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wimpy diode

Started by John Larkin December 7, 2015
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. > > > > >
How come you seem to be the only one that shows READABLE (_not_ grey on grey) drawings? How dare you make things clear :)
On 07/12/2015 19:47, 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. >
How is that e-phemt Vgs bias point to change with temperature? Would the B-E junction TC even be in the right direction? piglet
On 07/12/2015 20:04, George Herold wrote:
> > An LED? or is that too much voltage? > > George H.
Or two schottky diodes in series? piglet
On Mon, 07 Dec 2015 11:47:48 -0800, 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.
I use a BAV199W if I want a high forward drop or vanishingly small reverse current. Vf is not very well constrained by worst case limits in the datasheet though. Allan
On 08/12/2015 08:08, John Larkin wrote:
> 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! > >
In some companies there could be a risk that someone in Marketing will change the LED colour to blue without asking you, because the blue LEDs fitted under his car look so good.
On Tuesday, 8 December 2015 02:03:27 UTC, John Larkin  wrote:
> On Mon, 7 Dec 2015 16:50:34 -0800 (PST), nt 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! :)
> I think that's equivalent to a current mirror. One diode and one > transistor does that, too.
Right, though you've got no emitter resistor then, and are wholly dependant on tr characteristics. | | R | | _|c +------(_) | |e D | | D D | | | +--------+---
> 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.
NT
On Tuesday, 8 December 2015 02:05:47 UTC, John Larkin  wrote:
> On Mon, 7 Dec 2015 16:52:20 -0800 (PST), nt 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.
If base diodes temp rise is half as much as the tr, you're good :) Easily arranged, but of course short term power dissipation variations will not go so well, and T_amb is uncompensated. NT
On Tuesday, 8 December 2015 02:34:16 UTC, Lasse Langwadt Christensen  wrote:
> Den tirsdag den 8. december 2015 kl. 01.52.25 UTC+1 skrev nt: > > 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
One of those 2mVs nicely compensates the tr. So halve the diode temp rise... NT
On Tuesday, 8 December 2015 11:22:17 UTC, Allan Herriman  wrote:
> On Mon, 07 Dec 2015 11:47:48 -0800, 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. > > > I use a BAV199W if I want a high forward drop or vanishingly small > reverse current. Vf is not very well constrained by worst case limits in > the datasheet though. > > Allan
I've got a graph from who knows where that shows 1N458 0.8v @ 10mA. FWIW. NT
On Tue, 8 Dec 2015 22:52:08 +1100, Chris Jones
<lugnut808@spam.yahoo.com> wrote:

>On 08/12/2015 08:08, John Larkin wrote: >> 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! >> >> > >In some companies there could be a risk that someone in Marketing will >change the LED colour to blue without asking you, because the blue LEDs >fitted under his car look so good. >
Not in my company! Manufacturing must use the BOM, and only an ECO can change the BOM, and I sign all the ECOs.