Switch and fight (leakage)?
in [Design]
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From: John Larkin on 15 Apr 2010 11:48 On Wed, 14 Apr 2010 21:51:15 -0700, Robert Baer <robertbaer(a)localnet.com> wrote: >John Larkin wrote: >> On Tue, 13 Apr 2010 22:29:06 -0700, Robert Baer >> <robertbaer(a)localnet.com> wrote: >> >> >>> So, instead of those Fairchild Ultra Low Leakage diodes, i should use >>> the C-B junction of a BFT25? >>> Better specs? >> >> 100x better on leakage, 10x better on price! >> >> John >> > Being Socially Insecure, the lower price spec is good to know. > Thanks. We created a PADS pcb symbol for the BFT25 as a diode. It's much less confusing on schematics. The c-b junctions of transistors tend to be very low leakage diodes, lots better than things born to be diodes. The smaller the junction, the better, usually. I recall that a 2N4402 leaks under a picoamp. You can buy a PAD-1, a 1 pA spec'd leakage diode, for a couple of dollars. It's actually a jfet with the source and drain connected. The channel resistance winds up in series with the gate diode junction, ballpark 2K ohms. For 30 cents, you can buy the BFT25 and get roughly 20 fA leakage and very low series resistance. Glass-packaged diodes leak when light hits them. That can be confusing. John
From: Robert Baer on 16 Apr 2010 04:27 John Larkin wrote: > On Wed, 14 Apr 2010 21:51:15 -0700, Robert Baer > <robertbaer(a)localnet.com> wrote: > >> John Larkin wrote: >>> On Tue, 13 Apr 2010 22:29:06 -0700, Robert Baer >>> <robertbaer(a)localnet.com> wrote: >>> >>> >>>> So, instead of those Fairchild Ultra Low Leakage diodes, i should use >>>> the C-B junction of a BFT25? >>>> Better specs? >>> 100x better on leakage, 10x better on price! >>> >>> John >>> >> Being Socially Insecure, the lower price spec is good to know. >> Thanks. > > We created a PADS pcb symbol for the BFT25 as a diode. It's much less > confusing on schematics. > > The c-b junctions of transistors tend to be very low leakage diodes, > lots better than things born to be diodes. The smaller the junction, > the better, usually. I recall that a 2N4402 leaks under a picoamp. > > You can buy a PAD-1, a 1 pA spec'd leakage diode, for a couple of > dollars. It's actually a jfet with the source and drain connected. The > channel resistance winds up in series with the gate diode junction, > ballpark 2K ohms. For 30 cents, you can buy the BFT25 and get roughly > 20 fA leakage and very low series resistance. > > Glass-packaged diodes leak when light hits them. That can be > confusing. > > John > Well, i think it is de-light-ful when someone that should know better gets startled by something known for at least 40 years.
From: Jim Thompson on 16 Apr 2010 10:49 On Fri, 16 Apr 2010 01:27:15 -0700, Robert Baer <robertbaer(a)localnet.com> wrote: >John Larkin wrote: >> On Wed, 14 Apr 2010 21:51:15 -0700, Robert Baer >> <robertbaer(a)localnet.com> wrote: >> >>> John Larkin wrote: >>>> On Tue, 13 Apr 2010 22:29:06 -0700, Robert Baer >>>> <robertbaer(a)localnet.com> wrote: >>>> >>>> >>>>> So, instead of those Fairchild Ultra Low Leakage diodes, i should use >>>>> the C-B junction of a BFT25? >>>>> Better specs? >>>> 100x better on leakage, 10x better on price! >>>> >>>> John >>>> >>> Being Socially Insecure, the lower price spec is good to know. >>> Thanks. >> >> We created a PADS pcb symbol for the BFT25 as a diode. It's much less >> confusing on schematics. >> >> The c-b junctions of transistors tend to be very low leakage diodes, >> lots better than things born to be diodes. The smaller the junction, >> the better, usually. I recall that a 2N4402 leaks under a picoamp. >> >> You can buy a PAD-1, a 1 pA spec'd leakage diode, for a couple of >> dollars. It's actually a jfet with the source and drain connected. The >> channel resistance winds up in series with the gate diode junction, >> ballpark 2K ohms. For 30 cents, you can buy the BFT25 and get roughly >> 20 fA leakage and very low series resistance. >> >> Glass-packaged diodes leak when light hits them. That can be >> confusing. >> >> John >> > Well, i think it is de-light-ful when someone that should know better >gets startled by something known for at least 40 years. Even those diodes (and transistors) packaged in black plastic exhibit photo effects. TI used to have a whole series of IR diodes (for TV remote links) that had no window. And, John, they don't "leak when light hits them"... what a dummy :-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | The only thing bipartisan in this country is hypocrisy
