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Subject: Follow Up on differential amplifier

Date: 08/28/03 at 5:44 AM
Posted by: Jason
E-mail: jason_class[AT55]yahoo.com (replace [AT55] with @ to reply)
Message Posted:

Hello Russ,

I would need your help again. Thank you.
I have change the circuit of differentuial amplifier which I asked you earlier on. Please see the below page
http://www.geocities.com/jason_class/pmos_nmos_diff_amp.gif

Note that the Vout position is changed. This make it an amplkifier as you have advised. Then for circuit (B), the connection of nmos current source is corrected which the source is connected to Vcc in order to turn on the transistor.

Circuit A is differential amplifier with pmos current source
Circuit B is differential amplifier with nmos current source

My questions are:
1) Is circuit A and B a type of common source amplfier as the Vin is feed to gate and the Vout is taken from drain to ground?
However, I read from books and it said that circuit B is a common drain/gate amplifier which I do not understand.
I knew that for common gate amplifier, the Vin is feed to source of mosfet and the Vout is taken from drain which I think circuit B is not common gate amplifier.
Then for common drain amplfier, input signal is feed to the gate and the output is taken from source. So again, circuit B is also not a common drain amplifier.

So if my understanding is right, then circuit a and b will have the same gain because they are both common source amplifier. Am I right or wrong? Kindly let me know why if I am wrong?

2) Some books teach us how to know if a circuit is a common source, drain or gate amplifier by seeing which terminal is ac ground. May I know what is ac ground?
It mention about ac ground in small signal. May I know which part of circuit will be ac ground when we trasnform to small signal model? Kindly teach me as I have read about small signal model but I do not know how to apply on determining the CS, CG or CD amplifier.

3) For analaysis of gain,
Is it true that both the circuit A and B are CS amplifier and their gain will use the analogy we have for a typical CS amplifier which goes as

Av= -gm2(rds2//rds4) for both circuit A and B
If my both circuit is not CS amplifier, kindly teach me how to see which common mode they are, and also how to determine the output resistance and then the gain.

3)Finally, why pmos current mirror amplifier is often used in text books?

I am sorry to ask a long questions. But I sincerely want you to know I am struggling to learn this graduate subject. Maybe I am not clever enough. But I am tryoing hard. I have been asking this question from current mirror to differential amp for many times. I read many books but just cant catch the concept. Kindly guide me more as I am just a beginner. The lecturer has teach till the end of a book and I am still scratching the first few chapters. I urgently need help. The exam is drawing near.

regards and thanks
Jason

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