Differential follower?

[abzug]

How can a differential follower be constructed (with common mode rejection)?

I have a circuit with THD between the differential outputs much lower than THD on each of the two outputs independently. I want to add buffer for both XLR and RCA connections, and so for the latter I need a way to make use of the much lower differential distortion. And no I don't want to use a transformer. I realize I can use a discrete opamp in unity gain configuration, but I want to minimize the number of additional stages.

[Nrik]

http://www.metaxas.com/pages/technology/diy.html
CP3

If you make the R2 and R3 to have the same value, you will have gain of 1 without feedback.

But remeber offcourse the 6dB gain of going from balanced to unbalanced.

[abzug]

Thanks. Any rough estimate on the CMRR?
One thing is I'm aiming for a DC coupled circuit though.

[Nrik]

I dont have any estimates for CMRR.
DC coupling should be possible, choosing the right FETs, and maybe switching the output for a P-channel.
But then you might face some thermal drift.

I am maybe not the right guy to continue this one, so if someone else would like to analyse and simulate, they are more than welcome.

[Tim__x]

In my opinion, the best way is with a fully-differential opamp like topology. Start with a LTP, add a VAS to each leg, add an output stage to both VASes. Then construct another LTP, make one input a CM reference, the other should see the average of the two previously mentioned outputs, sum the appropriate phase current output of this servo LTP with the tail of the first LTP. Voila

[Tim__x]

Upon rereading your post I see that perhaps you don't want a differential follower but instead a differential to single ended convertor.

Any DC coupled topology with remotely acceptable performance is going to need at least a differential input and a VAS and will end up being as complicated and will have as many gain stages as an opamp.

[abzug]

Why would a VAS be needed when no voltage gain is required?

[Tim__x]

Obviously you need to start with a differential stage, all standard dc coupled differential stages produce a current output that for reasons of compliance must be referenced to a rail.

You're right to say that you don't absolutely need a VAS but you do absolutely have to have a level-shifter, whether that means a VAS, a folded cascode, or simply a current mirror is up to you.

[abzug]

So there's no way to make a single stage follower into a differential pair? A differential pair can only be made with common emitter/source configuration?

[Tim__x]

Due to the nature of a transistor only the base (or gate) and the emitter (or source) are particularly useful as inputs, while only the emitter (or source) and the collector (or drain) are useful as outputs. By definition a differential must have two inputs, and though it may not at first be obvious, even larger more complex differential stage apply the differential input across a single transistor's base and emitter (or as in a LTP across multiple single transistors, the base-emitter junctions in the two transistors in an LTP are in series). Do to the fundamental nature of a transistor as a dissipative device and not a power generating device, a transistor must have sufficient voltage headroom.

The output of a single stage differential to single ended convertor will always be referenced to a different voltage than the input; and as such must always be followed or preceded by level shifting or AC coupling if low input to output DC offset is desired.