[sdiy] Unity Gain Diff-in Diff-out driver

[Ben Riggs]

Hi all,

I’ve been playing with differential driver circuits as part of a larger synth circuit I’m working on to seek a simple solution, what I’ve come up with just seems wrong but the sim has it working exactly as I need.

To start, I played with 2 cross-coupled input stock-standard differential op-amp circuits, then adding the “Superbal” feedback to both inverting and non-inverting, then I ended up playing with the MCI cross coupled output driver to reduce parts-count (drv135). With the MCI circuit, all reference to the output resistor of the MCI in my research was about output impedance (being quasi-floating was for short circuit current when one input was tied to 0V), I recall in other reading years ago it was also to reduce the cross coupled gain below 1 for stability, but then in another ESP article Elliot compensated for the loss in gain?? I’m not interested in the quasi-floating aspect and won’t be tying either output to 0V so I dropped that output resistor. Then I realised that the cross couple non-inverting resistor divider is (basically) in parallel with the inverting Rin/Rf on the visavi cross coupled op-amp so I reduced them and ended up with 2 basic inverting amplifiers with the op-amp inputs cross tied directly. A resistor tied from each output to ground to keep the output symmetrical, 2 op-amps and 6 resistors.

I hope that description makes sense.

The sim has 1V differential in = 1V differential out, 1V single ended in (one input tied to 0V) has symmetrical .5V on the non-inverted and -.5V on the inverted. This is exactly what I need!

It just seems wrong, and I’ve tried deliberately unbalancing the whole circuit in the sim, interchanging mix-matching op-amps and deliberately mismatching resistors but I think the SIM is failing me.

Is this actually a “stable” circuit, or is the simulator just telling me what I want to see? Surely life isn’t that simple..

Regards,
Ben.