Types of diode-connected BJTs
in [Design]
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From: John Larkin on 8 Jul 2010 20:35 On Thu, 8 Jul 2010 00:53:14 -0700 (PDT), C Egernet <egernet(a)hushmail.com> wrote: >What are the relative merits of diode-connecting a BJT by shorting >base and collector (using B-E junction) versus shorting base and >emitter (C-B junction)? > >Na�vely, I have assumed that one always uses the B-E junction but then >I saw this post from s.e.d by Phil Hobbs: > >> BFT25A C-B junctions are at least as good as 2N4117As as diodes. >> >> Cheers >> >> Phil Hobbs > >What gives? > > >Chris Most diodes-sold-as-diodes, like BAV99 and 1N4148 and such, leak nanoamps, and the glass ones are photosensitive. I took data on using BFT25A C-B junctions as diodes. They are fantastic. I measured about 20 fA reverse leakage at a few volts, log linearity from 1 pA to 10s of mA, and about half a pF. I didn't try the B-E junction, because it will zener at a few volts so isn't as generally useful. The measurements are tedious. It leaks less than a PAD-1, has much lower forward resistance and capacitance, and costs a lot less. You can hardly buy a diode that good. One exception is the Central CMPD6001S, a dual SOT-23 that leaks about 50 fA at -5 volts and room temp. It's probably a bigger junction than the BFT25, because it leaks more and is more like 2 pF. That's two diodes for about 16 cents John
From: Grant on 8 Jul 2010 21:09 On Thu, 08 Jul 2010 17:35:30 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Thu, 8 Jul 2010 00:53:14 -0700 (PDT), C Egernet ><egernet(a)hushmail.com> wrote: > >>What are the relative merits of diode-connecting a BJT by shorting >>base and collector (using B-E junction) versus shorting base and >>emitter (C-B junction)? >> >>Naïvely, I have assumed that one always uses the B-E junction but then >>I saw this post from s.e.d by Phil Hobbs: >> >>> BFT25A C-B junctions are at least as good as 2N4117As as diodes. >>> >>> Cheers >>> >>> Phil Hobbs >> >>What gives? >> >> >>Chris > >Most diodes-sold-as-diodes, like BAV99 and 1N4148 and such, leak >nanoamps, and the glass ones are photosensitive. > >I took data on using BFT25A C-B junctions as diodes. They are >fantastic. I measured about 20 fA reverse leakage at a few volts, log >linearity from 1 pA to 10s of mA, and about half a pF. I didn't try >the B-E junction, because it will zener at a few volts so isn't as >generally useful. The measurements are tedious. Does E connect anywhere? Or leave open? Grant. > >It leaks less than a PAD-1, has much lower forward resistance and >capacitance, and costs a lot less. You can hardly buy a diode that >good. > >One exception is the Central CMPD6001S, a dual SOT-23 that leaks about >50 fA at -5 volts and room temp. It's probably a bigger junction than >the BFT25, because it leaks more and is more like 2 pF. That's two >diodes for about 16 cents > >John >
From: Jim Thompson on 8 Jul 2010 22:23 On Fri, 09 Jul 2010 11:09:45 +1000, Grant <omg(a)grrr.id.au> wrote: >On Thu, 08 Jul 2010 17:35:30 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>On Thu, 8 Jul 2010 00:53:14 -0700 (PDT), C Egernet >><egernet(a)hushmail.com> wrote: >> >>>What are the relative merits of diode-connecting a BJT by shorting >>>base and collector (using B-E junction) versus shorting base and >>>emitter (C-B junction)? >>> >>>Na�vely, I have assumed that one always uses the B-E junction but then >>>I saw this post from s.e.d by Phil Hobbs: >>> >>>> BFT25A C-B junctions are at least as good as 2N4117As as diodes. >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>> >>>What gives? >>> >>> >>>Chris >> >>Most diodes-sold-as-diodes, like BAV99 and 1N4148 and such, leak >>nanoamps, and the glass ones are photosensitive. >> >>I took data on using BFT25A C-B junctions as diodes. They are >>fantastic. I measured about 20 fA reverse leakage at a few volts, log >>linearity from 1 pA to 10s of mA, and about half a pF. I didn't try >>the B-E junction, because it will zener at a few volts so isn't as >>generally useful. The measurements are tedious. > >Does E connect anywhere? Or leave open? I'll leave it for Larkin to pontificate, then I'll say :-) > >Grant. >> >>It leaks less than a PAD-1, has much lower forward resistance and >>capacitance, and costs a lot less. You can hardly buy a diode that >>good. >> >>One exception is the Central CMPD6001S, a dual SOT-23 that leaks about >>50 fA at -5 volts and room temp. It's probably a bigger junction than >>the BFT25, because it leaks more and is more like 2 pF. That's two >>diodes for about 16 cents >> >>John >> ...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 | Obama isn't going to raise your taxes...it's Bush' fault: Not re- newing the Bush tax cuts will increase the bottom tier rate by 50%
From: C Egernet on 9 Jul 2010 03:51 Many thanks to Jasen Betts, George Herold, whit3rd and John Larkin. I learned something useful. Chris
From: George Herold on 9 Jul 2010 09:48
On Jul 8, 8:35 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Thu, 8 Jul 2010 00:53:14 -0700 (PDT), C Egernet > > > > > > <eger...(a)hushmail.com> wrote: > >What are the relative merits of diode-connecting a BJT by shorting > >base and collector (using B-E junction) versus shorting base and > >emitter (C-B junction)? > > >Naïvely, I have assumed that one always uses the B-E junction but then > >I saw this post from s.e.d by Phil Hobbs: > > >> BFT25A C-B junctions are at least as good as 2N4117As as diodes. > > >> Cheers > > >> Phil Hobbs > > >What gives? > > >Chris > > Most diodes-sold-as-diodes, like BAV99 and 1N4148 and such, leak > nanoamps, and the glass ones are photosensitive. > > I took data on using BFT25A C-B junctions as diodes. They are > fantastic. I measured about 20 fA reverse leakage at a few volts, log > linearity from 1 pA to 10s of mA, and about half a pF. I didn't try > the B-E junction, because it will zener at a few volts so isn't as > generally useful. The measurements are tedious. That's great! It would be useful as a over-voltage protection 'diode' on a low noise front end. Any idea how much current it can handle. Seems like if used as over-voltage protection you might want to short the E and B and get a bit of current gain. George H. > > It leaks less than a PAD-1, has much lower forward resistance and > capacitance, and costs a lot less. You can hardly buy a diode that > good. > > One exception is the Central CMPD6001S, a dual SOT-23 that leaks about > 50 fA at -5 volts and room temp. It's probably a bigger junction than > the BFT25, because it leaks more and is more like 2 pF. That's two > diodes for about 16 cents > > John- Hide quoted text - > > - Show quoted text - |