VC Butterworth Question

[Ian Fritz]

Hi Mike --

Yes, as you say, it's the same as the Sallen/Key low-pass VCVS, except
that (1) the feedback gain is unity and (2) the cap that feeds back from
the output to the first stage is twice the value of the cap (to ground)
in the second stage. So in the usual notation, R1 = R2, C1 = 2*C2, and
A=1. This is the "C-2C" Butterworth configuration.

In the implementations I've seen before, the effective R is replaced by
a CA3080 OTA, with, typically, 100k / 200 Ohm input voltage dividers.
I'm using a CA3280 with linearizing diode current of 1 mA and series
input resistors of 7.5 k (no resistance to ground). The C's are 560 pF.
It looks like I get a Butterworth response if C1 = C2, but if C1 is 20%
more than C2 there is noticable peaking. There is no significant
frequency dependence (f0) to the response, so I don't think it is
anything like a Q enhancement due to spurious phase shifts. 

Thanks for your interest and any ideas.

  Ian


Mike I wrote:
> 
> Hi Ian,
> I have not seen this circuit-is it a variation of the voltage-controlled
> voltage-source ( VCVS ) filter topology where a voltage follower is used
> since the  amplifier needs a gain of exactly one for the circuit to
> operate (with the OTA's replacing the fixed resistors)?