Current mirror circuit to produce biasing currents in nA's

[hsanghera]

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.

 

[FvM]

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.

 

[hsanghera]

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?

 

[millwood]

current mirror na current

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