What other factors determine jfet opamp cutoff

Discussion in 'Homework Help' started by epcole, May 13, 2010.

  1. epcole

    Thread Starter New Member

    Nov 18, 2009
    8
    0
    I'm working on the circuits in this article: http://grad.uprm.edu/tesis/naranjobueno.pdf
    The filter circuit is on pg. 80 of 156

    I used 1/(2pi*RC) to figure out the component values for the filter circuit. I ran a simulation on LTSpice with the components and the LF356 sub and the results were good. Then I built the circuit using the values and its not giving the same results.

    some information on the wanted functions:

    Low Pass Cutoff = 200KHz
    High Pass Cutoff = 4kHz

    Here is what I'm actually getting:

    1st stage
    High Pass cutoff= 30kHz R=6.8k C= 5.6n

    2nd stage
    High Pass cutoff= 30kHz R=18k C=2.2n

    3rd stage
    95% of voltage is lost across input resistor
    R=8.2k C=100n

    I'm using ceramic capacitors and carbon film resistors. The capacitors I've had for a long time and could be bad or something but the resistors are fairly new.
    Please can anybody tell me what I did wrong? Any suggestions or comments would be very helpful.

    Thanks.
     
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Nothing makes sense here:
    1. Sallen and Key Butterworth filters don't have equal-valued resistors AND capacitors. Here are low pass and high pass calculators.
    2. For your low pass values, R=8.2k, C=100n has a 200Hz cutoff, not 200kHz. As I said, though, use the calculator.
     
Loading...