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2N3904 B-E junction as fast diode substitute?


 

While waiting for my Schottky diodes to repair my S-1 (and possibly my S-2), I did some searching for currently available substitutes. The late, great Bob Pease once wrote:

"One "diode" that does turn ON and OFF quickly is a diode-connected transistor. A typical 2N3904 emitter diode can turn ON or OFF in 0.1 nsec with negligible overshoot and less than 1 pA of leakage at 1 V, or less than 10 pA at 4 V. (This diode does, of course, have the base tied to the collector.) However, this diode can only withstand 5 or 6 V of reverse voltage, and most emitter-base junctions start to break down at 6 or 8V. Still, if you can arrange your circuits for just a few volts, these diode-connected transistors make nice, fast, low-leakage diodes. Their capacitance is somewhat more than the 1N914's 1pF."

Has anyone actually tried this for sampling diodes, especially in the (relatively) slow S-1? 100 ps sounds perfect. Not only do I have some handy, TO-92 packages are a lot easier to manipulate and solder than SOT-323, especially with my large fingers and aging eyes... I can't find any articles where it's actually been done though.

Of course I could just go tack a couple in the already-disassembled S-1 and give it a try.


Bruce Griffiths
 

Diode connected BJTs (base connected to collector) have been used in low close in phase noise double balance mixers operating at 10MHz.

Bruce

On 22 February 2021 at 13:48 Charles <charlesmorris800@centurytel.net> wrote:


While waiting for my Schottky diodes to repair my S-1 (and possibly my S-2), I did some searching for currently available substitutes. The late, great Bob Pease once wrote:

"One "diode" that does turn ON and OFF quickly is a diode-connected transistor. A typical 2N3904 emitter diode can turn ON or OFF in 0.1 nsec with negligible overshoot and less than 1 pA of leakage at 1 V, or less than 10 pA at 4 V. (This diode does, of course, have the base tied to the collector.) However, this diode can only withstand 5 or 6 V of reverse voltage, and most emitter-base junctions start to break down at 6 or 8V. Still, if you can arrange your circuits for just a few volts, these diode-connected transistors make nice, fast, low-leakage diodes. Their capacitance is somewhat more than the 1N914's 1pF."

Has anyone actually tried this for sampling diodes, especially in the (relatively) slow S-1? 100 ps sounds perfect. Not only do I have some handy, TO-92 packages are a lot easier to manipulate and solder than SOT-323, especially with my large fingers and aging eyes... I can't find any articles where it's actually been done though.

Of course I could just go tack a couple in the already-disassembled S-1 and give it a try.





Michael Dunn
 

C-B junctions can often be used as low-leakage diodes... I'm surprised that E-B are also considered low-leakage, given the low breakdown. Must do a comparison some day :-)


Roy Thistle
 

On Mon, Feb 22, 2021 at 04:41 AM, Michael Dunn wrote:


E-B are also considered low-leakage, given the low breakdown.
Diode connected BJT (where that mean base and collector are connected) are frequently used in current mirrors (in integrated circuits) ... but that's for temperature compensation reasons.
Diode connected BJTs can sometimes be seen in designs, where for some reason (cost?), the designer specs a diode... but, production uses a transistor.
Those uses for a Diode connected BJT seem common enough.
But using the base to emitter junction as a diode... maybe that's not so common?
Anyway, for a 2N3904 (planar epitaxial NPN silicon BJT, ON Semiconductor)... if you keep the reverse bias below 6 volts (ON gives this as the breakdown), I expect the current to be less than a few pA... maybe less than 1 pA. (One can do the calculations.)
I reckon, the current in a base to emitter configuration is going to be at least an order of magnitude less than a base to collector, Diode connected BJT. ... because the current scales with the area of the junctions.
If you don't fly by math, or by simulation either... then doing the measurements would be great... but, that might not be so easy... but, it would be interesting to know.


Tom Lee
 

The B-E diode certainly recovers very fast (my samples typically measure about twice the value Pease reports, but that’s still fast). The junction capacitance is large (several pF) though, so that makes them less attractive as sampling diodes. You get quite a lot of blow-by.

Tom

Sent from my iThing, so please forgive typos and brevity.

