Do active filters even care about input/output impedances?

Thanks guys -- this has really cleared up my questions on active filters.

Best Regards,

-Bill

The output of an active filter usually comes from the output of an op-amp. This can drive any load impedance within reason. It doesn't change the output significantly. In this respect, it's no different from an op-amp inverting or non-inverting amplifier.

The input needs to be driven by a source impedance that's significantly less than the input impedance of the filter, if it's not to upset the response. The design software may assume zero source impedance i.e. a perfect voltage source. If you have a resistor at the filter input, you can reduce it by the size of the source impedance, since they are effectively in series. In this way the non-zero source impedance is "absorbed" by the input resistor.

You can't learn everything in four years. I think it's better to stick to fundamentals (math, physics, circuit theory, signals+systems, control theory) in school, and learn the practical stuff on the job. Heck, most profs don't know the practical stuff anyhow.

Anybody can learn the practical stuff and be a tech, but to design serious electronics you need some theory.

John

Right on, John. The thing that limits me is theory, since I have a Chemistry degree instead of EE. (Why did I do that?!?!)

Theory takes a lot of time to learn, and few people can do it on their own outside of formal instructional environments.

There are things which just can't be done without it.

I'm back to a designing a new PLL motor control project, so I know.

All active filters require to be driven from a low impedance. This is kinda 'assumed to be known'.

Any active filter using op-amps or even a discrete active gain block will normally have a low output impedance and therefore doesn't care much about the load - within the normal limits of what the op-amp etc can drive.

Graham

Active circuits design assumes that the driving impedance is zero and that the terminating impedance is infinite.

It is up to follow-on circuit designers to include analysis of the effects of the surrounding impedances in which a paricular circuit is embedded.

If you are not capable of doing this then you are not qualified to do the job anyway. Pass it on to somebody who knows what he is doing.

Hi Reg,

most ' kids out of college / university ' aren't taught to do much that's useful or practical these days that you and I would probably have taken for granted.

A sad reflection on the state of education.

Graham

True. I guess. I'm suppose I'm unusual. I started learning theory and practice from about age13 on out of personal interest.

Much of what I see taught gives me little confidence though.

Graham

I agree with you. Another point is that universities are supposed to teach fundamentals, not solely prepare people for work. Preparing people for a particular trade is what trade schools are for.

I got my BSEE fairly recently (2000), and many times I heard students complain that the EE curiculum wasn't practical enough. But the professors (and, I guess, the accrediting folks) place a lot of emphasis on EE fundamentals.

--Mac

You're doing well to have observed this enough to ask the question, OTOH if you really knew your circuits analysis (and had tough EE profs like I did) you'd know how to analyze the circuits and decide what impedances would affect the filter. But there's a way you can redeem yourself...

Read Don Lancaster's "Active Filter Cookbook." (Do Not Pass Go, Do Not Collect $200) All your questions will be answered, and no, he's not paying me to say this. This is NOT an AC circuits analysis or design book, though it does have the formulas and can allow a working engineer to fake a lot of good knowledge in a lot of situations.

-----