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

Guy McCusker guy.mccusker at gmail.com
Fri Dec 11 20:25:22 CET 2020

```Depending on exactly what you mean by 1/gain (I would just call this
value the gain of the input attenuator -- some fraction quite a lot
lower than 1) this seems right. You just want to know how many amps
out per volt in to the OTA block taken as a whole. I think what was
confusing you in the previous calculation was that you weren't
dividing by the input voltage to get the transconductance in
amps-per-volt (aka mho or Siemens).

On Fri, Dec 11, 2020 at 7:05 PM Chris McDowell <declareupdate at gmail.com> wrote:
>
> you need to take into account the voltage divider at the input of the OTA.
>
>
> ah, okay, I thought I understood that, but was not doing anything about it! it appears to simply be, for the 13700 at least:
>
> gm = 19.2 * Iabc * (1 / gain of input attenuator)
>
> Sound reasonable? Spreadsheet seems to like it ¯\_(ツ)_/¯
>
>
> On Dec 11, 2020, at 12:38 PM, Tom Wiltshire <tom at electricdruid.net> wrote:
>
> I think Guy means that if you’re looking for the unity-gain point, you need to take into account the voltage divider at the input of the OTA. There’s a big attenuation going in, so the gain required to achieve unity gain overall will depend on the component values you choose to bring the input down to the right level. However, it *won’t* depend on the input level itself.
>
> Is that right, Guy?
>
> Tom
>
>
>
> On 11 Dec 2020, at 18:18, Chris McDowell <declareupdate at gmail.com> wrote:
>
> How would I go about this without making the cutoff frequency seem voltage dependent? If I derive gm from Vin/Iout, then the resulting frequency is dependent on the input voltage, but we know it is not. Is it just some algebra I'm overlooking to separate it out?
>
>
>

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