On Feb 21, 2021, at 4:48 PM, Charles <charlesmorris800@centurytel.net> wrote:

While waiting for my Schottky diodes to repair my S-1 (and possibly my S-2), I did some searching for currently available substitutes. The late, great Bob Pease once wrote:

"One "diode" that does turn ON and OFF quickly is a diode-connected transistor. A typical 2N3904 emitter diode can turn ON or OFF in 0.1 nsec with negligible overshoot and less than 1 pA of leakage at 1 V, or less than 10 pA at 4 V. (This diode does, of course, have the base tied to the collector.) However, this diode can only withstand 5 or 6 V of reverse voltage, and most emitter-base junctions start to break down at 6 or 8V. Still, if you can arrange your circuits for just a few volts, these diode-connected transistors make nice, fast, low-leakage diodes. Their capacitance is somewhat more than the 1N914's 1pF."

Has anyone actually tried this for sampling diodes, especially in the (relatively) slow S-1? 100 ps sounds perfect. Not only do I have some handy, TO-92 packages are a lot easier to manipulate and solder than SOT-323, especially with my large fingers and aging eyes... I can't find any articles where it's actually been done though.

Of course I could just go tack a couple in the already-disassembled S-1 and give it a try.





 

My boss taught me a trick back in the early 1970s when Zener diodes were expensive and some transistors were almost as cheap as "pop-corn". The reverse biased base-emitter junction has a very sharp breakdown voltage around 5 to 7 volts. He used them in the circuits our company designed as a low cost alternative wherever he needed a voltage reference.
Since then I have learned that it is essential to keep the reverse-bias current low because it slowly degrades the junction over time.

Dennis Tillman W7pF



--
Dennis Tillman W7pF
TekScopes Moderator


Tom Lee
 

And Zeners of around 6V have a positive TC about equal in magnitude, but opposite in sign, to that of a forward-biased diode. Stack the two to create a low-TC reference. Useful for those times when you don’t have a bandgap ref around, or if you just want to party like it’s 1965.

Tom

Sent from my iThing, so please forgive typos and brevity.

On Feb 22, 2021, at 11:01 AM, Dennis Tillman W7pF <dennis@ridesoft.com> wrote:

My boss taught me a trick back in the early 1970s when Zener diodes were expensive and some transistors were almost as cheap as "pop-corn". The reverse biased base-emitter junction has a very sharp breakdown voltage around 5 to 7 volts. He used them in the circuits our company designed as a low cost alternative wherever he needed a voltage reference.
Since then I have learned that it is essential to keep the reverse-bias current low because it slowly degrades the junction over time.

Dennis Tillman W7pF



--
Dennis Tillman W7pF
TekScopes Moderator





Jim Ford
 

I'd want to know about the tempco of the breakdown voltage vs a Zener, Dennis.  IIRC, a 5.1 V Zener has a tempco near zero.       Jim Ford Sent from my Verizon, Samsung Galaxy smartphone

-------- Original message --------From: Dennis Tillman W7pF <dennis@ridesoft.com> Date: 2/22/21 11:00 AM (GMT-08:00) To: TekScopes@groups.io Subject: Re: [TekScopes] 2N3904 B-E junction as fast diode substitute? My boss taught me a trick back in the early 1970s when Zener diodes were expensive and some transistors were almost as cheap as "pop-corn". The reverse biased base-emitter junction has a very sharp breakdown voltage around 5 to 7 volts. He used them in the circuits our company designed as a low cost alternative wherever he needed a voltage reference. Since then I have learned that it is essential to keep the reverse-bias current low because it slowly degrades the junction over time.Dennis Tillman W7pF-- Dennis Tillman W7pFTekScopes Moderator


Roy Thistle
 

On Sun, Feb 21, 2021 at 04:48 PM, Charles wrote:


diode-connected transistors
One can also do this with JFET, and FET, transistors too.


Dave Wise
 

?I seem to recall that the degradation takes the form of low beta at microamp currents. I hope that's all it does, because at certain settings, my HP 740B's zener a 2N3053 switch 20000 times a second. (It breaks down at about 10V.) One of several oddities that I consider to be design goofs.


Dave Wise

________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Dennis Tillman W7pF via groups.io <dennis=ridesoft.com@groups.io>
Sent: Monday, February 22, 2021 11:00 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 2N3904 B-E junction as fast diode substitute?

