Pro Audio Design Forum

Visualizing the Birt Line Receiver As A Fully Differential Amp

The Birt Line Receiver is a very useful circuit but the topology is confusing particularly when the polarity inverting stage is drawn backwards. When drawn this way the Birt looks like M.C. Escher's "Drawing Hands." By rearranging the circuit elements and drawing a big triangle around it, the Birt looks suspiciously like a Fully Differential Amplifier. The Birt topology emulating a Fully Differential Amp has many practical applications.

"Drawing Hands" rendition of the Birt Line Receiver as shown by Jung et al.

The circuit below can be used as a balanced line reciver or a balanced line driver.
The Birt topology has unique characteristics which include a driven reference leg that places the inverting amplifier's noise contribution in common mode.



Re-visualizing the Birt Line Receiver with uncommitted connections makes it appear like an FDA.


Visualizing the Birt Line Receiver As A Fully Differential Amp

David Birt "Electonically-Balanced Interfaces" https://proaudiodesignforum.com/images/ ... rfaces.pdf

"Evolution and Origin of the SuperBal Variable Gain Input" https://proaudiodesignforum.com/forum/p ... php?p=4822

More to follow...

Some Applications of the Birt Fully Differential Amp



The modulator driver has the "internal" inverter sense input brought out and connected after the upper stage's build-out resistor.

That’s a very flexible circuit. I wonder how it compares to something like an OPA1637?

They're functionally very similar.
I think the advantage of the Birt using op amps is that it is lower cost and not single-sourced.
For DIY there is also the through-hole advantage.
The overall CMRR performance of both is limited by the resistors.

Good points.