[volt-nuts] How to measure micro-amp currents and have low impedance?

[Bruce Griffiths]

Bob Paddock wrote:
> On Sat, Sep 26, 2009 at 6:56 PM, Bruce Griffiths
> <bruce.griffiths at xtra.co.nz> wrote:
>   
>> Use the Patterson transdiode connection with npn and pnp transistors as
>> feedback elements.
>>     
>
> I'm not familiar with Patterson's work, and Google was not much help.
>
> Will you point me to a specific paper or schematic please?
>
>   
Its not particularly new, you have probably seen the circuit many times
(the name has been around for at least 50 years or so) see:

http://www.electronics.dit.ie/staff/ypanarin/Lecture%20Notes/DT021-4/6LogAntiLogAmplifiers.pdf

The problem is that transistor bulk resistance and opamp output
limitations restrict the maximum operating current to 10mA or less.

Bruce