From: John Larkin on 16 Apr 2010 11:54 On Fri, 16 Apr 2010 01:27:15 -0700, Robert Baer <robertbaer(a)localnet.com> wrote: >John Larkin wrote: >> On Wed, 14 Apr 2010 21:51:15 -0700, Robert Baer >> <robertbaer(a)localnet.com> wrote: >> >>> John Larkin wrote: >>>> On Tue, 13 Apr 2010 22:29:06 -0700, Robert Baer >>>> <robertbaer(a)localnet.com> wrote: >>>> >>>> >>>>> So, instead of those Fairchild Ultra Low Leakage diodes, i should use >>>>> the C-B junction of a BFT25? >>>>> Better specs? >>>> 100x better on leakage, 10x better on price! >>>> >>>> John >>>> >>> Being Socially Insecure, the lower price spec is good to know. >>> Thanks. >> >> We created a PADS pcb symbol for the BFT25 as a diode. It's much less >> confusing on schematics. >> >> The c-b junctions of transistors tend to be very low leakage diodes, >> lots better than things born to be diodes. The smaller the junction, >> the better, usually. I recall that a 2N4402 leaks under a picoamp. >> >> You can buy a PAD-1, a 1 pA spec'd leakage diode, for a couple of >> dollars. It's actually a jfet with the source and drain connected. The >> channel resistance winds up in series with the gate diode junction, >> ballpark 2K ohms. For 30 cents, you can buy the BFT25 and get roughly >> 20 fA leakage and very low series resistance. >> >> Glass-packaged diodes leak when light hits them. That can be >> confusing. >> >> John >> > Well, i think it is de-light-ful when someone that should know better >gets startled by something known for at least 40 years. I didn't say it confused *me* I first noticed something like this when GE introduced their first plastic transistors. They had a round black plastic header (e-c-b inline) and a flatted, sort of TO-92, cylindrical body made of brownish epoxy. The top was concave, where they apparently poured the brown gunk into a mold. They would leak nA of current in healthy room light. I designed a nice 1 uV/degC opamp using them and told my boss to come over and see. Of course he leaned over and blocked the light and messed up the offset tweak. Before long GE went all black on their packaging. I was still a teenager, so it *was* over 40 years ago. The other plastic style back then was TO-5 looking, with a round open-top grey ceramic base and a black epoxy glob top. There was an 8-pin version for early RTL parts. The latest semiconductor discovery that startled me, aside from the fA leakage of some RF transistors, is the parasitic PIN diode layer I discovered in some Optek laser diodes. It was just like having a substantial inductance in series with the part. Bad news for pulse applications. And the National substrate diode SRD thing of course. John
From: Jasen Betts on 17 Apr 2010 04:10
On 2010-04-16, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: > On Fri, 16 Apr 2010 01:27:15 -0700, Robert Baer ><robertbaer(a)localnet.com> wrote: > Even those diodes (and transistors) packaged in black plastic exhibit > photo effects. > > TI used to have a whole series of IR diodes (for TV remote links) that > had no window. I've seen some like that, a bare silicon wafer. however photdiodes packed in black plastic are packaged in plastic that is transparent in the infrared, black in the visible wavelengths cuts down on interferance from fluorescent lights etc... > And, John, they don't "leak when light hits them"... what a dummy :-) --- news://freenews.netfront.net/ - complaints: news(a)netfront.net --- |