AC and DC voltage source

Question

Why do we consider DC voltage source as short in AC analysis? There is some explanation in Malvino's book but I am unable to understand, can somebody explain this in a lucid manner?

score 2 · Answer 1 · answered Dec 09 '20 at 19:25

2

We regard all voltage sources as having zero impedance output, even AC ones.

In the case of AC analysis, we neglect DC biassing, so we only have to respect the zero impedance across any DC source. A short circuit is the simplest way to show this.

answered Dec 09 '20 at 19:25

Neil_UK

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score 2 · Answer 2 · answered Dec 09 '20 at 19:46

OK, I will try to explain it "in a lucid manner"...

Imagine a circuit of three elements - AC voltage source, resistor and DC voltage source. When the AC voltage varies, the current will vary according to Ohm's law... but the voltage across the DC source will not vary... so its (differential) resistance is zero.

Now replace the DC voltage by a piece of wire (short circuit) and vary the AC voltage again. As above, the current will vary according to Ohm's law... but the voltage across the wire will not vary... since its resistance is zero.

So, from the point of view of AC analysis, the two elements (voltage source and piece of wire) are equivalent... and you can replace the more complex with the simpler...

AC and DC voltage source

2 Answers2