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How to analyse this circuit?

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Full Member level 5
Sep 7, 2005
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Hi, all: how to analyse this circuit? It likes a OPA, but I think it is not a OPA. How to analyse this circuit?

Robert Qi: can you explain this circuit detailed?

M17 , M18 are the source-coupled input transistors,and M12 is a bias (tail current source);
M47 and M48 are the cascode transistors, and M49 & M50 are biases;M13 through M16 constitue a cascode current load.

but Robert Qi, do you know what is the ac analysis result of this circuit?
look at the attachment please, this result is so strange!!!

so I do not know how to analyse this result!!

Is there any warning information in your simulation? Perhaps you can check the output.log file.

This is indeed a folded cascode OTA.

Your simulation shows rubbish, it looks as if you're using/simulating the amplifier in a wrong way. You typically get these kind of results when there is no gain available.

Possible reasons :
-bad current source biasing (not in saturation)
-bad cascode biasing (not in saturation)
-simulation out of the input range

First check whether all transistors are saturated.

rgds, JT

To simulate the circuit, you must be sure that all transistors be correctly biased and that the offset voltage Vos is in the linear area of the transfer curve. if not , you shold solve the problem and give the circuit a appropriate dc operational point.
because the op may have very hign gain, so you should be careful when doing ac analysis.
Best Reagards,

Hi, all: I have already why the simulation result is wrong. The circuit is right, but when I simulate this circuit, I forget connecting the source of M49 to ground.

many books analyze the circuit in detail .

you can refer the books

it is a very classical configuration.making sure every transistors working in saturated state is so important.


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