Input Impedance: Low-Pass vs High-Pass Filter

[fizziks]

Would the input impedances be the same for a low-pass and high-pass filter at very high and very low frequencies?

I know how the input impedance would be for a low-pass filter but not sure about a high-pass.

 

[berkeman]

For the case of simple series RC LPF and HPF, yes, the Zin for each is identical over frequency -- just the series R and C combination, it doesn't matter which comes first and second when they are in series.

This is true as long as the input impedance of the next stage (which taps off at the midpoint between the R and C) is very large compared to the value of the resistor. But if the next stage is loading the first filter stage, then that finite impedance would have to be taken into account, and the input impedance of the LPF and HPF would generally be different.

 

[fizziks]

Just what I thought it was. Thank you very much.

 

[berkeman]

Just what I thought it was. Thank you very much.

You're welcome. Keep in mind that I was talking about single-stage RC filters, though. As soon as you go to multi-stage filters, you'd have to analyze them to see what their Zin(f) plots looked like.