# help explaining an answer

Discussion in 'Homework Help' started by lokeycmos, May 5, 2013.

1. ### lokeycmos Thread Starter Active Member

Apr 3, 2009
432
7
I have attached a scanned page from a practice test. I was wondering if someone could explain the answers for questions 23 and 25. the correct answers are 1 and 2 respectively.in question 23 what is the purpose of c2? just looking for an explanation. thanks!

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2. ### tubeguy Well-Known Member

Nov 3, 2012
1,157
197
Hint for #23:
C2 is a bypass capacitor. It acts like another resistor in parallel with Re as far as the AC
signal is concerned.

Last edited: May 5, 2013
3. ### panic mode Senior Member

Oct 10, 2011
1,328
305
Hint for 25:
which transistor pin is common/fixed?

4. ### WBahn Moderator

Mar 31, 2012
18,079
4,917
On questions like this you often have to make reasonable assumptions about some things that are otherwise unstated. Sometimes this is intentional and sometimes it is sloppiness and frequently it is impossible to tell which.

In the case of #23, they are expecting you to have an understanding of what the role is that is normally played by capacitor C2 and, hence, the impact it would have on the small-signal equivalent circuit at the frequencies of interest.

So, try to explain what that role is and how it affects the small-signal circuit and we can hopefully help you zero in on the correct conclusion from there.

For #25, consider what change on the output voltage a change on the input voltage would have. As before, you are expected to know what the normal roles of the two capacitors are and how they affect the small-signal circuit at the frequencies of interest.

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