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

[Ben Riggs]

Thanks Richie,

I did play with this and can be made to do what I need, but the gain of this circuit is always greater than 1. to get unity gain the gain resistor needs to be removed and you’re left with basic op-amp followers or voltage dividers need to be added at the inputs which them makes it look like 2 difference amps (like the output stage of the instrumentation amp).

Ben.

> On 18 Mar 2020, at 1:46 am, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
> 
> 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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