Small Signal Analysis- why ground all power connectors ?

[karakurto]

Small Signal Analysis

Hi,

When we try to find the small signal equivalent of a circuit, we apply AC and all the DC power supply nodes(like Vdd and Vss) pretend as if they are ground. At least in text books this is what is taught, but no explanation for that(or I couldn't find). I can't figure out the reason behind that? What are the physical or electrical explanations for that? I mean why DC nodes are accepted as gorund for ac analysis?
Thanks

 

[tyassin]

Re: Small Signal Analysis

Hi

It is because you can not have any AC signal on something which has a constant value.

So an ideal DC-supply of 12V cannot be higher or smaller than 12V i.e. the voltage over it can not change.
The small-signal voltage across will then be zero, this is why it is considered as a ground node i small signal analysis.

Hope it helps

 

[karakurto]

Small Signal Analysis

Thanks tyassin, your answer is clear and makes sense.