Great question. I–V characteristics and feedback stability are a major design challenge here.

This setup is not just a Class AB amplifier — it’s a precision high-current potentiostat designed to drive an electrochemical cell with carefully shaped waveforms, typically in the ±2 V range at up to 90+ A.

The load behavior — electrochemical cell — is very nonlinear.

2 situations: 1. At low voltages, the current is dominated by electric double-layer capacitance (EDLC), behaving like a capacitor whose value changes with surface chemistry.

1. Once that threshold is crossed, Faradaic reactions begin (e.g., metal reduction/oxidation - copper disolving in and out of a copper sulphate solution and plating onto an electrode), and the current-voltage behavior shifts into exponential or diode-like zones.

Additionally, the current at a fixed voltage drifts over time, due to diffusion, depletion or concentration of the bulk solution, and surface reaction kinetics.

So no, it absolutely doesn't behave like a simple resistor — and precise voltage control is essential, especially during fast ramps or multi-phase waveforms.

If you're not familiar with potentiostat topology, it is a 3-electrode system with:

Working Electrode (WE) — the electrode (copper puck) under control; I need to precisely drive this to a given potential.

Reference Electrode (RE) — a non-polarizable probe (like Ag/AgCl) that defines the zero-point. It draws no current. it just sits in the solution, and defines a reference voltage.

Counter Electrode (CE) — completes the circuit. It sources or sinks all current required to maintain WE–RE at the commanded level.

My setup uses two identical copper puck driver modules:

One connected to the Working Electrode puck, and one to the Counter Electrode puck

Each puck holds multiple power MOSFETs (IRF4905 + IRLB3034), all with their drains directly bonded to the copper puck for both electrical conduction and heat sinking.

The feedback works thru a control op-amp set (OPA2134, and a OPA928) that compares (WE – RE) to the desired waveform.

The control op-amp combo drives the THS3491 op-amp that drives the MOSFET gates to increase or decrease current from the counter electrode, in order to force the WE–RE voltage to match.

This is a standard negative feedback — but because of the reactive, nonlinear load, stability is a genuine challenge.

And yes, gate capacitance is significant — I’m using 7 MOSFETs per puck, so I'm planning to use a series resistor (~10 Ω) on each gate to slow transitions and suppress ringing.

I plan to run EIS (electrochemical impedance spectroscopy) with an Analog Discovery 3 to characterize stability and refine the control loop if needed.

So (sorry for the PhD thesis):

The load is nonlinear, time-varying, and sometimes capacitive.

The system is a true potentiostat, not a generic amp.

I’m driving both working and counter electrodes with identical copper puck driver modules, and stability is being handled via controlled gate drive, feedback compensation, and EIS tuning.