Well it works, with a minor tweak. Somewhere along the way with the simulations I made a few oversights with the servo constants. Firstly, when modifying the servo topology from the first basic sim I erred with the loop gain scaling by 20dB making the unity loop gain frequency of the servo one decade higher that what it should have been. Secondly I did not re-investigate the servo loop gain and phase margins after adding the BJT cascodes to the JFET input stage to mitigate JFET Miller-effect capacitance. Cascoding the JFET input stage quite largely increases input stage transconductance, which pushes the unity loop gain frequency of the servo loop out proportionally. As it turns out, due to the sum of these oversights, the servo loop with the 1uF integrator capacitor wasn't close to stable.
For adequate phase margin in this topology the unity loop gain frequency of the servo really needs to be not higher than one third the input frequency pole formed by the 1uF input coupling capacitor and the 2M2 resistor; which is thus currently at 0.072Hz. Increasing the servo integrator capacitor from 1uF to 1000uF got the unity loop gain frequency down to ~0.02 Hz where it should be and the amplifier was then stable. I don't like a servo with a 1000uF capacitor. That large it has to be electrolytic which will no doubt be leaky and therefore contribute very low frequency drift all on its own accord. So the design will require a revision, but fudging in the 1000uF for now is a quick way the get the amplifier operational to evaluate its noise performance and other parameters.
Here it is (the tall skinny electrolytic in the middle is the 1000uF. It was the smallest 1000uF I had at hand):
![](https://www.eevblog.com/forum/projects/low-noise-amplifier/?action=dlattach;attach=280145;image)
I used my home brew noise measurement test set to measure the amplifiers noise:
![](https://www.eevblog.com/forum/projects/low-noise-amplifier/?action=dlattach;attach=280147;image)
Due to mains hum pickup and other interference of the bare amplifier PCB sitting on the bench top there are limits to which I can evaluate the noise performance prior to installing the amplifier into a shielded steel enclosure, so for now I used audio A-weighting filtering, primarily to get rid (mostly) of the 50Hz and HF crud coming from the local AM broadcast transmitter.
This scope shot shows the A-weighted noise at the output of the amplifier (the amplifier having its input shorted) amplified by 80dB (10,000 Av) (actually only by 60dB, but the DSO channel input coupling was set to 10:1 while the input connection was 1:1).
![](https://www.eevblog.com/forum/projects/low-noise-amplifier/?action=dlattach;attach=280149;image)
The noise measures 42 uV rms on my test set. The A-weighting filter has an equivalent noise bandwidth of 13.5 kHz. So the equivalent output noise, expressed in a 1Hz bandwidth = 42 uV / SQRT(13500) = 361 nV rtHz.
The amplifier has a fixed closed loop gain of 60dB (1000 Av), so the actual input-referred noise is 0.361 nV rtHz, which is getting quite close to the theoretical target. It can only be improved with a proper shielded enclosure. The noise on the scope has an obvious 100 Hz component (rectified mains) - most likely contributed by the crappy bench supply currently providing the +/-15V.