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
    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.

  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
    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.