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

Neil Johnson neil.johnson71 at gmail.com
Tue Mar 17 19:13:40 CET 2020

```Too many words, need picture :)

Cheers
Neil

On Tue, 17 Mar 2020 at 17:23, Ben Riggs <benalog1977 at gmail.com> wrote:

> 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.
>
>
>
>
>
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