I am currently trying to design a bandpass filter in a simulator for a university assignment to the following criteria:
I have come up with the design seen in the attached image but I am getting an unexpected peak outside of my pass band (See Bode plots). I don't understand why this is happening or how to combat it. It seems linked to the amplification on the low pass filter as the peak gets much bigger when i use the variable resistor to increase the gain to 10dB compared to 0dB.
I have an idea it may have something to do with the op amp i am using. I'm not sure how to select an op amp that will still be effective for the requirements I have but not have this problem. Which parts of the op amp spec should i be looking at?
Whilst i am asking for help I also haven't been able to do the maths for calculating the cut off frequencies for no attenuation in the pass band, i have just used 1KHz and 250KHz as my cut off frequencies. Is it possible to explain the maths to work out the cut off frequencies so i can do that?
If anyone takes the time to read thank you!
- Frequencies below 1KHz and above 250KHz should be attenuated.
- Note that you will need to calculate the cut off frequencies given the pass band (1KHz to 250KHz) should not be attenuated.
- This filter should have roll off of -40dB per decade.
- The filter also needs to have a variable gain for the pass-band, of between 0dB and 10 dB.
- All capacitor and resistor values must be from the E12 E series.
- Expected amplitude of the input voltage is up to 1 volt peak to peak, (you will need positive and negative supply; assume +9, –9 volts and ground are available, and can be connected to your circuit using an appropriate adaptor of your choosing.
I have come up with the design seen in the attached image but I am getting an unexpected peak outside of my pass band (See Bode plots). I don't understand why this is happening or how to combat it. It seems linked to the amplification on the low pass filter as the peak gets much bigger when i use the variable resistor to increase the gain to 10dB compared to 0dB.
I have an idea it may have something to do with the op amp i am using. I'm not sure how to select an op amp that will still be effective for the requirements I have but not have this problem. Which parts of the op amp spec should i be looking at?
Whilst i am asking for help I also haven't been able to do the maths for calculating the cut off frequencies for no attenuation in the pass band, i have just used 1KHz and 250KHz as my cut off frequencies. Is it possible to explain the maths to work out the cut off frequencies so i can do that?
If anyone takes the time to read thank you!
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