How does an opamp make an active filter so great?

Question

I want to understand active filters and why op-amps make them function so well.

I gave up on trying to derive a transfer function for a four pole butterworth filter. Can you describe how it works or link to a website that derives a transfer function for an active filter? Thanks

making the filter is not a problem for me, I want to understand why the active filter can have such a steeper transfer function than a passive filiter

Answer 1

"making the filter is not a problem for me, I want to understand why the active filter can have such a steeper transfer function than a passive filiter"

Amplification of the inband part of the signal while attenuating the outband part. The combination can produce an impressive result. Passive components can only perform the latter, so your inband/outband ratio just isn't as good (less Q).

Answer 2

The op amp lets you synthesize active components like L and C that are too extreme to be real. Huge inductances and capacitances, and thus make a filter that would not be at all practical to build using actual catalog devices. I personally can't do the synthesis, but rely on a handbook to come up with the designs. It is the "Manual of Active filter Design", by Hilburn and Johnson, McGraw-Hill. A cookbook. All diagrams and charts, no heavy math. Butterworths and Chebyshev. For the designer that just needs a design that works and isn't going to get homework points for synthesis, it is a good book and its designs have worked for me. I recommend it.

Answer 3

Low pass filters are not capacitively coupled, therefore they pass DC as well as AC up to the cutoff frequency (where the half-power point is reached, or 3dB down from the set gain). You can tell the gain of the filter by putting in a fixed DC voltage and look at the output voltage. This should be the gain for all frequencies up to the cutoff frequency. A high pass filter is capacitively coupled, so it will not pass DC, as the capacitor blocks the DC.

Answer 4

hit the library and find the latest ARRL handbook. plus there are about 5 books i have on filter design using opamps. you can combine slewrates also. <500 Hz try 741's ,above use higher slewrates. CA3140 , etc. , use a bipolar supply for high CMRR. makes a difference. also copius quantities of coffee help.,,,also a pair of VR's is a good idea, the little 100 mA type.... +/- 15V will be fine. max Eapp i think is 35-40 V

Answer 5

Here's a great little application from Analog Devices:

http://www.analog.com/Wizard/filter/filterUserEntry/

It used to be a downloadable app. but they changed it to a web-based app. recently.

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