# LPH and HPF

Discussion in 'Homework Help' started by omerysmi, May 23, 2015.

1. ### omerysmi Thread Starter Member

Oct 10, 2014
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0
Hi, I have a question about LPF and HPF in RC circuit.
I know that in LPF when τ>>t the capacitor is close to be a short.
but what happens in HPF when τ>>t ? that capacitor is close to be a short also?

2. ### DickCappels Moderator

Aug 21, 2008
2,664
634
After T seconds very little current would be flowing through the capacitor, so it could be generalized as being open circuit.

3. ### omerysmi Thread Starter Member

Oct 10, 2014
44
0
But in HPF when τ>>t the output signal is very similar to the input signal so it means that the capacitor is short and the input voltage pass to the output no?

4. ### MrAl Well-Known Member

Jun 17, 2014
2,440
492
Hi,

In the HPF, for very short times the output equals the input so the cap looks more like a short.
For very long times the output goes to zero so the cap looks like an open.

Looking at it from a frequency perspective, for high frequencies the cap looks like a short, and for low frequencies the cap looks open.

Since time is the inverse of frequency:
T=1/f

high frequency is similar to a short time period, where the period is relative to the RC time constant tau.