Active Filters (how are they different from passive filters)

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waqasai

New Member
Hello,
I am having a hard time in understanding the use of Op AMps in filtering circuits, I read that it is useful because it can provide filtering of very low frequencies but i cant understanding how is it done, and we are using the same capacitor in Op Amp filter (Active filter) as well as in passive filters so how is Op Amp making it special?

MikeMl

Well-Known Member
Do you understand the concept of a mathematical transfer function for a circuit? There are active filters (consisting of resistors, capacitors, opamps) that have identical transfer functions as filters implemented using only inductors, capacitors and resistors. It is all about the math.

Read this to see what active filters can do.

MrAl

Well-Known Member
Hello,
I am having a hard time in understanding the use of Op AMps in filtering circuits, I read that it is useful because it can provide filtering of very low frequencies but i cant understanding how is it done, and we are using the same capacitor in Op Amp filter (Active filter) as well as in passive filters so how is Op Amp making it special?
Hi,

Op amps have a large internal gain, and that gain is used to enhance various types of circuit including filters. A passive filter has no active gain while an op amp filter does so it can enhance the filtering action by making it either sharper or better in some other way. The output impedance can be lower too so that may be just enough advantage for some circuit to make it a good idea to use an op amp rather than strictly passive elements.

crutschow

Well-Known Member
You can use both passive and active filters to achieve the same filter function.

The problem with passive filters at low frequencies, is that the required inductor sizes become large and expensive. Also such large inductors tend to have less than ideal characteristics which reduces the performance of the filter. Thus such filters are usually only used at low frequencies for high power requirements, such as speaker crossover networks, where an active filter cannot be used.

Active filters use only capacitors and resistors with an amplifier. The amp allowing you to achieve the same filter transfer functions as you would get with inductors, but doesn't require any. The capacitors used for low frequency active filters are inexpensive and have near ideal characteristics, so the filter performance can be close to that predicted by the filter transfer function.

Gary B

New Member
I think the easiest way to understand it is that an Op amp can invert a signal so if you connect a capacitor between the output of an Op amp and its inverting input, you have something that acts like an inductor which is the inverse of a capacitor (XL=1/XC). Now, if you connect another capacitor and a resister to that same inverting point you have something that acts like an inductor connected to a capacitor and resister which is exactly what a passive filter is.

crutschow

Well-Known Member
I think the easiest way to understand it is that an Op amp can invert a signal so if you connect a capacitor between the output of an Op amp and its inverting input, you have something that acts like an inductor which is the inverse of a capacitor (XL=1/XC). Now, if you connect another capacitor and a resister to that same inverting point you have something that acts like an inductor connected to a capacitor and resister which is exactly what a passive filter is.
Not true.

The addition of gain can allow the generation of an active filter transfer function that mimics the operation of a filter with inductors, but signal inversion has nothing to do with that.

You can build an active filter with an amp that has a gain of +1 (see the Salen-Key configuration).

A capacitor from the output to the inverting input input of an op amp generates the integrate function, not an inductor function.

But there is an op amp circuit that does simulate the operation of an inductor. It's called a Gyrator.

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