<?=$alt; ?>
<?=$alt; ?>

Transistor Matching

Buy the PCB:

Transistor Matching PCB
Transistor Matching PCB
Price: $10.00

FUNCTION:

This is a Moog™ circuit used to match the characteristics of two transistors for use where such a pair is required. VCO core comes to mind.

USE:

In the course of building a synthesizer, a pair of transistors with similar electrical characteristics is often required to help stabilize temperature fluctuations, such as in a VCO core. Some use them in other spots, but they are not necessarily needed, but it also doesn’t hurt. In the Synthasystem, the only place such a pair is required is in the VCO core, although Nyle confessed to me that in later production, they stopped matching them. I do, however, have a very vivid memory of matching transistors while working for Nyle, and a special drawer with matched pairs taped together.

In the resurrection of the Synthasystem, there are a lot of “paired” transistors in various modules, notably the VCO and VCF. I chose for no real reason, to use a footprint compatible with most monolithic paired transistors but which will also work with discrete transistors. In the Steiner VCO core, you also have to have a PNP transistor in thermal contact with the matched NPN pair. This PNP does not have to match the NPNs. I don’t really know a good way to match a PNP to a NPN anyway.

To use this PCB, you need a volt meter with mili-volt precision and accuracy or better. I recommend you solder some wires to the “transistor to match” locations on the PCB with a small piece of solid wire, or other pin on the free end of each wire. There are some mini patch cables made for bread boards that are great for this as they already have straight solid pins on each end. cut them in half and solder one end to the PCB. Moog™ has some for their Werkstatt synthesizer, Adafruit has them, too.

Next, line up a bunch of transistors on a bread board and let them stabilize in temperature. Take the three leads you just soldered and connect the free end to the breadboard where the first transistor is and measure the voltage at the output. Note the voltage and go to the next transistor. When you have two transistors with a voltage within 2mV, you have a good pair.

Here is the original Moog™ schematic with the unsurprising error of a missing resistor between the two transistors on the PNP side, sigh. You need a 10K resistor on the PNP matching circuit comparable to the NPN circuit; between the two transisors.

Transistor matching Schematic

Inputs/Outputs:

The PCB has power input, +/- 12 or 15 volts. It has two output connectors, one for the NPN and one for the PNP voltage. Put something here that will make it easy to connect a volt meter.

GENERAL CONNECTIONS:

COMPONENT SELECTION:

See the Component Notes page for more information.

ICs:

Any decent OpAmp is fine.

Transistors:

Any small signal transistors can be used.

BUILD NOTES:

Soldering:

I assume you know the basics of soldering. I like to insert the low lying parts first, like resistors, diodes, etc. After these, I install the IC sockets. Next capacitors, transistors, connectors. Use a good solder, either an organic flux, which you should wash regularly, or a no-wash flux.

Take a break every so often, wash off the flux if you are using a flux which required cleaning. Double and triple check orientations, pins, and solder joints.

Power Supply Regulation/Filtering:

Any balanced power supply can be used, +/-12 to +/-15 volts.

COMPONENT PLACEMENT:

Mounting:

There are four holes which are sized for the snap in end of these spacers, Eagle Products #561-MTP250. These don’t seem to be available at Mouser anymore, but any spacer with snap in on the end should work.