Electronic – Why cascode transistors contribute little to overall noise in an OTA

If I were to guess, I would say that the circuit is falling victim to its wacky biasing scheme. A valiant attempt is made in the negative reference voltage generation to ensure good power supply rejection, with lots of bypassing (with capacitors of several sizes) and buffering. But this might not compensate for the op-amp being on wildly assymetric power rails. Everything in the datasheet assumes symmetric power rails.

Never mind that. Let's look at something else. In the specifications table in the datasheet we see something interesting under Common Mode Voltage Range. Namely, it can be as bad as plus/minus \$V_s - 10\$! Uh oh! This means that your circuit should ensure that the inputs of the PA84 do not go within 10 volts of either power rail. I suspect that the -VREF line alone could be violating that, even with no signal. Your manipulations of the potentiometer could be taking the device beyond the common mode input range.

They do not contribute the the noise because of the degeneration. Having emitter/source degeneration in effect introduces negative feedback, greatly improving the performance of the transistors. For a more quantitative discussion see the end of this presentation.

You end up with the following equation for the noise: $$\frac{i_D}{1 + g_m r_o} \approx 0$$ Where \$r_o\$ is the degeneration. For high frequencies \$C_{gs}\$ provides an alternative current path, reducing the degeneration.