My boss taught me a trick back in the early 1970s when Zener diodes were expensive and some transistors were almost as cheap as "pop-corn". The reverse biased base-emitter junction has a very sharp breakdown voltage around 5 to 7 volts. He used them in the circuits our company designed as a low cost alternative wherever he needed a voltage reference.
Since then I have learned that it is essential to keep the reverse-bias current low because it slowly degrades the junction over time.

Dennis Tillman W7pF



--
Dennis Tillman W7pF
TekScopes Moderator


Ed Breya
 

A reversed E-B junction is approximately a Zener in breakdown. Silicon transistors tend to have about the same reverse E-B breakdown near 5-7 V, regardless of size. There's a low level clamping topology that's pretty slick, that I've seen in a number of places. You put two same type small signal transistors with their bases tied, and the emitters go between the clamping points. At low voltage, you get a pretty low leakage diode blocking in either direction, but when the voltage reaches the Zener range, either transistor will clamp, depending on polarity.

Everything that we commonly call Zeners above around 7 V are actually avalanche breakdown devices, but we don't normally bother about the distinction. For low leakage diodes with higher breakdown, use the B-C instead, or G-DS of a JFET.

Ed


 

You are correct about 4 to 5V being the magic zero TC. That wasn't my point.
In 1970 the reverse B-E junction cost less than 1/20th of a Zener diode. No need to use reverse junctions today. Zener's are cheap now.
As Tom Lee just pointed out the reverse biased junction has a positive TC about equal in magnitude, but opposite in sign, to that of a forward-biased diode. Stack the two to create a low-TC reference.

Dennis Tillman W7pF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Jim Ford
Sent: Monday, February 22, 2021 11:15 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 2N3904 B-E junction as fast diode substitute?

I'd want to know about the tempco of the breakdown voltage vs a Zener, Dennis. IIRC, a 5.1 V Zener has a tempco near zero. Jim Ford Sent from my Verizon, Samsung Galaxy smartphone
-------- Original message --------From: Dennis Tillman W7pF <dennis@ridesoft.com> Date: 2/22/21 11:00 AM (GMT-08:00) To: TekScopes@groups.io Subject: Re: [TekScopes] 2N3904 B-E junction as fast diode substitute? My boss taught me a trick back in the early 1970s when Zener diodes were expensive and some transistors were almost as cheap as "pop-corn". The reverse biased base-emitter junction has a very sharp breakdown voltage around 5 to 7 volts. He used them in the circuits our company designed as a low cost alternative wherever he needed a voltage reference. Since then I have learned that it is essential to keep the reverse-bias current low because it slowly degrades the junction over time.Dennis Tillman W7pF-- Dennis Tillman W7pFTekScopes Moderator







--
Dennis Tillman W7pF
TekScopes Moderator


Jim Ford
 

OK.  Point taken.    JimSent from my Verizon, Samsung Galaxy smartphone

-------- Original message --------From: Dennis Tillman W7pF <dennis@ridesoft.com> Date: 2/22/21 12:07 PM (GMT-08:00) To: TekScopes@groups.io Subject: Re: [TekScopes] 2N3904 B-E junction as fast diode substitute? You are correct about 4 to 5V being the magic zero TC. That wasn't my point. In 1970 the reverse B-E junction cost less than 1/20th of a Zener diode. No need to use reverse junctions today. Zener's are cheap now. As Tom Lee just pointed out the reverse biased junction has a positive TC about equal in magnitude, but opposite in sign, to that of a forward-biased diode. Stack the two to create a low-TC reference.Dennis Tillman W7pF-----Original Message-----From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Jim FordSent: Monday, February 22, 2021 11:15 AMTo: TekScopes@groups.ioSubject: Re: [TekScopes] 2N3904 B-E junction as fast diode substitute?I'd want to know about the tempco of the breakdown voltage vs a Zener, Dennis.  IIRC, a 5.1 V Zener has a tempco near zero.       Jim Ford Sent from my Verizon, Samsung Galaxy smartphone-------- Original message --------From: Dennis Tillman W7pF <dennis@ridesoft.com> Date: 2/22/21  11:00 AM  (GMT-08:00) To: TekScopes@groups.io Subject: Re: [TekScopes] 2N3904 B-E junction as fast diode substitute? My boss taught me a trick back in the early 1970s when Zener diodes were expensive and some transistors were almost as cheap as "pop-corn". The reverse biased base-emitter junction has a very sharp breakdown voltage around 5 to 7 volts. He used them in the circuits our company designed as a low cost alternative wherever he needed a voltage reference. Since then I have learned that it is essential to keep the reverse-bias current low because it slowly degrades the junction over time.Dennis Tillman W7pF-- Dennis Tillman W7pFTekScopes Moderator-- Dennis Tillman W7pFTekScopes Moderator


Ed Breya
 

Dennis is right about the transistor characteristics suffering in reverse B-E breakdown. If you do commit a Q as a Zener, keep it that way, and avoid breakdown in transistors that are used in high performance "normal" applications.

