[sdiy] how to calculate cutoff in an OTA filter?

[Michael E Caloroso]

Make sure you do not exceed the maximum control signal.  IE for
LM13700 OTA max is 2mA.

MC

On 12/11/20, Guy McCusker <guy.mccusker at gmail.com> wrote:
> Unless I have become confused again, the centre frequency of the SVF
> bandpass (which is not the same as the 3dB point of the lowpass, but
> is probably the right frequency to think about) is the same as the
> unity gain frequency of the integrators. This assumes you have the
> integrators set for the same gain, which you don't have to, and fun
> can be had if you don't.
>
> So for a typical OTA integrator, I think you would use your equation
> but the gm would be calculated to take into account attenuation at the
> input of the OTA.
>
> Guy.
>
> On Fri, Dec 11, 2020 at 5:06 PM Chris McDowell <declareupdate at gmail.com>
> wrote:
>>
>> The subject says it. How do we calculate the cutoff frequency in a filter
>> with OTAs, or VCAs for that matter. Maybe this is more obvious than I
>> think but I'm not making sense out of it.
>>
>> So for a state variable filter, if transconductance, gm = 1/R, then R =
>> 1/gm, then why is the cutoff not just 1 / ( 2 * PI * (1/gm) * C) ?
>>
>> Cheers and thanks in advance,
>> Chris
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