# how to reduce the cutoff frequency of a high pass filter?

Discussion in 'The Projects Forum' started by PANKAJ DHINGRA, Jun 27, 2013.

1. ### PANKAJ DHINGRA Thread Starter New Member

Jun 27, 2013
1
0
I m new here and this is my first thread!!

My problem is:

I have made a simple high pass filter (first order) which is just a series combination of a capacitor and a resistor of values 100pf and 35kohm resp.
so, by using the formula of cutoff frequency fc = 1/(2*pie*r*c), i found that the cutoff frequency of my circuit is 45.5khz (approx). Now, I want to reduce this cutoff frequency without changing the values of R and C. I want to know that how can we do that with the help of transistors or opamps.

P.S. : I don't want to connect capacitors and resistors in parallel to these C and R of my high pass filter .......REPLY ASAP!!

2. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
You cannot construct a passive filter with a different cutoff frequency without changing R or C.

You can make an active filter with opamps and R's and C's but it will not have the same topology as the simple passive single pole filter. A common implementation is called Sallen-Key

http://lmgtfy.com/?q=Sallen-Key

All you ever wanted to know about filters is in van Valkenberg, Analog Filter Design

3. ### LDC3 Active Member

Apr 27, 2013
920
160
Without changing the capacitance or the resistance, the cutoff frequency will remain the same. Transistors and op-amps are designed to work with even higher frequencies, so they will not decrease the cutoff frequency.
BTW, if you want an audio signal, you would want a low-pass filter with a cutoff at 45.5kHz. A lot of people cannot hear tones above 35kHz.

4. ### wayneh Expert

Sep 9, 2010
12,391
3,246
If by "a lot of" you mean 100%, yes. Few humans and no old ones can hear up to 20kHz.

5. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
@OP - where did you get the quaint notion that you could actually do what you asked about -- changing the frequency without changing the values? That is equivalent to saying the original formula is wrong or the answer is not unique. Mathematics just doesn't work that way.