Biquad Implementation

What is the general trend of using opamp topology to implement biquad-filter? I mean, is two-stage or cascode or folded-cascode opamp being used? How I can reduce the output impedance factor?

Regards
Samiran.

Hi samiran_dam,

The basic component of an active filter in majority,is an operational amplifier.Based on the specs you are given for your filter you must reduce your implementation procedure to the estimation of the opamp specs that will give a robust filter design.So,you can start your biquad design with a 1-pole opamp model and specify the desired gain and unity gain frequency and after having those fundamental values you can choose an appropriate opamp topology for your needs.A rule of thumb is that the opamp should have UGF 10 times the filters passband frequency so that the virtual ground at the inputs of your opamp will be valid due to sufficient opamp gain at that frequency (~20dB approximately).

If you don't mind what is the filter order you will design and under what power supply voltage and technology??

Best Regards,
Jimito13

I will be designing a second-order LP filter with 50 KHz (-3dB) Bandwidth.

The internal topology of the opamp chosen is not important as long as the transit frequency is at least - let's say - 5 MHz.
Thus, you have a bandwidth-to-UGF ratio of 100 which is sufficient in most cases. The output impedance of the opamp plays no major role if the resistors connected at the output are larger by a factor of at least 100 (in practice: some kohms)