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

[Tim Ressel]

Hi Grant,

Thanks for the tip. After looking at the datasheet,
I'm wondering how good this part would be. The top of
datasheet claims 0.5% accuracy, but later it clams 1%
precision and 1% matching. It is the matching number
we want. I believe that one can do better with
discrete. With 0.1% resistors I can get 0.2% matching.
And if I has a really good ohmmeter to further match
the resistors, I could get much better numbers.

By the way, do you know how close transistors need to
be? What is a good matching number to shoot for?

--tr

--- Grant Richter <grichter at asapnet.net> wrote:
> 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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> 


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