how to generate Ibias =200pA/1nA for OTA ?

[fatima1_s]

Can anyone please help regarding this issue?

 

[Dominik Przyborowski]

You need a two/three or more current mirrors to drop a µA current to nA range.

 

[fatima1_s]

Thanks for your reply.
Would you please tell me about the sizing of transistors as well as their region of operation (weak inversion and moderate inversion)in designing current mirror circuit so as to get Ibias=200pA or 1nA.

 

[Dominik Przyborowski]

You need a spaghetti fets ;-). Current mirrors works good only in strong inversion but it is hard to obtain for pA currents. You didn't mentioned nothing about your technology node, so I'm not able to calculate a proper dimensions of your mosfets.

But, try to start from the last current mirror, calculate from a IC=10 (inversion coefficient) the W/L dimensions for 200pA, assume the minimum W and made first current mirror with ratio for example equal to 20. In next steps You get dimensions for other current mirrors.

 

[prestonee]

Current mirrors should always be in saturation, a slight mismatch in the exp curve is big deal in subthresh compared to near flat in sat.
also instead of mirroring down multiple times we use subtraction(yes math) make a current 5i and 4i then mirror both down say 5 i, then subtract the 4i from the 5i, youd get a small predictable current say 1/5i if i is 1 uA then new i and then you would do it again. but 1nA is very hard to do. many models arent accurate to that point and even if you have a device turned all the way off you can still get a nA flowing through.

 

[fatima1_s]

Thanks for your reply. This is the paper in which self biased current source is mentioned. Please have a look at this andkindly help me ... i need this to work out in final project. i'm working in 90nm technology with supply voltage =1V. Ibias is used to implement OTA filter for biomedical applications (ultra low power and low voltage).

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I've attached another image of main circuit, please give me an idea

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Please have a look at this main picture, this is main circuit in which I have to implement ibias.