[sdiy] Tempco, was Transistor help

[Bob Weigel]

Great page Jim.  I've never seen that somehow. I added it to my general 
links on the front page..in fact it inspired me to actually put in such 
a section :-).  One note...DigiKey no longer has CA3280 or E coming up 
on their links.  Not sure why.  Those are still in production as per 
Intersil's page.  I just got some stock from Arrow in case people need 
them while ordering other things or I need them along the repair trail. -Bob

James Patchell wrote:

> http://www.oldcrows.net/%7Epatchell/matcher/matcher.html
>
> At 04:30 PM 4/26/2005 -0700, Harry Bissell Jr wrote:
>
>> 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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>> >
>
>
>         -Jim
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