[sdiy] Tempco, was Transistor help

[Harry Bissell Jr]

Really, what you want to do is match for the same
base-emitter voltage drop at some particular test
current that is in the range you will use. Usually
100uA is a good place to start.

Pass 100uA through the base-emitter junction and
measure the voltage. Pair those that are within
1mV or so.

There are schematics online for the test jigs that
Bob Moog etc used to do this. Fess' up if you've
got them on your site.

Higher beta is better for other reasons, but usually
no for matching.

DO NOT touch the transistors you will test, the warmth
of you hands will change the reading VERY much. Use
tweezers or something like that.  I have used the
Pomona "mini hooks" to pick up the part by its leads,
Don't test in sunlight, or drafty place, or near heat
vent.

Once you match transistors, use them in pairs and its
best to tie them together with epoxy, or a copper clip
(hard to find these days) or thermal compound between
them... or I sometimes use thermal compound and a
little piece of heat shrink tubing over them.

If it is for an expo converter, the tempco should be
thermally coupled as well.

I use the 2SC1583 (NPN pair) and 2SA798 (PNP pair) as
alternatives to matching (they ARE matched) but have
the emitters tied to one pin (good for expos, bad for
Moog ladder...)  The 2SC3381 and 2SA1349 are good
pairs that are separatr.  The CA3046 is a five
transistor package in a 14 pin dip, matching and
thermal coupling are quite good.  Cost of the 3046 can
be as low as $.25 in 100pcs. lots  (cheap  !)

Good luck

H^) harry


--- Fiercefish <fiercefish at btinternet.com> wrote:
> Thanks to all who replied, I have tested a bunch of
> the transistors and the hFE varied from about 130 to
> 200, I found about 8 that matched at 166, do you
> reckon this would be ok to use in the soundlab
> VCO's? From what I can gather a matching hFE is
> better than just randomly sticking any pair in but
> not as precise as the traditional method, no?
> 
> Also re the tempco's would these be suitable?
> 
>
http://www.maplin.co.uk/Module.aspx?ModuleNo=2218&TabID=1&source=15&WorldID=&doy=26m4&Cache=#CacheID
> 
> The design calls for 2k PTC but would I be able to
> use the 4.7k above if I changed the circuit a bit?
> Any thoughts welcome!
> 
> BTW I promise to post some pics up when I finally
> finish the Soundlab, it will look pretty cool in a
> nice old hardwood cigar box with an aluminium panel
> and cool wide skirt numbered knobs.
> 
> FF> No virus found in this outgoing message.
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