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kevin gilmore 25th February 2013 09:51 AM

unbalanced/balanced to balanced converter
i've been working on this for a while, i wanted an input
stage that performs well both for unbalanced and balanced inputs
without feedback, fully dc coupled etc. I came up with this which
sounds really good and has a thd of <.01%
buildable with standardly available parts from mouser.
performs even better with jfets as the inputs.

tvrgeek 25th February 2013 09:49 PM

So, how is the CMRR? That's the bugger. Thermal stability?

googlyone 25th February 2013 10:12 PM

without spending time analysing the detailed circuit I can't see how you are controlling the overall gain.

The input diff pairs are driving current mirrors - good. The current mirror drives into the common base cascade, which between the two pairs you develop the outputs in push pull.

The diff pair gain should be quite high given its load. How is this set? Similarly I note the output would normally have very high gain, those 430 odd ohm resistors on the output would "pull this down" but I suspect that in the absence of overall feedback the gain and more general performance would be quite load dependant.

Have you tried the heresy of upping the gain and adding overall feedback? :)

kevin gilmore 26th February 2013 09:57 AM

thermal stability seems rock solid with the output stage servo's.

the gain is set by the 475 ohm output resistors. current mirror into
a resistor is fixed gain.

Handles about 5 volts of common mode. That is what the 1k tail
resistors are for.

adding feedback would just lower the input resistance a bunch
so not sure that is a good idea.

Ken Newton 26th February 2013 08:31 PM


Reminds me a bit of the enhanced cascode gain circuit in fig.9 of the below linked Malcolm Hawksford paper. You've added a nice little local feedback-loop via the current-mirror implementation. I like it.

For me, the most interesting and, perhaps, challenging area of solid-state gain stage design involves low-ish distortion global open-loop circuits. Especially, those which do not greatly depend mostly on very precise device matching to achieve low distortion.

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