question on Miller Effect equivalent circuit

[sgperzoid]

the equivalent circuit of a miller item lets say a gate-to-drain capacitance of a MOSFET is well know.

The effect of Cgs can be replaced by a gate to ground capacitance and a drain to ground capacitance with the equation given.

However, intuitively, I think this is only an approximation of the Miller effect, the two miller equivalent capacitor do not readily act as the behaviour of Cgd throughout the frequency range, expecially at high frequency.

That is my intuitive observation, but I can not clearly and describe under what kind of situation, the Miller equivalent model is not a good approximation of the Miller capacitor.

Can anyone here help me? Hope you can provide a clear and complete answer

thank you in advance!

 

[osbourne]

Hi,

the Miller equivalent circuit is only valid as long as the conditions that existed in the network when the gain was determined are not changed. At higher frequencies Cgd increasingly leads to coupling between input and output, so gain changes.

In the equivalent circuit with capacitors to ground at the input and output, there is no coupling betwwen input and output. So the real circuit and the equvalent circuit should behave differently.