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Basically I've got an exam in circuitry this thursday and I've noticed that every single exam has one of these amplifier circuit excercises in them where the first part is usually finding the small-signal equivalent of said model and the other parts are based on this.

I am however finding it rather hard seeing how I'm supposed to go from circuit model to small-circuit equivalent, as the book never mentions it thoroughly, it basically just says; "hey here's a circuit, here's the equivalent, have fun".

So anyway, I'll illustrate an example and say what I think I've understood of it.

Here's an amplifier circuit with a MOSFET transistor:

http://img262.imageshack.us/img262/4987/amplifierol5.jpg [Broken]

And then I'm supposed to create the small-signal equivalent. The first thing I think I know is that when constructing the small-signal equivalent is that we're now only interested in the small signal, and for the small signal we can view any capacitors as short-circuited.

Alright. At the same time, Ithinkwe can short-circuit all voltage supplies and remove all current supplies, and then basically evaluate where the current from the small-signal Vsig will travel to see how the equivalent small-signal circuit will be. The transistor should also be replaced with a voltage-controlled current source.

Am I missing anything here now?

Basically following what I've said, Vsig should be in series with Rsig, and the voltage-controlled current source should be paralell with RD and RL.

I know this is supposed to be how it looks:

http://img259.imageshack.us/img259/6354/smallcircuitia2.jpg [Broken]

The thing is, even with what I know, I could never ever have eventually ended up at this solution by myself. Do you think anyone here can give me a nudge in the right direction on how to design these small-signal equivalent circuits or direct me to a webpage which could describe it for me?

Any help would be greatly appreciated.

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# Homework Help: Small-signal equivalent circuit models - How?

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