Hi all,
I am currently working on a range-switched photodiode design for use in our laboratory [1]. It took me a while, but I've arrived at a design I'm fairly happy with (in no small part thanks to Phil Hobbs' helpful writings on the topic):
My current draft uses a simple BF862 source follower at the summing node of a ADA4817-based 2 M?/2 k? transimpedance amplifier to bootstrap the
40 pF of photodiode capacitance (its output being AC-coupled into the PD bias node). On paper and in SPICE, I am getting just over 2 MHz bandwidth on the 2 M? range at reasonable noise levels (limited by the single BF862's e_n), assuming fairly realistic parasitics. On the lower gain range, however, I'm only predicting ~75 MHz of bandwidth, as the BF862 stage quickly loses steam driving the 40 pF load due to its output impedance.This gets better when putting an active sink on the JFET source, but I thought I would spend some time optimizing the bootstrap design while I'm waiting for parts to arrive (~100 MHz bandwidth would be nice to have). It's not hard to come up with a circuit that provides >0.99 gain into the given load out to a few hundred MHz in simulation, for example by adding an RF(-ish) PNP transistor to make a complementary feedback pair, or simply tacking on an NPN emitter follower (bootstrapping the JFET drain for better effective gain as necessary).
However, while those bootstrap designs look good in isolation, it seems like they invariably render the complete circuit unstable on the 2 k? range in simulation (although they work brilliantly on the 2 M? range). I'll have to go through the maths properly, but this appears to be due to the two-pole rolloff caused by having two transistors in the path. On the 2 M? range this happens far above the op-amp loop bandwidth anyway, but this is no longer true on the 2 k? range. The bare BF862, on the other hand, has a lower gain to start with and rolls off earlier, but the single pole doesn't seem to degrade the phase margin enough to cause issues.
Building a sub-nV emitter follower that is good out to multiple GHz obviously can't be the right solution. I feel like I'm just missing an easy fix in the form of a lead-lag-type tweak in the right place, but I can't quite figure out where that would be, and much less how to make that work without degrading the noise performance too much.
Since my intuition for analog design is not great, I thought I'd ask here whether I missed something obvious. I can also post a sketch or LTspice file of the relevant part of the circuit if that helps, but I figured the question would be trivial to answer for the photodetector veterans among you, as presumably you would have run into this before.
Thanks!
? David
[1] Quantum optics. The idea is to build small (well, small as far as lab gear goes ? ~ 35mm x 30mm x 25mm) modules with built-in beam sampling optics for laser intensity monitoring and stabilisation all across the visible and near-IR. And yes, the range switching led to some rather expendable gymnastics, but the alternatives would have been even more painful.