# Why Vdd is also the AC ground when doing small signal equivalent circuit analysis?

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#### lookforjob

##### Newbie level 6
for a simple common source amplifer, why Vdd is also AC ground when doing small signal equivalent cirduit analysis?

Thanks

Re: a simple question

when doing small signal (AC) analysis.. only alternating (that change with time) voltages are considered. since DC voltage source is not varying (fixed value) it is considered zero and is short circuited to ground.

similarly if its a DC current source it is open circuited..

hope it helped

#### barath_87

##### Full Member level 3
Re: a simple question

While doing small signal analysis (AC) we are bothered about the small changes in the circuit(remember there is no change in your DC supply which is large compared to your i/p and hence called large signal analysis.) so we make the DC sources effect on the circuit zero by shorting voltage sources and open circuiting current sources.....

Hope my answer helped you in some way......

#### A.Anand Srinivasan

Re: a simple question

it is basically due to superposition theorem.... in AC analysis they omit the DC supplies once they have found the circuit is properly biased in DC analysis....

#### vivek_raj_verma

##### Junior Member level 2
a simple question

why do we open the dc currrent sources ??

#### A.Anand Srinivasan

Re: a simple question

while applying superposition we replace the sources with their equivalent impedance and it is open circuit of current source and short circuit for voltage source...

#### lijianheng

##### Full Member level 2
Re: a simple question

Because VDD ac value is zero, it is a DC constant value. No matter how much and how fast current flow through it, its voltage value maintain constant.

#### Hunter_BJUT

##### Member level 2
a simple question

for ac, vdd is small signal ground

#### ambreesh

##### Full Member level 5
Re: a simple question

there is no AC component in the supply.
We find the PSRR of the circuit for some AC harmonics in the power supply.
When we do AC analysis we only concider AC sources

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