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

Richie Burnett rburnett at richieburnett.co.uk
Tue Mar 17 19:46:13 CET 2020

```If you're after a buffer with balanced inputs and balanced outputs, then
take a look at the input stage part (balanced part) of a conventional three
op-amp "instrumentation amplifier" arrangement.  In other words the bits
that surrounds the two op-amps on the left of this schematic:

https://en.wikipedia.org/wiki/Instrumentation_amplifier#/media/File:Op-Amp_Instrumentation_Amplifier.svg

You don't need the op-amp on the right and it's accompanying four resistors
that make up the differential amplifier unless you want to convert the
balanced output to single-ended.

This arrangement of two op-amps, two feedback resistors and one gain setting
resistor tied between the inverting inputs has the following properties:

1. Balanced input and output.
2. High input impedance (good for buffer)
3. Low output impedance (good for a buffer)
4. Gain is adjustable by varying a single resistor.
5. Good stability.

Hope this helps,

-Richie,

-----Original Message-----
From: Ben Riggs
Sent: Tuesday, March 17, 2020 5:21 PM
To: SDIY List
Subject: [sdiy] Unity Gain Diff-in Diff-out driver

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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