Many synth circuits require matched pairs of transistors... often for differential amplifiers.
There are lots of transistor matching circuits and devices out there.
Many ways...... but the question is which to use. ???
Vbe (base-emitter voltage) matching is the most common type of matching done. It is what is used for matching transistors in an exponential converter for example. It can be done with a circuit like Ian Fritz's, or can be roughly done using the diode setting on a volt meter (Ian's circuit gives a more accurate result).
I like using a Atlas DCA Pro DCA75
Why choose to measure Vbe ?
Ie = Emitter current
Ies = the reverse saturation current of the base emitter diode.
Vbe = the Base-emitter voltage
Vt = thermal voltage
Ies depends on how the transistor is made and can be different from one transistor to the next.
It also changes with temperature.
The goal of transistor matching is to find pairs or transistors with the same Ies.
It's very difficult to measure Ies, but we can instead measure the Vbe if we pass a known current through the transistor. If we can keep the temperature constant, then matching Vbe is equal to matching Ies.
So the solution (if you are to build a circuit) is to have 3 things:
1. an accurate current source.
2. an accurate multimeter.
3. a constant temperature.
Whether you are using a circuit or the Atlas peak, remember that the temperature of the transistor will effect the measurements. I like to leave the transistors alone for about 5 minutes after handling them
and them carry out the measurements.
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The TTSH build required matched transistors in the filter section.
I matched them via hFE ... not by the usual the Vbe (base-emitter voltage).
I'm hoping I'll get away with it for that instance. However, the TTSH VCA section requires more matched trannies. So it's time I guess to bite the bullet.
Moog has a classic transistor checker schematic (from the mini-moog manual).
Here one current drives another current through the transistors and you need to measure the voltages between the base & emitter. It calls for a +/- 10V supply and needs a really accurate volt meter.
There is a variation of this circuit on DragonflyAlley.com
It uses a +/-15V supply, a 741 op-amp & 3 resistors.
The Op-amp is a 741. (Pin 6 of the 741 connects to the base of the 3904)
It seems to work though there is lots of voltage fluctuation
Very temp sensitive.
Very temp sensitive.
Ian Fritz also has a transistor matching circuit.
You can download it here.
This is a breadboard of the Fritz transistor matcher.
The crossed resistors need to be matched as closely as possible. I guess a variable trim-pot could also be used to get a closer matching between the resistors.
The crossed resistors need to be matched as closely as possible. I guess a variable trim-pot could also be used to get a closer matching between the resistors.
I kinda like it more than the Moog circuit. It's definitely much simpler using just 3 resistors & a diode.
This particular batch of 3906s seem to vary by 0.5 to 4 millivolts.
For completeness here are a few more links to blogs, websites which discuss transistor matching.
Hopefully you will find this useful.
Muffs has a cool thread for DIY Transistor matching if you finally wish to go down this path..
matching transistors - DIY
The famous minimoog tester of Dr. Robert Moog.
MFOS has a great page on how to build transistor matchers.
(MFOS Practical Transistor Matching)
I might try this MFOS matcher later if the earlier attempts aren't successful.
Links
+ Transistor - Basic info
+ JFETS - matching for synthesizers
+ Muffs
+YATMA Yet Another Transistor Matcher
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