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Current mirror circuit to produce biasing currents in nA's

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hsanghera

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current source na

Hi. I was just wondering if anyone knew of any transistor circuits that could reliably produce a constant current in range of 10-100 nA and also have a very high output impedence. The reason for this is that I'm trying to do resistance measurements of a highly resisitive semiconductor material (> 10 Mohm) and need a really small biasing current. I've read up on the Widlar current source but am just wondering if there are any others.

Thanks.
 

current source circuit nanoamp

I don't see a problem to operate a transistor current source in the nA range. You have to check for sufficient low leakage currents of course. But I wonder, if it's an appropriate way to build a current source for a precise measurement. The current transfer ratio even of monolithic current sources is rather variable. I think that high ohmic resistors in the 100 Mohm upto Gohm range allow to design higher accuracy current source as expected in instrumentation applications.
 

widlar current source

Does anyone have another idea of how to make a current source in the 10's of nA range. It doesn't have to be really accurate, just super stable.

I have tried making a Widlar current source using BJTs. I have got a fairly stable current (~10 nA) upto load resistances of 100kOhms. I haven't tested pasted that yet. What I really want is something with an output resistance of at least a couple of GOhms. Is it better to use MOSFETs for the Widlar design to get higher output resistances as compared to BJTs?
 

current mirror na current

it will be very difficult to compensate for thermal drift at those levels.
 

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