# Designing 1st order filter with Op-AMP

Discussion in 'Homework Help' started by rsix, Jun 15, 2013.

1. ### rsix Thread Starter New Member

Jun 15, 2013
1
0
Hello people, I´d like to ask for help with solving problem attached in screenshot. Objective is to determine values of R1,R2,C1 and fc.
I really don´t know where to start with solving. I guess I can choose one of parameters and key to designing is probably calculating cutoff frequency (in picture fm), then it would not be problem calculate rest of the parameters. But how to do that? I would be greatful for any kind of hint or advice. I´m sorry for not the best english, its not my native language so pls try to use as simple language as possible.

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2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Do you know voltage gain for inverting amplifier? And form this gain solve for R1 and R2.
Next read Fm frequancy from the graph and solve for C.

3. ### LvW Well-Known Member

Jun 13, 2013
778
105
I rather would suggest NOT to start with the low frequency gain (that means: neglecting C), but instead with the correct transfer function using the gain formula

H(s)=-X2/R1 with X2=R2||1/sC2

Then rewrite the formula to get a "1" in the denominator.
Now you have the so called "normal form" of H(s) and you simplxy can read ´the expressions for the dc (and low frequency) gain as well as the 3-dB frequency (reciprocal of the time constant associated with "s" in the denominator.