OpAmp with high output current and capacitive load

Question

I am trying to solve the following problem which - until today - I thought to be a trivial one: I have multiple reference voltages (from different sources) that I need to buffer and/or invert, filter with a large cap (10uF) and needing to supply around 100mA to the load:

^{simulate this circuit – Schematic created using CircuitLab}

The input references range from 0V to 4V and the supplies should be either single supply +5V (for the buffers) and +/-3.3V dual supply (for the inverting ones). Furthermore, it should be two opamps per chip.

The canonical solution to the problem is using an op-amp in (a) unity gain configuration (b) inverting amplifier:

^{simulate this circuit}

I wasted my whole day finding the right opamp. There are many that can drive arbitrary caps (I do not want to deal too much with compensation networks) and there are many that can supply >100mA. But the intersection is zero.

For example, the AD8655 provides +/-220mA but drive max. 500pF (and that rings already). The ADA4807 is stable for CL>100nF without compensation but the output current is limited to about 50mA. Many op amps are explicitely built to be stable for all capacitive loads, like the AD826 but again, current too low.

Am I missing something?

Answer 1

Here is one approach

^{simulate this circuit – Schematic created using CircuitLab}

The massive capacitance [I used at least 10uF] on emitter of the output buffer causes phase shifts and BODE plot rolloff and likely instability; including R6 (to isolate) opamp (-)pin from the emitter, and including C2 (needed to provide a look-ahead on the phase shift) provides a control-loop "zero".

In my original implementation, we used bipolar-input-device opamps with the inevitable input bias currents; by making R5 = R6, the nominal voltage drops are cancelled and the temperature-dependent deltaIbias/deltaTemperature error is reduced, to zero if the two input transistors of the opamp's diffpair are indeed matched.

R3 and C1 provide high frequency filtering of incoming power trash; the control-loop performs well at low frequencies. I knew the end-user-system was a high-sample-rate video imager and the focal-plane diodes have ZERO ability to reject power-supply-noise; system raw power were Switching Regulators at high-chopping frequency for efficiency, and I selected R1 and C1 to provide 20 to 40 dB additionally filtering at the SwitchReg chopping frequency.

Result of including R3/C1? There were no herringbone beatnotes on the display video screen, even with system gain turned up (via SPI) to the 8X max value. To delight of the program manager, we did achieve the 17 nanoVolts/rtHz noise floor. The pixels were quiet.

Answer 2

A voltage regulator is basically voltage reference + error amplifier + pass transistor, with the error amplifier compensated correctly to work with driving (through the pass transistor) a lot of capacitance.