# Active Bandpass Filter Dual Opamp Based Active Bandpass Filter

## Introduction

A dual opamp based active bandpass filter, I’m using the LM358. The board uses one half of the LM358 to create a virtual ground and then the other one to amplify between a low pass and high pass RC network. It’s laid out like this to simplify as much as possible design of each stage. The two 100K resistors could be of any equal value, ideally large for power efficiency.

I think I might re-jig the board to use a cascade filter on both sides for sharper cut offs as when using it for a narrow band you end up with reduced gain at the centre frequency.

The last version was made by OSHPark. The cost was $9.10 for 3 boards. ### Pin Compatible opamps I think the layout is pretty standard. • LM358 • NE5532 • TLC27M7 ## Schematic A full PDF for the schematic is attached below. Between the RC filter networks is a non-inverting opamp amplifier with gain, Av = 1 + $$\frac{R_F}{R_G}$$. The RC filters have a -3dB frequency at $$\frac{1}{2\pi{}RC}$$. ## PCB The KiCad and Gerber files are attached below, the layout is simple. The power planes are fairly random with a small capactive effect for the VCC and $$\frac{V_{CC}}{2}$$ supplies. ### Professionally Made I ordered a set from OSHPark, as always the purple and gold combo is absolutely banging.  Manufacturer OSHPark JLCPCB Layers 2 2 Width 45.3mm 45.3mm Height 26.0mm 26.0mm Boards 3 5 Cost$9.10 \$9

OSHPark is a little more expensive (you get 5 boards from JLCPCB for the price of 3 from OSHPark), and the boards form OSHPark come with these nasty sprues that you have to file off (see in figure 3).

That said I am very happy with the quality. I’ll likely order a set of boards from JLCPCB with a few changes, mainly using fullsized resistor footprints (1/4 W) and braking out $$V_{CC}/2$$.

## Example Usage

I’ve used it for a few small purposes, it was easy to solder even with the small resistor spacing. The new version will use full sized resistor spacing.

## Changes

02-05-2021
Breakout $$\frac{V_{CC}}{2}$$
01-05-2021
Included OSHPark examples
08-04-2021
Widen tracks, why so thin???
05-04-2021
Switch back to dual OPAMP with single supply. Ordered from OSHPARK.
04-04-2021
Switch to quad opamp with virtual ground.
03-04-2021
High pass first to act as coupling stage.

## Other Formats

Date: 3rd of April 2021

Email: blog@shoryuken.me

Created: 2021-05-02 Sun 22:37