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How to analyse this circuit?

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wjxcom

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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?
 

wjxcom

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Robert Qi: can you explain this circuit detailed?
regards!
 

Robert Qi

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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.
 

wjxcom

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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!!
 

jwfan

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

JT

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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
 

Robert Qi

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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,
Qi
 

wjxcom

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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.
 

beckchm

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many books analyze the circuit in detail .

you can refer the books
 

daisyinfall

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

regards!
 

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