I have a simple op-amp circuit (picture) designed to drive a capacitive load (~1nF) with a variable ac voltage up to 40Vpp and variable frequency up to 150kHz. The circuit takes signal from a function generator, amplifies it with a gain of 2, and passes it to the load. The op-amp used is an LTC6090, with rails connected to a (9V in) +24V, -24V DC-DC converter than can supply 21mA.
The circuit works well for the above requirements, but I need to limit its maximum output current to 2mA for safety reasons. The danger is that the user could bypass the capacitive load and get a shock.
I have considered a current sense amplifier interfaced to the output disable pin of the op-amp (difficult with bipolar signals), a 10k resistor in series with the load (limits the frequency by forming a low-pass filter with the load capacitance), a 10k resistor in series with each of the supply rails (spoils op-amp performance and adds noise) and I'm running out of ideas. I've also considered constant-current diodes, but I'm pretty unfamiliar with these and I'm unsure of their limitations in ac circuits (any ideas?!). Also, I can't seem to find any DC-DC converters at these voltages that have small enough maximum currents to limit it inherently.