_________________________________________

                         ACTIVE BANDPASS FILTER
                Dual Opamp Based Active Bandpass Filter

                               Ali Raheem
                           blog@shoryuken.me
               _________________________________________


                           3rd of April 2021


Table of Contents
_________________

Introduction
.. Pin Compatible opamps
Schematic
PCB
.. Professionally Made
Example Usage
Files
Changes
Other Formats





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.

  <./active_bandpass_filter_3D_view.png>


[OSHPark] <https://oshpark.com>

Pin Compatible opamps
~~~~~~~~~~~~~~~~~~~~~

  I think the layout is pretty standard.
  - LM358
  - NE5532
  - TLC27M7


Schematic
=========

  A full PDF for the schematic is attached [below].

  <./active_bandpass_filter_schematic.png>

  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 $\frac{1}{2\pi{}RC}$.


[below] See section Files


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 V_CC 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).

  <./active_bandpass_filter_OSHPark.jpeg>

  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.

  <./active_bandpass_filter_example.jpeg>


Files
=====

  - [KiCad and Gerber Files]
  - [Schematic PDF]
  - [Home Etch PDF]


[KiCad and Gerber Files] <./active_bandpass_filter.zip>

[Schematic PDF] <./active_bandpass_filter_schematic.pdf>

[Home Etch PDF] <./active_bandpass_filter_home_etch.pdf>


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
=============

  - [Web]
  - [Text]
  - [PDF]


[Web] <./index.html>

[Text] <./index.txt>

[PDF] <./index.pdf>