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What is small signal AC analysis?

  1. Mar 23, 2013 #1
    I have designed a common emitter amplifier using a BJT and I need to perform a DC analysis, followed by a small signal AC analysis, on it.

    Before I continue, I will admit that this is homework, but please don't stop reading yet. I am not looking for anyone to do these analyses for me, but I merely want to know if it is even possible to perform them by hand, and if so, then what is meant by these terms so that I can do it myself. If I put this question in the homework help forum, probably no one would know the answer.

    Now, I think I know how to do this using a simulator, but in the outline of the task, it mentions that simulation is optional, making me think that it is possible to do these by hand. Am I right in thinking that for DC analysis, you just replace any AC voltage sources with a short circuit, and replace capacitors with an open circuit, and then just perform standard circuit analysis? Then for AC analysis, I'm much more lost. I guess you would replace DC voltage sources with a short circuit, but then what defines a 'small' signal, what source frequency do you choose to analyse with, etc?

    Please help, I need a reply ASAP because I can't start this until I know what it is I'm even supposed to be doing!
  2. jcsd
  3. Mar 23, 2013 #2
    For DC analysis you assume the BJT to be in some possible state, e.g. active region with known base-emitter voltage for npn, and you solve your circuit for unknown voltages and currents (you need to check your assumptions after).

    This gives you an operating point (DC bias for the BJT) for small signal analysis where you build a (linear) small signal model from whatever (nonlinear) high-fidelity BJT model you're using by Taylor expansion of any nonlinearities. You can build an equivalent circuit of your small signal model and proceed with AC analysis like you would with any other circuit, but you need to make sure not to perturb your small signal model far away from its operating point where it might be very inaccurate, hence _small_ signals.

    For a tonne of examples:

    I have to wonder, though, didn't you cover this stuff in your course beforehand?
    Last edited: Mar 23, 2013
  4. Mar 23, 2013 #3
    Thanks for the reply; I'll check out those examples.

    While we just got up to the small signal model for the BJT in lectures, it wasn't explained clearly and I'm still wrapping my head around it. Besides that, only the very basic principles of a BJT have been covered so I'm very new to dealing with the device. Possibly that explains my confusion.
  5. Mar 23, 2013 #4
    You're welcome and you're certainly welcome to post in the homework section with regards to the specifics of your problem if you get stuck.
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