Question about BJT AC (small signal) analysis

buzaiandras

Joined Jul 18, 2011
54
Hi

I have a question related to BJT AC (small signal) analysis.
Why, when performing this analysis, all the DC voltage sources a shorted to ground? (ex: +5V DC is shorted to GND).
I understand why capacitors are replaced with closed circuits but
unfortunately I was not able to understand why the rest of the voltage sources are shorted to ground.

Thank you,

Buzai

Brownout

Joined Jan 10, 2012
2,390
Because ideal voltage sources have zero impedance. Therefore, to an AC signal, they are direct shorts to ground.

Jony130

Joined Feb 17, 2009
5,243
The ideal DC voltage source has 0Ω internal resistance. And that's why AC-signals are shorted by DC voltage source.
DC voltage is always constant so for any change in current there is non change in voltage. So there is 0 Ohm internal resistance.
And additional in real life circuit we always uses a bypass capacitor connect parallel to DC Voltage. And this capacitor will short all AC-signal to the ground.

For example we have a 9V DC voltage source, now if we smoothly change the current that is draw from dc-voltage source, from 40mA to 20mV (we change the resistance from 225Ω to 450Ω). We create ac-current. But the battery voltage has not changed.
So the dynamic resistance of dc-voltage source is equal to
rd=0V/20mA=0Ω
And that why we say that DC-Voltage is a short for AC-signals.

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