Do active filters even care about input/output impedances?

Hi All,

I've been looking at a lot of active filter design software and design formulas, and none seem to care too much about the input and output impedances that the filter will want to "see" when placed in a circuit. When designing *passive* filters, we would typically specify these impedance values as a matter of course (normally 50 ohms). Why don't active filter design programs even ask what the input and output impedances are that the filter will have to work with, nor state what it is after the circuit is synthesized? Does it even matter; or will the frequency response and gain just not be affected by most normal values? Or is it assumed that the active filter will be placed between certain impedance values? If not, then how can I tell what the "optimal" impedance values should be for an active filter? This has me baffled!

Thanks,

-Bill

Reply to
billcalley
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Thanks Bob and Ban for the great explanations! Now I understand much more about what active filters like to see impedance wise.

Best Regards,

-Bill

Reply to
billcalley

You have to consider that active filters are usually used for *low* frequencies from DC to maybe 10MHz, where the wavelength is much longer than the mechanical dimensions of the components. We do not have reflections and the capacitors are much bigger than the parasitic circuit capacitances. That is why it's possible to have relativly high impedances involved, in order to decrease capacitor size. Active filters do not use inductors (coils) as do passive filters. It is also a convention that these filters are used with 0 ohms driving and >10k loading impedances. The low O/P impedance of an opamp stage will not affect freq. response or gain, because usually the values are in the 3k3 to

100k region, but for a precision measurement device real values have to be plugged in. Many programs not only neglect the in/out impedances, but are based on an ideal opamp model, with infinite gain and node impedances. Though a good simulation program like spice can take all these into account.
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ciao Ban
Bordighera, Italy
Reply to
Ban

Most active filters are designed to be driven by constant voltage meaning nearly zero impedance. This is the case when they are driven from an op-amp in the circuit before the filter. It's output impedance is nearly zero ohms. If there is much resistance or impedance in the driving circuit, it will affect the response of the active filter and must be included in the response calculations. Think of it this way: Suppose the input resistor on a certain filter is 10K. Now suppose you drive it with a 5K source, the real input resistance is now 15K not 10k of the original design and it affects the response. Driving from the low impedance output of an op-amp alleviates this issue.

The output side of active filters is usually the output of an op-amp and, therefore, provides nearly zero impedance to the following circuits. In other words the active amplifiers "buffer" the circuits from one another.

Passive filters usually do not have amplifier buffering and cannot be designed for near zero impedance. Therefore, they are designed for a specific impedance, say 50 ohms. But, because this is not zero ohms it must be included in the filter's calculations. Bob

Reply to
Bob Eldred

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