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wimpy diode

Started by John Larkin December 7, 2015
On 12/08/2015 01:53 PM, John Larkin wrote:
> On 08 Dec 2015 11:22:12 GMT, Allan Herriman > <allanherriman@hotmail.com> 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 > > If you want crazy low reverse current, consider the c-b junction of a > BFT25. > > https://dl.dropboxusercontent.com/u/53724080/Parts/Transistors/BFT25.JPG > > That reverse leakage was about at my measurement resolution limit. > >
I'm using one in that super low cost TIA (with the LM324). It's in parallel with a 200M feedback resistor, to give a logarithmic overrange indication. Your plot has the transistor in normal bias for the inverted configuration (collector negative), which probably helps the log conformance a fair amount. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
On Wed, 09 Dec 2015 05:28:49 -0500, Spehro Pefhany
<speffSNIP@interlogDOTyou.knowwhat> wrote:

>On Wed, 9 Dec 2015 01:51:53 -0800 (PST), the renowned >tabbypurr@gmail.com wrote: > >>On Wednesday, 9 December 2015 04:10:16 UTC, John Larkin wrote: >> >>> Thanks, but I don't need that precision, and we have the BCX70 in >>> stock. >>> >>> Putting more parts into stock is generally avoided. We have 6469 parts >>> already, 3.03 million pieces. >> >>Ouch. JIT. >> >> >>NT > >I think at one time JL said they had more than 100 types of LDO alone >in stock. > > >--sp
Yeah, too many voltage regulators. About 100, of all kinds, including switchers. Most of those 3e6 parts are resistors and caps on reels.