[sdiy] OTA diodes

Donald Tillman don at till.com
Sun Jun 15 07:08:57 CEST 2014

On Jun 14, 2014, at 12:01 PM, Justin Owen <juzowen at gmail.com> wrote:

> In Electronotes EN62 pg12 (one of the pages that is publicly available AFAIK...) there is a schematic of a basic OTA where the top current mirror hangs off a 680R resistor with 3 diodes to GND. I remember seeing this in another discrete OTA schem but never in the simplified schematics of the major production OTA's.
> Recently I noticed that the simplified 2164 schematic uses a similar config - but in a different way (I think...).
> What are these doing, what purpose do they serve, what are they improving? I'd be particularly interested in answers from an OTA POV.

Circuits that rely on matched transistors work much better when those transistors are operating in identical, or near identical, electrical environments.  In this case the issue is Vcb, and you'll lose the effects of matching if the transistors are operating at different Vcb's.

An example would be the current mirror at the top of EN-62 p12.  The 3 transistor Wilson current mirror is superior to the 2 transistor vanilla current mirror because the third transistor assures that the Vcb of the two matched transistors are within one diode drop.  So the Wilson current mirror is a win.

For the diff amp here, the second transistor's collector is at 0 volts as it's a virtual ground.  A current mirror operating at a high voltage would mean that the collector of first diff amp transistor would be way up high.  So running the current mirror at a low voltage assures that the diff amp collectors are within one diode drop of each other.

The SSM2164 circuit works exactly the same way.


Ironically... with the EN62 circuit running the current mirror at this low voltage... the Wilson current mirror is no longer an advantage over the plain vanilla current mirror.  Heh-heh.

  -- Don

Don Tillman
Palo Alto, California
don at till.com

More information about the Synth-diy mailing list