Unity frequency and Miller capacitor

Thread Starter

anhnha

Joined Apr 19, 2012
880
I am not sure how Cgd is included in the formula below.
Cgd is a Miller capacitor. How is it converted to parallel with Cgs?

Unitiy frequency.PNG
 

AnalogKid

Joined Aug 1, 2013
8,209
In the AC analysis of analog circuits, one of the primary assumptions is that there is zero ohms AC impedance between all power supply rails and GND. So from an AC point of view, the drain and source are shorted together in the above drawing. This places Cgs in parallel with Cgd, so they add.

In a real-world circuit with inadequate power source bypassing, the above assumption is not true and all of the standard circuit equations change, leading to distortion, oscillations, etc.

ak
 
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