Forums

wimpy diode

Started by John Larkin December 7, 2015
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

On Monday, December 7, 2015 at 2:47:59 PM UTC-5, John Larkin 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. > > > > > > -- > > 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.
On Mon, 7 Dec 2015 12:04:08 -0800 (PST), George Herold
<gherold@teachspin.com> wrote:

>On Monday, December 7, 2015 at 2:47:59 PM UTC-5, John Larkin 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. >> >> >> >> >> >> -- >> >> 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
On Monday, 7 December 2015 19:47:59 UTC, John Larkin  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.
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
On Monday, December 7, 2015 at 3:33:47 PM UTC-5, John Larkin wrote:
> On Mon, 7 Dec 2015 12:04:08 -0800 (PST), George Herold > <gherold@teachspin.com> wrote: > > >On Monday, December 7, 2015 at 2:47:59 PM UTC-5, John Larkin 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. > >> > >> > >> > >> > >> > >> -- > >> > >> 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.
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.
> > 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
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: > > >On Monday, December 7, 2015 at 2:47:59 PM UTC-5, John Larkin 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. > >> > >> > >> > >> > >> > >> -- > >> > >> 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.
Aren't 2 diodes cheaper? NT
On Mon, 7 Dec 2015 12:49:17 -0800 (PST), tabbypurr@gmail.com 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: >> >> >On Monday, December 7, 2015 at 2:47:59 PM UTC-5, John Larkin 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. >> >> >> >> >> >> >> >> >> >> >> >> -- >> >> >> >> 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. > >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. https://dl.dropboxusercontent.com/u/53724080/Circuits/Current_Sources/LED_Isrc_bb.JPG https://dl.dropboxusercontent.com/u/53724080/Circuits/Current_Sources/LED_Isrc_data.JPG Plus, it glows in the dark! -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Monday, December 7, 2015 at 12:49:23 PM UTC-8, tabb...@gmail.com 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: > > > > >On Monday, December 7, 2015 at 2:47:59 PM UTC-5, John Larkin wrote: > > >> I need a wimpy diode, with lots of forward drop at a few mA, to bias > > >> up an e-phemt.
> > >An LED? or is that too much voltage?
> > I want about 0.8 volts
> Aren't 2 diodes cheaper?
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.
On Mon, 7 Dec 2015 13:26:40 -0800 (PST), whit3rd <whit3rd@gmail.com>
wrote:

>On Monday, December 7, 2015 at 12:49:23 PM UTC-8, tabb...@gmail.com 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: >> > >> > >On Monday, December 7, 2015 at 2:47:59 PM UTC-5, John Larkin wrote: >> > >> I need a wimpy diode, with lots of forward drop at a few mA, to bias >> > >> up an e-phemt. > >> > >An LED? or is that too much voltage? > >> > I want about 0.8 volts > >> Aren't 2 diodes cheaper? > >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
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