

Thanks for comments. I don't understand the concept of Miller effect in the paper I attached. The word of Miller effect has been marked in the paper. I would be grateful if somebody could tell me what that means.

 

[MrAl]

Hello there,

In short, the Miller Effect is an action by an amplifier that results in a capacitance multiplication. The physical capacitor looks larger due to the Miller Effect. So a capacitance of 1uf might look like a 100uf cap with a 100x gain factor.

 

[Tony Stewart]

the effective or Miller capacitance, CM is the physical C multiplied by the factor

but it applies to any feedback impedance. Zin=Zf /