[sdiy] linearize OTA for VCF?

[Ian Fritz]

Terry --

Thanks. With the circuit you have described, I believe that the input and
the output will clip at the same point when Iabc is .5 mA. But the input
always clips when Iin = Id, so if Iabc is less than .5 mA the input will
clip before the output does.

I believe one generally needs to design so that the input structure doesn't
clip. This leaves a bit of a dilemma. Should the input be set to clip when
the nominal system level (+/-5V, +/-10V or whatever) is reached, or at a
higher level to allow some overhead? The most conservative approach (when
designing a filter with feedback, say) would seem to be to design so the
input structure cannot clip until the input signal reaches the rails. This
of course will eat into whatever improvements in distortion and S/N the
linearizing diodes give you.

  Ian


----- Original Message -----
From: "Terry Michaels" <104065.2340 at compuserve.com>

>> The way I got around this in my VCA design, which uses the CA3280 with
> diode bias activated, is I set the gain levels so that the OTA clipping
> point coincides with the output buffer reaching the power supply rail.
> You're going to clip there anyway.
:
:
The diode bias pin has a 68K resistor to +15 volts, the output of the
CA3280 drives the inverting input of an opamp, the opamp has a feedback
resistor of about 30K.  Iabc has a maximum of about 0.5 mA.