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Design >> Analog Design >> Cascaded Current mirror low current
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Message started by neuro11 on Apr 3^rd, 2013, 1:14am

Title: Cascaded Current mirror low current
Post by neuro11 on Apr 3^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

Title: Re: Cascaded Current mirror low current
Post by neuro11 on Apr 3^rd, 2013, 1:15am

and the current output in differential stage

Title: Re: Cascaded Current mirror low current
Post by neuro11 on Apr 3^rd, 2013, 1:16am

and the current output in output stage.

Title: Re: Cascaded Current mirror low current
Post by raja.cedt on Apr 3^rd, 2013, 1:29am

is P12 gate connection correct????

Title: Re: Cascaded Current mirror low current
Post by neuro11 on 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.....

Title: Re: Cascaded Current mirror low current
Post by sheldon on 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

Title: Re: Cascaded Current mirror low current
Post by neuro11 on 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

Title: Re: Cascaded Current mirror low current
Post by sheldon on 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