Current Buffer/Amplifier

[T8P1]

Hi
I search for a discrete design idea for an current buffer/amplifer with 0 to 50uA DC current and 0 to 30pA AC current with a Gain of 1 to 5 or more. Important is that the Current Input noise have to be under 3pA/Hz^(1/2). Important for me is, that the volltage at the input have to be constant.

Now, i amplify the dc and ac current separatly.

 

[FvM]

As a "small" handicap, the noise density of 50 uA DC current source will be already 4 pA/√Hz. so some resreictions apply...

 

[T8P1]

Thats one reason, why i separatly amplify the ac and dc signal. Now, i look for an buffer only for the dc part to separat the source from the amplification. At this time i use an activ low pass filter, but the voltage at the input varry about some uV. A like a constant voltage there, but i have no idea how. The only idea i have, is to use a Current-Current Opamp as a buffer.