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

[Chris McDowell]

> 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 <mailto: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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