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wimpy diode

Started by John Larkin December 7, 2015
On Monday, 7 December 2015 21:26:45 UTC, whit3rd  wrote:
> On Monday, December 7, 2015 at 12:49:23 PM UTC-8, nt wrote: > > On Monday, 7 December 2015 20:33:47 UTC, John Larkin wrote: > > > On Mon, 7 Dec 2015 12:04:08 -0800 (PST), George Herold > > > <gherold@teachspin.com> wrote:
> > > >An LED? or is that too much voltage? > > > > I want about 0.8 volts > > > Aren't 2 diodes cheaper?
that's misleading editing. I was talking about use in current sources, not the former application that wanted 0.8v
On Mon, 7 Dec 2015 16:50:34 -0800 (PST), tabbypurr@gmail.com wrote:

>On Monday, 7 December 2015 21:09:04 UTC, John Larkin wrote: >> On Mon, 7 Dec 2015 12:49:17 -0800 (PST), nt wrote: >> >On Monday, 7 December 2015 20:33:47 UTC, John Larkin wrote: >> >> On Mon, 7 Dec 2015 12:04:08 -0800 (PST), George Herold > >> >> >An LED? or is that too much voltage? > >> >> 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. >> > >> >Aren't 2 diodes cheaper? > >> The right LED at the right current will have a temperature coefficient >> that almost cancels the Vbe of a transistor. > >I was wondering about that. I suppose in theory you could use 2 diodes and the emitter resistor was also a diode! :) > > >NT
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
On Mon, 7 Dec 2015 16:52:20 -0800 (PST), tabbypurr@gmail.com wrote:

>On Monday, 7 December 2015 21:26:45 UTC, whit3rd wrote: >> On Monday, December 7, 2015 at 12:49:23 PM UTC-8, nt wrote: >> > On Monday, 7 December 2015 20:33:47 UTC, John Larkin wrote: >> > > On Mon, 7 Dec 2015 12:04:08 -0800 (PST), George Herold >> > > <gherold@teachspin.com> wrote: > >> > > >An LED? or is that too much voltage? >> >> > > I want about 0.8 volts >> >> > Aren't 2 diodes cheaper? > >that's misleading editing. I was talking about use in current sources, not the former application that wanted 0.8v
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
Den tirsdag den 8. december 2015 kl. 01.52.25 UTC+1 skrev tabb...@gmail.com:
> On Monday, 7 December 2015 21:26:45 UTC, whit3rd wrote: > > On Monday, December 7, 2015 at 12:49:23 PM UTC-8, nt wrote: > > > On Monday, 7 December 2015 20:33:47 UTC, John Larkin wrote: > > > > On Mon, 7 Dec 2015 12:04:08 -0800 (PST), George Herold > > > > <gherold@teachspin.com> wrote: > > > > > >An LED? or is that too much voltage? > > > > > > I want about 0.8 volts > > > > > Aren't 2 diodes cheaper? > > that's misleading editing. I was talking about use in current sources, not the former application that wanted 0.8v
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
John Larkin <jjlarkin@highlandtechnology.com> Wrote in message:
> > 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. > > https://dl.dropboxusercontent.com/u/53724080/Parts/Transistors/BCX70_as_Diode.JPG > > 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 > >
The 1N914/1N914A is quite flaccid. -- ----Android NewsGroup Reader---- http://usenet.sinaapp.com/
In article <itnb6bhrlf6c1461v8rlchvms5c432ks52@4ax.com>,
 John Larkin <jjlarkin@highlandtechnology.com> wrote:

> 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. > > https://dl.dropboxusercontent.com/u/53724080/Parts/Transistors/BCX70_as_Diode. > JPG > > 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.
Vishay makes the TZMC1V0. The datasheet is extremely short but it looks like a wimpy diode. -- I will not see posts from astraweb, theremailer, dizum, or google because they host Usenet flooders.
On Monday, December 7, 2015 at 4:11:42 PM UTC-8, John Larkin wrote:
 
> 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.
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.
On Mon, 7 Dec 2015 20:57:02 -0800 (PST), whit3rd <whit3rd@gmail.com>
wrote:

>On Monday, December 7, 2015 at 4:11:42 PM UTC-8, John Larkin wrote: > >> 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. > >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.
On Mon, 7 Dec 2015 18:34:11 -0800 (PST), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

>Den tirsdag den 8. december 2015 kl. 01.52.25 UTC+1 skrev tabb...@gmail.com: >> On Monday, 7 December 2015 21:26:45 UTC, whit3rd wrote: >> > On Monday, December 7, 2015 at 12:49:23 PM UTC-8, nt wrote: >> > > On Monday, 7 December 2015 20:33:47 UTC, John Larkin wrote: >> > > > On Mon, 7 Dec 2015 12:04:08 -0800 (PST), George Herold >> > > > <gherold@teachspin.com> wrote: >> >> > > > >An LED? or is that too much voltage? >> > >> > > > I want about 0.8 volts >> > >> > > Aren't 2 diodes cheaper? >> >> that's misleading editing. I was talking about use in current sources, not the former application that wanted 0.8v > >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
Yeah, that's the link I posted: https://dl.dropboxusercontent.com/u/53724080/Circuits/Current_Sources/LED_Isrc_data.JPG That 90 PPM tempco could be all in the emitter resistor. This was good enough, but could certainly be tweaked.
On Mon, 07 Dec 2015 20:52:36 -0800, Kevin McMurtrie
<mcmurtrie@pixelmemory.us> wrote:

>In article <itnb6bhrlf6c1461v8rlchvms5c432ks52@4ax.com>, > John Larkin <jjlarkin@highlandtechnology.com> wrote: > >> 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. >> >> https://dl.dropboxusercontent.com/u/53724080/Parts/Transistors/BCX70_as_Diode. >> JPG >> >> 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. > >Vishay makes the TZMC1V0. The datasheet is extremely short but it looks >like a wimpy diode.
Bizarre. They call it a 0.75 volt zener diode, but it sure looks like a regular PN diode, used forward biased.