ac ground in MOSFET small signal models

[giri_lp]

mosfet ground

Hi,
In MOSFET small signal models, analysis are being carried out when Substrate, gate and drain are at ac ground. Though I could able to understand what dc ground is, ac ground seems to be puzzling.

Will highly appreciate if analog veterans can throw some light on this.

Thanks in advance,
Giri

 

[sutapanaki]

mosfet small signal analysis

A DC ground, of course, can be also an AC ground. Apart from that any node that doesn't carry an ac signal and only has a DC value is considered at AC ground. For example, VDD is AC ground.