Frascati pre-amp: gain increased to 200

Yesterday, with Mauro I., we tried to increase the gain of the Frascati pre-amp. We simply changed the feedback resistor, as shown in the figure.



At first, we used a 2.5kOhm resistor, but it turned out that the pre-amp is not working anymore: no output signal at all. Probably this is due to the fact that increasing the feedback resistor too much unbalances some signal levels.

After several tests, it turned out that it is possible to increase the gain only by a factor of 2.
So now the gain is approximately 200 (measured with an oscilloscope, to be better measured in TOV), the noise at the output is around 200nV/Hz^1/2, which means that the input referred noise is around 1nV/Hz^1/2. All this must be checked in TOV.

I think that we should compare the noise with gain of 100 with the noise with gain of 200.
Why do you want increase the gain ?

Back to Tor Vergata, I measured the transfer function of the pre-amp: the gain is 235.
I also report the input referred noise: it seems really good, below 1nV/Hz^1/2 above 10 Hz.