Why can you treat a power supply like an AC ground? (5-Transistors)

Let's find the input impedance for a common emitter and a common collector amplifier. We can neglect the resistance looking into the base, but why can we treat the DC power supply like a ground? Let's discuss this in detail to make it clear.
Aaron Danner is a professor in the Department of Electrical and Computer Engineering at the National University of Singapore.
danner.group
Video filmed and edited by Cheryl Lim.
@randomcheryl

[@ihabsglei]

It was once explained to me that a DC voltage source can be treated like a very large capacitor. And because the latter represent a short circuit for alternating voltages, this also applies to DC voltage sources. This explanation was enough for me, but i like the solution you presented much more. 👨‍🏫

[@Enigma758]

Great explanation, thank you!

[@andrejtih7373]

спасибо!!!))

[@SergiuCosminViorel]

Now a problem, and please do not approximate like for a small signal amplifier!
A case in which superposition does not apply. Not any superposition i've seen others formulate.
A Power amplifier, remember, no simplification by excluding 5% or so contributions,
an amplifier used to amplify radiofrequencies,
being supplied through an inductor, to the collector or drain,
that outputs through a PI-Lowpass Filter, made of an inductor and two capacitors,
capacitors to the ground,
the output with an isolating capacitor, very low reactance.
I know for a fact, superposition does not apply, but you can try to convince me

[@VandalIO]

I didn’t get it

[@mdesm2005]

I have in common with you an interest in electronics. I'm still not sure what the emitter has in common with either the base of the collective (in a "common emitter" topology), other than being part of the same transistor. The base and collector have something in common, they are part of the "signal path". The emitter seems 'excluded' from all the AC fun. Who came up with that name ...

[@mrlapras024]

It’s called common emitter because the emitter is common to both input and output of the amplifier topology. Common base and common collector/emitter follower follow this criteria as well. Btw, Professor Danner is part of the faculty at University of Singapore, I would say his knowledge extends past interest

[@mdesm2005]

I'm not questioning that it's called 'common emitter'. I've known this for 35 years, having worked as an electrical engineer for that time. The choice of that word common always seemed odd. Still does. The emitter is 'excluded' from the input and output function, sure. @@mrlapras024

[@mdesm2005]

for instance, it's not that the base current and the collector current both go to the emitter that makes the base & collector have "something in common that involves the emitter" (one possible spin), because that's true of the common collector configuration.
@@mrlapras024