[sdiy] improved dynamic range diffpair-vca

[JH.]

> The dolby's patent shows a variable resistance cell using a 
> differential
> pair with a lot of diodes at the common emitter (Fig. 6). 
>   http://www.pat2pdf.org/patents/pat4068139.pdf
>   An OTA with 100% feedback works as a variable resistor.

Wow, that's an interesting way to look at it indeed!

My first reaction was: This has nothing to do with an OTA or
differential pair; it's just a cleverly folded diode stack. (Maybe to
avoid using a straight diode stack, who knows? EMS used  stacks of 10
diodes for the Phaser in their HiFli, for instance.)
It's also described as a diode stack in the patent text (column 7, lines
19ff).
And yet you are right: The same circuit can be considered as an OTA with
a "diode-degenerated emitter" differential pair, and 100% feedback (node
38 being both the OTA output and the inverting input).

Amazing!

JH.