# Homework Help: Op-Amp Bandpass Audio Filter

1. Nov 8, 2012

### hutch3ka

I am attempting to build an Op-Amp circuit in which a sinusoidal input is used. Below is the description:

An active, op-amp bandpass filter circuit for audio frequency inputs. The
implemented filter circuit must provide a maximum gain of 14 dB inside the pass-band and less than 9 dB of gain below and above the critical frequencies of 1000 Hz and 5000 Hz.
Design and implement an LED indicator circuit that uses the output of the bandpass filter circuit as the input. Light a green LED when the frequency of a 1 V peak-to-peak amplitude sinusoidal signal is between 1000 Hz and 5000 Hz. Light a red LED when the frequency of the input signal is outside of this range.

Any help greatly appreciated!

2. Nov 8, 2012

### yungman

Get an Active Filter Cook Book and look up Sallen Key:

http://en.wikipedia.org/wiki/Sallen%E2%80%93Key_topology

What your requirement should be quite easy. As for the gain, put an op-amp gain stage before the filter to get the total gain you want.

Don't worry about all the math, all you have to do is follow the formulas to find the R and C to get the effect. I have the old Active-Filter Cookbook by SAMS, it is easy and it is good.

As for LED, did not give enough info. Is the input of the filter constant amplitude, only the frequency change? If so just use a peak detector and use a comparator to get the LED driving signal. If the input amplitude varies, then it would be difficult.

3. Nov 8, 2012

### dlgoff

Single supply LM124/LM224/LM324/LM2902 Low Power Quad Operational Amplifiers are good for audio.

And other tips can be found here: "www.ti.com/lit/ds/symlink/lm124-n.pdf" [Broken]

Last edited by a moderator: May 6, 2017
4. Nov 8, 2012

### Staff: Mentor

Welcome to the PF.

In the future, please be sure to show your attempt at solving your schoolwork question/project, or you post will likely be deleted. Thanks.