why assume dc voltage as ac ground when doing ac analysis?

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chunlee

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why normally we assume the dc bias voltage as ac ground when we doing ac analysis? if we design a voltage bias circuit using transistor to bias another circuit, how close the voltage bias node will close to ac ground?
 

Re: why assume dc voltage as ac ground when doing ac analysi

Generally, the AC resistance of the bias circuit is almost zero.
 

Re: why assume dc voltage as ac ground when doing ac analysi

ac analysis is a kind of equivalent analysis method. Because under the condition of ac analysis, all dc voltage does not change.So the dc voltage is equivalent with ac ground.
 

Re: why assume dc voltage as ac ground when doing ac analysi

why when i use resistors to form a voltage divider circuit to bias my MOSFET gate, the zin and zout of the circuit at 2.4GHz see different (if compare to use ideal dc voltage source?)

when i add 10uF capacitor to the gate of MOSFET to gnd, the zin and zout become similar to the circuit i use ideal dc voltage source.

thanks a lot!
 

Ideal dc voltage source has ideal great source resistor, so it has no influence on the zin and zout of the circuit. Resistors is not so.

Capacitor is important for input impedance in RF circuits. you can refer to the basic theory about RF impedance.
 

Re: why assume dc voltage as ac ground when doing ac analysi

chunlee said:
why normally we assume the dc bias voltage as ac ground when we doing ac analysis? if we design a voltage bias circuit using transistor to bias another circuit, how close the voltage bias node will close to ac ground?
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In AC, DC voltage just use to bias the transistor in saturation region. As long the transistor in saturation, we only care about AC signal.
 

Re: why assume dc voltage as ac ground when doing ac analysi

Because the bias voltage is being constant and the current is being linear (constant) always, we say the effect of it to be AC ground because the DC is not changing and like having a constant zero potential voltage.
 

Because there are capacitor at the supply rail which act as AC ground.
 

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