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neuro11 Junior Member Offline Posts: 30	Cascaded Current mirror low current Apr 03^rd, 2013, 1:14am Hi, this is a differential input Opamp with cascaded current mirror. The mirror circuit looks complex but i know it is supposed to provide a stable output current of 6uA ~7uA. But i am getting very low current in fact nearly zero. Can some one please suggest the reason..... I have attached the circuit
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neuro11 Junior Member Offline Posts: 30	Re: Cascaded Current mirror low current Reply #1 - Apr 3^rd, 2013, 1:15am and the current output in differential stage
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neuro11 Junior Member Offline Posts: 30	Re: Cascaded Current mirror low current Reply #2 - Apr 3^rd, 2013, 1:16am and the current output in output stage.
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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: Cascaded Current mirror low current Reply #3 - Apr 3^rd, 2013, 1:29am is P12 gate connection correct????
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neuro11 Junior Member Offline Posts: 30	Re: Cascaded Current mirror low current Reply #4 - Apr 3^rd, 2013, 1:59am thanks for looking.... ya the connection is alright. And in the circuit i posted i made a mistake of not connecting the differential inputs base to VDD, but that was not the reason i am not getting the right current.....
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sheldon

Community Fellow

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Posts: 751

Re: Cascaded Current mirror low current
Reply #5 - Apr 3^rd, 2013, 3:45am

Neuro11,

1) The connection for P12 is incorrect. P12 should be connected to the
gate of P10. You have connected it to the source of P11, this
voltage is one threshold voltage less than the gate voltage of
P10 and is approximately the same as the gate voltage of P9.
2) You should consider using a low dropout mirror, see Johns and
Martin.
3) Next, why are P6, P11, and N7 in the circuit? What purpose do
they serve.
4) Why are you using a diode connected transistor to set the bias
current? If the supply voltage goes up or down the bias current
will vary significantly. It is worth your time to build a real
bias current generator.

Best Regards,

Sheldon

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neuro11

Junior Member

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Posts: 30

Re: Cascaded Current mirror low current
Reply #6 - Apr 3^rd, 2013, 6:06am

Thanks a lot for the detail response. I was following a TSMC tutorial and using exactly this current mirror they recieved a stable current.So i tried, for your questions:

1. i exactly thought the same way and rechecked but it seems they were not using p12 for mirroring which is surprising because p12 directly connects to differential pair.
2. thanks for reference i will check
3. i assume they used N7 and p11 to reduce the voltage at which current gets stable. (i am not expert so not sure)
4.ya now i think i should study to make my own. what book you recommend. i want to design a voltage buffer opamp and a differential input opamp. what book you suggest for that.

Thanks

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sheldon Community Fellow Offline Posts: 751	Re: Cascaded Current mirror low current Reply #7 - Apr 3^rd, 2013, 7:15pm Neuro, I think that the NMOS mirrors for the diff pair bias and load were originally cascoded and N7, P11 biased the cascode. Sheldon
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