I once constructed a biquad filter (using 3 op amps). It requires a set of matched capacitors and resistors. If you want to tune its frequency, you'll need a ganged potentiometer.
Have you found any op amps that perform at the frequencies you're talking about?
A passive RC filter is easy to tune its rolloff frequency, simply with one potentiometer.
You posted in the analog IC design section, does this mean that you intend to implement the filter completely on-chip, not just design the OP?
Which tuning method?
Provided you plan on chip filter and electronical (e.g. voltage controlled) tuning, this would suggest gm-C topology.
For Sallen-Key cut-off frequency is calculated by w=1/(sqrt(R1*C1*R2*C2)). So theortically it should work by tuning these values. I mean no much dependencies on the Opamp.
However you should be aware that Sallen-Key is sensitive to small variation of R's and C's values.
I don't know what other biquads you can work with (becuase there are tow-thomas, KHN, ..) but if you mean tow-thomas it hasn't this sensitivity.
One advantage for tow-thomas is you can control gain for your input signal and filter the noise.
So tow-thomas is less sensitive, immune to noise, configurable gain.
Sallen-key is simple, small area and power.
Regards,
Will a single stage folded cascode with 60dB gain be sufficient?
May I add a note please that he wants to work as a Butterworth filter.--> 60dB gain at your desired cutoff frequency should be sufficient.
--> 60dB of DC gain: most probably not.
May I add a note please that he wants to work as a Butterworth filter.
Any idea on how much should be the open loop gain for the OPAMPS for this topology? Will a single stage folded cascode with 60dB gain be sufficient?
What I can read from first post in this thread that you need 0dB filter.
So be aware that Transfer function of Sallen-Key depends on gain of the opamp (H= G/ (second order filter equation)). so you should work with gain between 1 and 10 (according to your specs).
Regards
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