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!!

    THANK U IN ADVANCE!! :)
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,138
    1,786
    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
    12,100
    3,034
    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
    10,138
    1,786
    @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.
     
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