Hello Everyone,

I have been finding this website helpful for a long time but this is my first time posting here, so apologies in advance for possibly posting in the wrong section.
I have this preamp circuit and I have a question about it, the nodes 1 2 is a current source and the circuit seems to me to be a simple IV converter with 1G gain (1G resistor). However, it has these inputs to the opamp from an obscure set of circuitry that I am having difficulty making heads and tails of. I am suspecting that they are some kind of impedance converter but I am not sure.
Any suggestions or advice as to the function of this circuit or resources I should look up would be very welcome.

Thanks.

 

The U430 is a matched pair N-channel FET.

You can get a datasheet here:
http://www.datasheetcatalog.com/datasheets_pdf/U/4/3/0/U430.shtml

 

Thanks, I figured that out, but I am not really sure what purpose it serves.

 

Perhaps the elaborate discrete component front end electronics is intended to add improved high input impedance / high common mode rejection. You might add low drift and low offset - who knows. Anyway, someone has gone to a lot of trouble so there would have to be good reasons for doing this. What's the purpose / application of the circuit?

 

The GAA3R512 is a 0.35pF to 3.5pF trimmer cap.

Usually caps that small aren't used unless you're working with RF.

 

This circuit is used to amplify the tunneling current signal from an STM microscope and convert it to a voltage signal, the tunneling current is very small (~10pA), hence the need for a 1G resistor, the bandwidth required is around 10Khz, therefore a regular opamp usually won't do the trick. However, I don't really see the function of this circuit.

 

It's an interesting circuit .....

The front end looks like a long-tailed pair arrangement (2N3904's) with a cascode input setup - the U430 matched pair each have the 2N2904 as a load. The 2N3904 bases are AC grounded as one would expect in a cascode setup.

The 2N4401 setup looks like a current source to feed the "tail".