I need a wimpy diode, with lots of forward drop at a few mA, to bias up an e-phemt. Thought I'd try a small-geometry NPN transistor.
Looks good. The b-e and b-c junctions look symmetric.
The middle config looks like the classic 60 mV/decade-current, and the others are maybe a little ohmic. We pay 2.4 cents each for these, so I don't mind wasting the transistor action.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
An LED? or is that too much voltage?
George H.
I want about 0.8 volts, to be a little above the gate turn-on voltage of the enhancement phemt. It would be cool to see it glow, but a visible LED would be 1.8 volts or so.
I have used LEDs as the voltage reference for simple current sources, where they are just right.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
When I tested a bunch of mixed unmarked diodes, one dropped 1.35v at 0.86mA. Don't know what it was, perhaps something HV.
Here are the measured Vfs at around 0.86mA. This was using an uncalibrated meter.
540 x3 560 x3 570 595 605 620 625 630 x7 640 x2 660 x2 670 x2 690 735 x2 740 x5 750 850 860 1350!NT
Would one of those Vbe multiplier circuit work? (I don't know why I ask you things like that... I figure you know 100 times more electronics than I do.. the chances I'll have something to add are small.)
But even the blind pig sometimes finds an acorn :^)
George H.
Aren't 2 diodes cheaper?
NT
The right LED at the right current will have a temperature coefficient that almost cancels the Vbe of a transistor.
Plus, it glows in the dark!
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Some HV diodes are actually two in series ( UF4007), and that used to be a common bias component- called a 'stabistor'. Alas, those actually had SPECIFIED limits on forward voltage, not just a worst-case maximum.
An easy solution: a transistor with C-B and B-E bias resistors.
I posted the BCX70 data. It looks perfect, using the b-e junction alone.
I doubt that a transistor with lower Vbe, and a couple of Vbe-multiplier resistors, would be any more accurate.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
I was wondering about that. I suppose in theory you could use 2 diodes and the emitter resistor was also a diode! :)
NT
that's misleading editing. I was talking about use in current sources, not the former application that wanted 0.8v
I think that's equivalent to a current mirror. One diode and one transistor does that, too.
The LED thing is not a mirror; the output current is fairly insensitive to supply voltage. I think one more resistor could null that out, too.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
OK, but that will have a pretty bad tempco, roughly 0.3% per degree C. The LED thing is way better. 2 diodes is OK if accuracy doesn't matter too much.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
without thinking too much about I think 2 diodes will have ~2x2mV/K with the right color LED I think you can find one that is closer to ~2mV/K
-Lasse
The 1N914/1N914A is quite flaccid.
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Vishay makes the TZMC1V0. The datasheet is extremely short but it looks like a wimpy diode.
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But, that (if I read the B-E versus (B+C)-E data correctly) depends on Rbb. It's not a well-controlled parameter. The BCX70 is made by Infineon, Fairchild, NXP, Rohm; it doesn't have to always be the same as you have measured.
Well, we have 2600 in stock, probably off a reel of 3000. I'm just trying to turn on (and off!) an Avago ATF-50189, which isn't terribly picky. I'd expect anybody's BCX70J to be pretty similar; they must all have similar geometry and doping, to meet the high/tight beta specs.
Yeah, that's the link I posted:
That 90 PPM tempco could be all in the emitter resistor. This was good enough, but could certainly be tweaked.
Bizarre. They call it a 0.75 volt zener diode, but it sure looks like a regular PN diode, used forward biased.
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