What is small signal AC analysis?

[mememe653]

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!

 

[milesyoung]

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:
https://www.google.com/search?q=bjt+dc+analysis
https://www.google.com/search?q=bjt+small+signal+analysis

I have to wonder, though, didn't you cover this stuff in your course beforehand?

 

[mememe653]

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.

 

[milesyoung]

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.

 

[alphysicist]

Small signal AC analysis is a technique used to analyze the behavior of a circuit at different frequencies around its operating point. It is commonly used in electronic design to determine the frequency response and stability of a circuit.

In your case, for the DC analysis, you are correct in replacing any AC voltage sources with a short circuit and capacitors with an open circuit. This allows you to determine the DC operating point of the circuit, including the biasing and quiescent values of the components.

For the small signal AC analysis, you would then apply a small AC signal to the circuit, typically with a frequency of 1kHz or lower. This signal is considered "small" if it does not significantly affect the DC operating point of the circuit. You can use a function generator or signal generator to provide this AC signal.

Next, you would use techniques such as nodal analysis or mesh analysis to determine the small signal AC response of the circuit. This includes calculating the voltage gain, input and output impedances, and frequency response.

In summary, small signal AC analysis is a method for analyzing the behavior of a circuit at different frequencies around its DC operating point. It can be done by hand, but it is often more efficient and accurate to use a simulator. I hope this helps clarify the concept for you.