[sdiy] Transistor Matching (was moog high pass flter)

[Grant Richter]

There is a really great chip from Burr-Brown (now TI) called the REF200. It
is two floating 100 uA precision constant current sources and a current
mirror in one package:

http://focus.ti.com/docs/prod/folders/print/ref200.html

While not cheap, this is a WAY good chip for building a transistor matching
rig. The Moog schematic is just a constant current source/sink (for PNP/NPN)
and you put a volt meter across Vbe. You can match transistors at any
current. Some people use 1 mA, I've had good luck with 100 uA which is a
decade below max current into a CA3080 pin 5.

If you put the 100 uA current source inside a full wave rectifier, you have
a bi-directional current source (see the REF200 datasheet). You can take a
transistor socket with a BDCS and a DPDT switch and match PNP or NPN in the
same socket with the same current source.

I'll try to find time to draw a schematic. It's just the same as the Moog
circuit only using the REF200 instead of the op-amp controlled transistor.

Get a large sheet of conductive foam, stick paper labels over it with mv
marked from 560 to 640 (depending on room temperature). Use plastic tweezers
to handle the transistors and sort 100 each PNP and NPN into the correct
bins at one sitting. Then just grab two from the same bin and they are
matched.

The REF200 is just great to have around. For instance, a REF200, 555 and a
cap makes a nice fixed frequency linear ramp oscillator (for a dither
oscillator).


> From: Tim Ressel <madhun2001 at yahoo.com>
> Date: Mon, 9 Dec 2002 17:02:54 -0800 (PST)
> To: synth-diy at dropmix.xs4all.nl
> Subject: [sdiy] Transistor Matching (was moog high pass flter)
> 
> Yo,
> 
> Question: How close does the trannies have to be
> matched for say a ladder filter? is 6mV close enough?
> 
> By the by, I breadboarded, for lack of a better name,
> a differential transistor matcher. I used a CA3096 to
> make both an NPN and PNP matcher. The idea is this:
> you can run two transistors at the same time, and tie
> them together to keep them at the same temperature.
> Also the voltmeter now reads the difference directly.
> Seems to work pretty well.
> 
> --tr
> 
> 
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