[sdiy] OTA filter stage transfer function

[David G. Dixon]

> Can anyone clear up a question I'm having?  When an OTA like the 13700
> has a capacitor to ground on the output in a typical filter stage
> (rather than feeding an opamp with the cap in the feedback loop), is the
> normalized transfer 1/s because it's a current output, rather than
> 1/(s+1)?

If the output of the cap (via the buffer) is fed back to the negative input
terminal of the OTA with the same gain (i.e., through an identical divider)
as the input which is fed to the positive terminal, then it's a lowpass
filter with a TF of 1/(s+1).  Without that feedback, it's a non-inverting
integrator with a TF of 1/s.

The fact that the OTA produces a current output has nothing to do with it.
Any filter or integrator requires a current source, even if it's simply a
voltage source across a resistor.
 
(I've sent you a document via private email which explains these concepts in
detail, and much else besides, with derivations of the transfer functions
for many different filters and integrators based on OTAs and VCAs; refer to
items 4 and 5.)