Open OTA
Created April 28, 2013

April 28, 2013

    This is a project I think has been needed for a while.  The OTA is probably the single most important circuit module needed to make Analog Music Synthesizers.  And it is fast becoming nearly impossible to find.  The last high quality OTA disappeared back in 2005 (the CA3280).  It is still possible to get the LM13700 and the NE5517 and a few others, but these are just not very good..  And even so, the quality of an IC is never as good as discrete as far as noise levels go anyway, but the convenience of an IC has always outweighed any of the problems that an IC would have.  Plus, it is a lot easier to get well matched and thermally tracking components on an IC which is pretty difficult to do with discrete transistors.

This is the first version of the OTA I am going to do.  It is basically just a CA3080 with a linearizing diode.  This schematic was drawn with KiCad, which is a totally opensource EDA program.  This way, the user will be able to modify  it to his hearts content.  All work on this project I will make the greatest effort to use only open software.
So, exactly what are the issues here?  It is a very simple circuit.  Should be a breeze to build.  And it is.  But getting the performance one wants from this circuit is not so easy.  My initial version is going to be built from 2n3904 and 2n3906 transistors in TO-92 packages.  Pairs of transistors are going to need to be matched and how well they are matched is going to determine just how good it will work.  The biggest problem is having Iabc feed through to the output as Iabc changes.  Even some of the IC versions had a lot of trouble in this arena.  So most of the exploration is going to be finding out what is the best way to match the transistor pairs.  Not all of the transistors have to be matched.  The current mirrors are Wilson current mirrors and the transistor that is odd man out does not have to be matched. to the other two.  Still, the circuit is going to require 8 pairs of matched transistors.  And that is going to be a lot of work for just one OTA.

    Her is a link to the schematic file: Open OTA schematic

Creative Commons License
Open OTA by Jim Patchell is licensed under a Creative Commons Attribution 3.0 Unported License.