Electronics > Beginners

Virtual ground push-pull

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Zero999:

--- Quote from: mikerj on April 28, 2019, 01:04:40 pm ---Just use one of the electrolytic cap models on the output, no need for separate resistors then.  Stuff often oscillates if you use a perfect capacitor model in spice.


--- End quote ---
It can oscillate in real life too.

--- Quote from: imo on April 28, 2019, 10:52:10 am ---@Zero999: the both caps had/have 20mOhm ESR (in all sims above and here).

--- End quote ---
All of those are under damped, so should oscillate.

Use a squarewave to test whether it really will oscillate.

The inductor makes the load look like a 1R8 resistor at high frequencies, where it's going to oscillate. R3 can't be removed or made too high, otherwise it will resonate with L1, C1 and C2. Increasing C1 and C2 will also cause oscillation. The inductor could be omitted, but the DC current would drop across R3, which might not be a problem, at low currents.

Kalvin:
Here is my shot at the circuit. The C3 and C4 are used to tweaking the step response and stability. The transistors' quiescent current is well below 1uA. The circuit is stable at least with C1 and C2 ESR in range 0 ... 50 milliohms.

iMo:
Pulsing loads and RS v3A.
4.7u 47u 470u caps 50mOhm esr.

mikerj:

--- Quote from: Zero999 on April 28, 2019, 01:27:09 pm ---
--- Quote from: mikerj on April 28, 2019, 01:04:40 pm ---Just use one of the electrolytic cap models on the output, no need for separate resistors then.  Stuff often oscillates if you use a perfect capacitor model in spice.


--- End quote ---
It can oscillate in real life too.

--- End quote ---

Quite, though my point was that zero ESR/ESL capacitors don't exist in reality.  Simulations will never be close to reality if you don't use models of real components.

Zero999:
It's easy to select capacitor models which fairly closely match reality.

The good thing here is that simulations will tend to use lower ESR capacitors, than what's available, therefore model the worst case.

It appears the class-B output stage was what was causing the instability, as the op-amp had a step change in its output, when changing from sourcing to sinking and back again. The above circuits are more class AB, which is more stable, but it's probably still prudent to test with different capacitor and ESR values, at different currents.

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