2N4955 operation characteristics? - UK Vintage Radio Repair and Restoration Discussion Forum

mole42uk · 2nd Jul 2015, 4:13 pm

I'm working with a lin/log converter which uses a 2N4955 or BFX10 dual NPN transistor. The two are connected as seperate diodes (b shorted to c) and I wonder if the only reason for using a dual transistor is for temperature matching.

If this is the case, could I expect a similar performance by strapping two ordinary transistors together with heat transfer paste between them?

Or could I just use a couple of diodes similarly strapped together?

Herald1360 · 2nd Jul 2015, 4:45 pm

I suspect other parameters might also be better matched in a dual device than in two singles.

If you played around to get best VF match in a pair over a range of currents, strapping a pair thermally might not be too bad.

Radio Wrangler · 2nd Jul 2015, 4:47 pm

The match is in temperature, diffusion temperatures and times, all sorts of things. If dual bipolars are hard to find, the much better monolithic 'supermatch' pairs from Analog devices and natsemi may be easier to find

Also analog devices do some good log/antilog converters.

David

julie_m · 2nd Jul 2015, 6:42 pm

There is also the CA3046, which is a 14-pin package with five matched transistors in thermal contact, two of which have their emitters joined together. It's obviously meant for building a custom op-amp, as another emitter is joined to the substrate and must be more negative than any other pin; those three would form the input, with the commoned-up transistors as the differential amp and the bottom one as a constant-current sink for the long tail, and the remaining two either as two single-ended stages or a further differential stage. But there's no reason to connect all five transistors, as long as pin 13 is tied negative (otherwise, weird effects can occur) .....

MotorBikeLes · 2nd Jul 2015, 9:54 pm

I junked an old HP dual timebase, and that had quite a few 6 leg dual NPNs (and probably some PNPs as well).
I could dig a couple out if you are stuck.
Les.

Radio Wrangler · 3rd Jul 2015, 8:15 am

The common HP-used NPN dual transistor was 1854-0221 usually marked 4-221 individual transistor behaviour was close to 2N3904, but I can't remember the common PNP dual, 1853-0075? these are what you'll find later on in the scopes. They used matched but separate JFETS mounted in S-clips at the front-end of many.

David

MotorBikeLes · 4th Jul 2015, 8:09 pm

David, I found these 6 leg duals in my junk board. (1821 T/B, maybe also a Y-amp from same mainframe.
3-075, presumably pnp dual
4-280, presumably npn dual
4-295, presumably npn dual
4-357, presumably npn dual
I have an old copy of the HP / jedec equivalents, but these appear to be studiously ignored.
Do you know the individual equivalent single versions?
Les.

2nd Jul 2015, 4:13 pm	#1
mole42uk Nonode Join Date: Aug 2010 Location: Resolven, Wales; and Bristol, England Posts: 2,608	2N4955 operation characteristics? I'm working with a lin/log converter which uses a 2N4955 or BFX10 dual NPN transistor. The two are connected as seperate diodes (b shorted to c) and I wonder if the only reason for using a dual transistor is for temperature matching. If this is the case, could I expect a similar performance by strapping two ordinary transistors together with heat transfer paste between them? Or could I just use a couple of diodes similarly strapped together? __________________ Richard Index: recursive loop: see recursive loop

2nd Jul 2015, 4:45 pm	#2
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: 2N4955 operation characteristics? I suspect other parameters might also be better matched in a dual device than in two singles. If you played around to get best VF match in a pair over a range of currents, strapping a pair thermally might not be too bad. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

2nd Jul 2015, 4:47 pm	#3
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,876	Re: 2N4955 operation characteristics? The match is in temperature, diffusion temperatures and times, all sorts of things. If dual bipolars are hard to find, the much better monolithic 'supermatch' pairs from Analog devices and natsemi may be easier to find Also analog devices do some good log/antilog converters. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

2nd Jul 2015, 6:42 pm	#4
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: 2N4955 operation characteristics? There is also the CA3046, which is a 14-pin package with five matched transistors in thermal contact, two of which have their emitters joined together. It's obviously meant for building a custom op-amp, as another emitter is joined to the substrate and must be more negative than any other pin; those three would form the input, with the commoned-up transistors as the differential amp and the bottom one as a constant-current sink for the long tail, and the remaining two either as two single-ended stages or a further differential stage. But there's no reason to connect all five transistors, as long as pin 13 is tied negative (otherwise, weird effects can occur) ..... __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

2nd Jul 2015, 9:54 pm	#5
MotorBikeLes Nonode Join Date: Dec 2012 Location: Kirk Michael, Isle of Man Posts: 2,350	Re: 2N4955 operation characteristics? I junked an old HP dual timebase, and that had quite a few 6 leg dual NPNs (and probably some PNPs as well). I could dig a couple out if you are stuck. Les.

3rd Jul 2015, 8:15 am	#6
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,876	Re: 2N4955 operation characteristics? The common HP-used NPN dual transistor was 1854-0221 usually marked 4-221 individual transistor behaviour was close to 2N3904, but I can't remember the common PNP dual, 1853-0075? these are what you'll find later on in the scopes. They used matched but separate JFETS mounted in S-clips at the front-end of many. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

4th Jul 2015, 8:09 pm	#7
MotorBikeLes Nonode Join Date: Dec 2012 Location: Kirk Michael, Isle of Man Posts: 2,350	Re: 2N4955 operation characteristics? David, I found these 6 leg duals in my junk board. (1821 T/B, maybe also a Y-amp from same mainframe. 3-075, presumably pnp dual 4-280, presumably npn dual 4-295, presumably npn dual 4-357, presumably npn dual I have an old copy of the HP / jedec equivalents, but these appear to be studiously ignored. Do you know the individual equivalent single versions? Les.