[sdiy] Importance of Matched Transistor Pair

[patchell]

    To keep it as simple as posible...

    In general, the matched pairs are used for the exponential converters (they
can also be used for other things, but when it comes to synths, expo converters
are the number one job, followed by ladder filter pairs, also found in the
TB303).  There are quite a few ways that transistors can drift with
temperature, by using a matched pair, all of these drifting parameters track
each other, so if you look at both at the same time, it apears there is no
drift....

    Generally, what needs to be matched in matched pairs is the Vbe of the
transistor.  Another important thing about matched pairs is they must be
thermally connected.  After you match a pair of transistors, you generally
should glue them together (I use epoxy).  However, the best way is to buy
matched transistors.  The most common one availiable is the CA3046 (actually
five matched transistors).  The CA3046 is also the least expensive.  Other
canidates are MAT02, MAT03, MAT04 (4 transistors), THAT200/220/240, some
japanese transistors I don't remember the numbers for (somebody else will come
up with them, last time I tried, I got them wrong).

Alex wrote:

> I'm kind of new at some of this, can anyone tell me what the importance of
> using a matched transistor pair in a synth circuit vs. two individual
> transistors?
>
> I know that the Roland TB-303 uses a matched transistor and there's a synth
> DIY site out of Scandinavia, that sites using a matched transistor pair for
> his mono phonic synth. I want to build a modular synth and then I saw this,
> can anyone provide me with some background on this on why it's used for
> synth design? There are only 3 or 4 matched transistors that NTE provides.

    I have purchased these in the past, they are good parts...

>
>
> Thanks.
>
> Alex

--
 -Jim
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