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