input capacitance of CMOS op-amp

[m_kuty]

Hello

1- I need your help to calculate the input capacitance of op-amp, And
2- how can I know the transistor that is represent the input capacitance.

the op-amp is two stage, also the second stage is class ab amplification

 

[shaiko]

I'm not an analog expert - but electrical theory rules should still apply:

1. The gate of a transistor is a capacitor.
2. The total capacitance = the sum of all capacitors in parallel (like resistors in series).

 

[FvM]

You make no fault if you assign the input capacitance to input transistors only.

There are two capacitance values, common and differential mode capacitance. Common mode capacitance is mainly set by Cgd, differential mode by Cgd + Cgs.

 

[m_kuty]

Thank you shaiko and FvM

But what is about the other capacitances. For instant, M5 (Cgs-Cdb) and for M3 (Cdb- Cgd), Are they related to input capacitance

 

[FvM]

Input-capacitance wise, the folded cascode can be considered as short, at least in the frequency range where the input is mostly capacitive.