[sdiy] Sallen-Key [was: Re: Digital pots as the gain element in a filter]

[lanterma at ece.gatech.edu]

On Jul 6, 2011, at 5:46 AM, Tom Wiltshire wrote:

> In the following paper:
> 
> 	http://focus.ti.com.cn/cn/lit/an/sloa024b/sloa024b.pdf
> 
> Section 3.4 (bottom of page 5) gives equations for an equal-components version of the Sallen-Key filter, and says that in this situation, the Fc and Q are independent (yay!).
> 
> The equations given are:
> 
> 	Fc = 1 / (2.Pi.R.C)
> 	Q = 1 / (3 - K)
> 
> Now, K I can't find a nearby reference too. It's mentioned earlier on in relation to the gain (back on page 4)
> 
> 	K = 1 + R4/R3, where R4 and R3 are the gain resistors providing feedback to the -ve input.
> 
> Is this what you meant? In this equal components version, does the Q only depend on the gain? Presumably, altering the gain also...umm...alters the gain? By which I mean passband gain.


Think of that op-amp-R4-R3 network as being a generic voltage gain stage of some sort. You can just tap your output *before* that gain stage. In Figure 5 in that document, just move your v_o marking from the output of the op amp to the positive input. (Of course this would need to be feeding something with a high input impedance to avoid loading down the R2-C1-etc.)

Tom Gamble does this here:

http://www.modular.fonik.de/files/EFMforum/orgEFMfiles/vcf8e.pdf

The Q is controlled by an OTA-type VCA in the feedback loop of the "Bach" Sallen-Key variation, but notice the output is taken before that VCA.

- Aaron