Why when we do Ac analysis of common emitter amplifier we assume that the DC power supply is ground?

 

Because, for AC, Vcc is ground. Vcc is connected to ground through several thousand microfarads or it's held to a steady DC level by a regulator chip. Either way, you can't inflict an AC current on Vcc and have it change the voltage very much, therefore, Vcc has the same AC potential as ground, which is about zero.

 

Because, for AC, Vcc is ground. Vcc is connected to ground through several thousand microfarads or it's held to a steady DC level by a regulator chip. Either way, you can't inflict an AC current on Vcc and have it change the voltage very much, therefore, Vcc has the same AC potential as ground, which is about zero.

So AC current passes through a DC battery the same way it passes through a capacitor?

 

AFA AC is concerned in an amplifier, VSS & VDD are both identical and common to each other, achieved via the decoupling between VSS & VDD.
Max.

 

Any ideal DC voltage source will have 0Ω internal resistance. And that's why AC-signals are shorted by DC voltage source.
Also ideal DC voltage is always constant, so for any change in current there is non change in voltage across the ideal voltage source.
And if we deal with ac-signals we uses, so-called "dynamic" resistance (ac-resistance) rd = ΔV/ΔI.
For exampel if we have a 9 volts ideal voltage source and if we suddenly change the current that is draw from this 9Vsource, let say from 40mA to 20mA (we change the load resistance from 225Ω to 450Ω) we "create" AC-current. Now we can calculate the dynamic resistance of dc-voltage source

rd =ΔV/ΔI = (9V - 9V)/(40mA -20mV) = 0V/20mA = 0Ω

And that why we say that DC-Voltage is a short for AC-signals.
Aditional in real life circuit we always uses a bypass capacitor connect parallel to DC Voltage. And this capacitor will short all AC-signals. Because the large-value capacitor is a short for AC-signal. Current in capacitor is proportional to the rate-of change of the voltage, the faster the voltage change, the larger the current flow through the capacitor.

 

So AC current passes through a DC battery the same way it passes through a capacitor?

Basically, yes.
Batteries and capacitors are non-perfect. AC current will pass through a battery, and it will pass through a capacitor. If the battery is not of the AC conductive quality you need, you should add a capacitor in parallel with it to assure that Vcc is very nearly equal to AC ground.

 

Thanks everyone! I understand now