Active low pass 2nd order sallen key Butterworth and Chebyshev filters: PLEASE HELP
Hi there guys. I am really stuck and need some major help. As part of something i have to do, one of the things i have to do is to design two filters.
Both have to be second order sallen key active low pass filters.
the first filter should have a Butterworth response and the second a chebyshev response. Both have to have a gain of 4 and both also have to have a cut off frequency of 1.5kHz. A capacitance of 0.1uF(100nF) is given. I have included a diagram at the end of this post that shows my filter setup. USING lm741
I first calculated the values and got the following:
Butterworth filter:
c1=100nF;
c2=200nf;
R1= 549.3 ohms
r2=1024.6 ohms
Rf= 6296 ohms
Rg=2098.6 ohms
I simulated above filter using TINA simulation packet and the bode plots were as expected. the gain of the circuit was equal to 4 and the cut-off freq was approximately 1.5kHz
CHEBYSHEV FILTER
c1=100nF;
c2=200nf;
R1= 760.66 ohms
r2=898.66 ohms
Rf= 6639.33 ohms
Rg=2213.33 ohms
I simulated above filter using TINA simulation packet and the bode plots were as expected. the gain of the circuit was equal to 4 and the cut-off freq was approximately 1.5kHz
THE PROBLEM STARTS HERE:
I then decided to build the circuit and test it. Because of component availibilty I used the following components. Using these components the circuit according to calculations is meant to give me a cut off frequency of 1.43Khz and retain the gain of 4.
Butterworth filter:
c1=100nF;
c2=200nf;
R1= 560ohms
r2=1 Kohms
Rf= 6296 ohms
Rg=2099 ohms
CHEBYSHEV FILTER
c1=100nF;
c2=180nf;
R1= 760 ohms
r2=898 ohms
Rf= 6639 ohms
Rg=2213 ohms
Both filters when tested have almost the same gain of 4 which is perfect. The problem comes with the cutt off frequency. They both almost have the same cut off frequency values, which is good. But the cut off frequency is sitting at somewhere between 30 and 40 Khz instead of at 1.43 Khz. Why????????/ Help me please. i have no idea what to do now. I was just using a sinewave of amplitude 1V as a input signal and the output is 4V till about 30Khz where it starts to drop.
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Both have to be second order sallen key active low pass filters.
the first filter should have a Butterworth response and the second a chebyshev response. Both have to have a gain of 4 and both also have to have a cut off frequency of 1.5kHz. A capacitance of 0.1uF(100nF) is given. I have included a diagram at the end of this post that shows my filter setup. USING lm741
I first calculated the values and got the following:
Butterworth filter:
c1=100nF;
c2=200nf;
R1= 549.3 ohms
r2=1024.6 ohms
Rf= 6296 ohms
Rg=2098.6 ohms
I simulated above filter using TINA simulation packet and the bode plots were as expected. the gain of the circuit was equal to 4 and the cut-off freq was approximately 1.5kHz
CHEBYSHEV FILTER
c1=100nF;
c2=200nf;
R1= 760.66 ohms
r2=898.66 ohms
Rf= 6639.33 ohms
Rg=2213.33 ohms
I simulated above filter using TINA simulation packet and the bode plots were as expected. the gain of the circuit was equal to 4 and the cut-off freq was approximately 1.5kHz
THE PROBLEM STARTS HERE:
I then decided to build the circuit and test it. Because of component availibilty I used the following components. Using these components the circuit according to calculations is meant to give me a cut off frequency of 1.43Khz and retain the gain of 4.
Butterworth filter:
c1=100nF;
c2=200nf;
R1= 560ohms
r2=1 Kohms
Rf= 6296 ohms
Rg=2099 ohms
CHEBYSHEV FILTER
c1=100nF;
c2=180nf;
R1= 760 ohms
r2=898 ohms
Rf= 6639 ohms
Rg=2213 ohms
Both filters when tested have almost the same gain of 4 which is perfect. The problem comes with the cutt off frequency. They both almost have the same cut off frequency values, which is good. But the cut off frequency is sitting at somewhere between 30 and 40 Khz instead of at 1.43 Khz. Why????????/ Help me please. i have no idea what to do now. I was just using a sinewave of amplitude 1V as a input signal and the output is 4V till about 30Khz where it starts to drop.
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