Transistor Biasing: How to combine AC and DC signals in amplifiers (8-Transistors)

When designing transistor amplifiers, it's necessary to have DC offsets in order to bias the transistor in the correct region of operation (forward active, not cut-off). But then the signal to be amplified is typically AC, so we need to combine AC with DC. This video shows how that is done. Then, I also explain how to extract an AC signal from a mixed AC + DC voltage in order to handle the output.
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

[@drew6017]

You should do a video on multiplying AC signals next. That would be a great partner-piece to this as a "math operations in circuit" kinda mini-series.

[@Francisco17Berrios]

Thank you so much for your lessons! They are always so useful!! :)

[@AjinkyaMahajan]

well explained, thanks !!

[@andymouse]

This stuff realy is solid gold ! Awesome tutorials....cheers!

[@ertreri]

Concerning thee video at the 5:01 mark:
You explained how the inductor can be replaced by a voltage divider with resistors R1 and R2. You then adjusted the Vout equation to account for the voltage divider. I understand that part.
My confusion is around how R1 replaces the inductor's function of blocking AC from the AC source from interfering with the DC source. You mentioned that R1 and R2 should be much smaller than the load resistance. Wouldn't this low resistance of R1 allow AC current to flow through it, defeating the purpose of blocking AC? I struggle to see how the voltage divider circuit achieves the same function as the original inductor in this scenario.
Thanks a lot.

[@andrejtih7373]

спасибо!!!)

[@ovalwingnut]

You R the "Electron Whisperer"

[@SergiuCosminViorel]

i wonder if there is another approach than the small signal approximation.
in power amplifiers the output impedance suffers huge variations.
we can follow the small signal approximation exclusively, and it is fine, if it is for beginners, but if we want more than the forever beginner level, we want to know more.
btw, your presentations are such a joy!

[@hidennseek1483]

I find difficult to understand what exactly you want but it sounds interesting. Do you mean that you want too see the other stages of an amplifier so you can drive different loads?

[@andymouse]

You want to factor in tolerance's of the components, temperature and other parameters as well is, that what you mean please?

[@stefanopassiglia]

The small signal model is used when the signal is not large enough to expose components non-linearity. What is "large" depends on the actual circuit, and not necessarily a power amplifiers cannot be analyzed with a small signal model. For instance, if the large signal does not affect the Q-point. If the signal, despite being "large" does not expose the non linearity of the circuit components then it can be analyzed with a small signal model. Hope this answers your questions.

[@dannmwangi3978]

when you replace the inductor with the resistors ,what stops the AC current from flowing to the dc source ?

[@VandalIO]

Maybe use an ideal diode (Opamp approximation) instead of Inductor

[@VandalIO]

I apologise for my NOOB question, but why most transistor biasing involves resistors? why can't we use voltage regulator to bias it ?

[@stefanopassiglia]

of course you can, but resistors are cheap and a lot simpler than a voltage regulator