regarding capacitors as shorts for ac

[miguelyogur]

Hello

This is my first post, I just registered in this website since I am trying to learn electronics by myself and I am running into some trouble from time to time, I would appreciate any help.

My first question is about the ac anaylsis of a circuit. In particular I am struggling with the common emitter amplifier. There, the capacitors used to block biasing dc are treated as shorts. However, for the application of low frequency where this amplifier appears, since the reactance of a particular cap in the signal path could be quite similar to the resistance of other resistors in series or parallel with it, I believe they should not be ignored, and their total impedance taken into account in the analysis. I have observed that in all these applications, at least in the books I looked at, caps are always taken as shorts and this is something I don't understand if its fully justified in every case. How does this work?

thanks in advance

 

[(*steve*)]

It may not be fully justified, however it makes the analysis easier.

In practice, you would choose capacitor values that have low impedance at the frequencies in use. If you don't, the deviations from your analysis may become significant.

In many cases you may be able to view the capacitor as part of a high pass or low pass filter and model the frequency response later (once other impedances are known)

 

[CDRIVE]

I disagree with any coupling caps being treated as shorts. The value of a base coupling cap for a BJT Common Emitter AF amplifier is going to be many magnitudes larger than a Gate coupling cap for a JFET. If we're talking rail bypass (decoupling) caps that's different matter. I can accept them as ideally being considered a signal short. That said, they won't be either.

Chris

 

[BobK]

Of course the same thing can be said about a wire. It is never truly a short. Or perhaps short means very low resistance / reactance, rather tha zero, in which case either a wire or a capacitor can be considered a short.

Bob

 

[miguelyogur]

Thnaks for the answers

It may not be fully justified, however it makes the analysis easier.

In practice, you would choose capacitor values that have low impedance at the frequencies in use. If you don't, the deviations from your analysis may become significant.

In many cases you may be able to view the capacitor as part of a high pass or low pass filter and model the frequency response later (once other impedances are known)

In that case how would i decide if the assumption that the reactance of the cap is small? I guess that first i would treat it as a short (for the base coupling cap) and then after the analysis check if the filter I get combining this cap with the imput impedance of the amplifier gives a very small gain loss at that particular frequency, is that the right approach?

I disagree with any coupling caps being treated as shorts.

In that case should I take the cap in the ac analysis and work with fasors? that doesn't seem to be how this is treated in most textbooks, is there a reason for that?

 

[(*steve*)]

Compare the reactance of the capacitor with the impedance that capacitor "sees". If it is very large, then you may be able to treat the capacitor as an open circuit. If it is very small you may be able to consider it to be a short.

 

[CDRIVE]

In that case should I take the cap in the ac analysis and work with fasors? that doesn't seem to be how this is treated in most textbooks, is there a reason for that?

I use a fairly elementary approach. Once I calculate the base input Z of BJT amp I'll select a coupling cap with Xc at least 10 times less than the base Z at the lowest input freq.

Chris

 

[miguelyogur]

Perfect, I get it, thanks everyone