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Active filters

Kevin Weddle

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An active filter adds some positive feedback at the cutoff frequency so the response is flat to the cutoff then a sharp slope.

A multiple-feedback bandpass filter can have any Q you want. A simple highpass filter then a simple highpass filter is a very poor bandpass filter. Both filters attenuate the frequency you want.


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  • 4 weeks later...

An active filter doesn't require positive feedback. Designing simple LR filters and RC filters in a cascade arrangement along with amplifiers might create less harmonics and varying harmonic frequencies.

Cascaded RL or RC filters have a droopy response. An active filter adds positive feedback to make the response sharp at the cutoff frequency. Then the response is flat up to the cutoff frequency then a sharp drop past the cutoff frequency.

The multiple-feedback bandpass filter opamp circuit I posted uses positive feedback to increase the Q of the simple RC filters. You cannot have a high Q without the positive feedback.

Todays opamps have extremely low harmonics that can barely be measured.
All the harmonics together plus noise of an OPA134 opamp is 0.00008% distortion.
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Since an LC filter can only produce 40db/decade attenuation, it can be replaced with a two pole RC or LC filter.

A simple low pass filter will always attenuate a higher frequency. A tuned filter might attenuate a higher or lower frequency within the fundamental frequency bandwidth depending on conditions. Resonance is often avoided in circuit design, but is also chosen to reduce signal loss.

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An RC low-pass filter is -3dB at the cutoff frequency and starts reducing the level at 1/5th its cutoff frequency.
Two buffered RC filters in series are -6dB at the cutoff frequency and start reducing frequencies that are 1/10th the cutoff frequency.

A Butterworth 2nd-order (two poles) active lowpass filter uses positive feedback to boost the cutoff frequency so it is down only -3dB and so that the respose is flat to about 1/4 the cutoff frequency.

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