Input Impedance: Low-Pass vs High-Pass Filter

  • Thread starter Thread starter fizziks
  • Start date Start date
  • Tags Tags
    Input
Click For Summary

Discussion Overview

The discussion centers on the input impedance characteristics of low-pass and high-pass filters, specifically comparing their behavior at very high and very low frequencies. The scope includes theoretical considerations related to simple RC filter configurations.

Discussion Character

  • Technical explanation, Conceptual clarification

Main Points Raised

  • One participant questions whether the input impedances for low-pass and high-pass filters are the same at very high and very low frequencies.
  • Another participant asserts that for simple series RC low-pass and high-pass filters, the input impedance is identical across frequencies, provided the input impedance of the next stage is significantly larger than the resistor value.
  • This same participant notes that if the next stage loads the first filter stage, the input impedances of the low-pass and high-pass filters would generally differ.
  • A later reply emphasizes the importance of considering single-stage versus multi-stage filters, indicating that multi-stage filters require further analysis of their input impedance frequency response.

Areas of Agreement / Disagreement

Participants appear to agree on the behavior of simple single-stage RC filters but acknowledge that the situation may change with multi-stage filters. There is no consensus on the implications of loading effects from subsequent stages.

Contextual Notes

Limitations include the assumption that the next stage's input impedance is very large compared to the resistor value, which may not hold in all scenarios. The discussion does not resolve the complexities introduced by multi-stage filters.

fizziks
Messages
239
Reaction score
1
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.
 
Engineering news on Phys.org
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.
 
Just what I thought it was. Thank you very much.
 
fizziks said:
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.
 

Similar threads

  • · Replies 4 ·
Replies
4
Views
3K
  • · Replies 6 ·
Replies
6
Views
3K
Replies
10
Views
3K
  • · Replies 14 ·
Replies
14
Views
4K
  • · Replies 13 ·
Replies
13
Views
4K
  • · Replies 11 ·
Replies
11
Views
2K
Replies
26
Views
7K
  • · Replies 15 ·
Replies
15
Views
2K
  • · Replies 6 ·
Replies
6
Views
2K
Replies
8
Views
2K