PHYS345 Electricity and Electronics

Answer for P14.6

To get the low-pass response, an integrator response is needed, thus a feedback capacitor and an input resistor. To get a low-frequency gain of thirty, add a feedback resistor in parallel with the capacitor; at low frequencies, XC will get very large and the parallel combination with the feedback resistor will approach Rf as a limit.

Op Amp Low-Pass Filter

The breakpoint of 3 kHz is set by having 2 pi f (RC)f = 1; that is, RfCf = 53 microsec. To get started, try Cf = 0.001 microFarad; the specified breakpoint would then require Rf = 53k. 51k is a standard 5% value, 53.6k is available in 1% tolerance.

Using a 1% 53.6k feedback resistor should have a 1.79k input resistor for a gain of 30. The standard 1% value of 1.78k is a good choice from the table of standard resistor values

Last updated Dec. 7, 1998.
Copyright George Watson, Univ. of Delaware, 1998.