Fast method for calculating voltage gain

Discussion in 'Homework Help' started by mhjallaboutcircuits, Apr 25, 2015.

Apr 25, 2015
6
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salaam Every body,

is there a faster method for gain(Av0) derivation than using KVL-KCL ? (using KVL-KCL is too slow and complex way for a multistage amplifier.)

I have seen a method used for calculating voltage gain but I don't remember it and it was something like:

output resistance/imput loop resistance

thanks.

2. crutschow Expert

Mar 14, 2008
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3,356
It depends upon the amplifier design.

Apr 25, 2015
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I want for simple amplifier like common source or common base

4. WBahn Moderator

Mar 31, 2012
18,079
4,917
For simple and common amplifier topologies, there are common formulas that have been arrived at via analysis and that most people just use. There are very simplistic formulas that rely on certain assumptions being valid and more complicated formulas that allow those assumptions to be relaxed.

But, in general, if you want to analyze a circuit then there are many ways to do it and which methods are optimal depends on the specific circuit topology and also on the component values because you have to decide which, if any, simplifying assumptions you could make.

5. Himanshoo Member

Apr 3, 2015
234
6
Do you really find...calculating gain by utilizing h parameters easy?

Apr 25, 2015
6
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well, I just forgot it.
(of course I have asked this question somewhere else and get some good answers)
but I now prefer to save in mind the voltage gain of all three mode: CE,CB,CE (equal to CS,CG,CD)

for example for ce: av=gm*rc/(1+gm*re)
rest of it here.

7. Himanshoo Member

Apr 3, 2015
234
6
As
Better you calculate voltage gain in terms of h parameters...
You have involved transconducatance (gm) in your formulation...as transconductance tell us the amount output current changes with a change in input voltage...as voltage gain involves change in output voltage with input voltage...
plus the input which you got somewhere else are quite vague..there are lot more to consider while formulating voltage gain of an amplifier..like amplifier impedances..and the moment you include amplifier impedance in your calcuation you will be forced to use h parameters...so h parameter kind of simplifies our task..

8. Jony130 AAC Fanatic!

Feb 17, 2009
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Apr 25, 2015
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no problem of gm because there would be a dc analysis before calculating ac gain.
and also as the Jony said, h parameters are not user friendly parameters
I'm looking for a fast and correct method for my university examinations.

10. WBahn Moderator

Mar 31, 2012
18,079
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I always preferred to see a student (or an interviewee) be able to analyze the circuit as a generic circuit as opposed to someone that throws a bunch of memorized formulas at a circuit. Of course, I am biased because of all the times I saw people throw memorized formulas at circuits do it wrong. So much so that I found that I could pretty much get everything I needed to know about an applicant's EE skills by asking them to analyze one circuit containing one resistor and one BJT transistor. Most people insisted on throwing a memorized formula at it and completely botched it.

Apr 25, 2015
6
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of course you are right , I myself hate of filling mind with formulas , but the problem is that the professor won't give me 2 hourse for every question on the final exam. or I don't have this much time for every HW questions.

12. WBahn Moderator

Mar 31, 2012
18,079
4,917
If you are proficient at circuit analysis, then it shouldn't take all that much longer to do it from basics. But I do understand where you are coming from and I recognize that some professors feel that students "know" the material as long as they can quickly regurgitate the right formula from a list. The problem is that the list becomes prohibitively large as soon as you move away from the basic circuit topologies. But if the exam is only going to have problems from the basic topologies, then about all you can do is memorize the formulas for each topology. If you understand where those formulas come from -- i.e., can start from scratch and easily derive them -- then remembering them correctly becomes a whole lot easier.

13. The Electrician AAC Fanatic!

Oct 9, 2007
2,300
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In the first post, you said "using KVL-KCL is too slow and complex way for a multistage amplifier." This leads me to think you want advice in solving multistage amplifiers, but so far all I see in this thread involves single stage circuits.

Do you in fact want to solve multistage circuits?

Apr 25, 2015
6
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yeah,
by "multistage" I mean a composition of these simple circuits like below. (I'm not sure if I use this word rightly)

solving this by kvl/kcl and pi model, will take more than 5 hours from me. (I'm very lazy )

15. The Electrician AAC Fanatic!

Oct 9, 2007
2,300
335
Does your instructor really expect you to solve such a circuit, setting up the kvl/kcl equations by hand? In only a few minutes on an exam?

When you solve a circuit using a simulator, the simulator is setting up kvl/kcl equations for you. Are you allowed to use a simulator?