Ed


Bert Haskins
 

On 2/22/2021 2:52 PM, Ed Breya via groups.io wrote:
A reversed E-B junction is approximately a Zener in breakdown. Silicon transistors tend to have about the same reverse E-B breakdown near 5-7 V, regardless of size. There's a low level clamping topology that's pretty slick, that I've seen in a number of places. You put two same type small signal transistors with their bases tied, and the emitters go between the clamping points. At low voltage, you get a pretty low leakage diode blocking in either direction, but when the voltage reaches the Zener range, either transistor will clamp, depending on polarity.
Heath used this trick as input protection clamping in some of their scopes, worked like a champ.

Everything that we commonly call Zeners above around 7 V are actually avalanche breakdown devices, but we don't normally bother about the distinction. For low leakage diodes with higher breakdown, use the B-C instead, or G-DS of a JFET.

Ed




Tom Lee
 

Heathkit generally implemented low-voltage, low-capacitance clamps with transistors in reverse avalanche mode, with bases floating. I can't recall ever seeing them use a version with the bases tied to anything (would be grateful for a link to a contrary example). Thanks to avalanching, such a transistor acts like an SCR. Once triggered, it clamps to a voltage well below the trigger voltage. As Bert says, it worked (and works) like a champ.

Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 2/22/2021 12:53, Bert Haskins wrote:

On 2/22/2021 2:52 PM, Ed Breya via groups.io wrote:
A reversed E-B junction is approximately a Zener in breakdown. Silicon transistors tend to have about the same reverse E-B breakdown near 5-7 V, regardless of size. There's a low level clamping topology that's pretty slick, that I've seen in a number of places. You put two same type small signal transistors with their bases tied, and the emitters go between the clamping points. At low voltage, you get a pretty low leakage diode blocking in either direction, but when the voltage reaches the Zener range, either transistor will clamp, depending on polarity.
Heath used this trick as input protection clamping in some of their scopes, worked like a champ.

Everything that we commonly call Zeners above around 7 V are actually avalanche breakdown devices, but we don't normally bother about the distinction. For low leakage diodes with higher breakdown, use the B-C instead, or G-DS of a JFET.

Ed







Morris Odell
 

The other useful property of a reverse biased BE "zener" is that it's usually noisier than a real zener. I have used them very successfully that way in random number generator applications.


 

Hi Morris,
So far I think there have been at least 7 or 8 things the humble B-E junction can be used for. I'm certain none of them were ever mentioned in any of my EE courses, and I would guess no-one else was taught about their many uses in their EE courses.

These are the kinds of things you learn on the job from talented fellow engineers who think outside the box or who, like Barrie Gilbert, was always wondering "What would happen if I hooked this to that? I miss his impish smile and his modesty. The first thing he would say when I would ask how many patents he had would be something like "Anyone could have thought of the things I did".

Dennis Tillman W7pF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Morris Odell
Sent: Monday, February 22, 2021 1:14 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 2N3904 B-E junction as fast diode substitute?

The other useful property of a reverse biased BE "zener" is that it's usually noisier than a real zener. I have used them very successfully that way in random number generator applications.








--
Dennis Tillman W7pF
TekScopes Moderator


Jim Ford
 

Yeah, I figure another good way to get ideas and start down a rabbit trail is to mix up a block diagram.  I ask myself, "Everybody does it this way; what happens if we do it a different way?"  And yes, I miss Barrie Gilbert, too, one of my heroes, along with Jim Williams and Bob Pease.  Pease was the only one with a degree, a lowly BSEE at that (like me).  Ok, Gilbert had at least one honorary PhD, but only Pease earned his.  Very cool electronics and music labs at Gilbert's house, too.  Wish I could have seen them in person.  I did have the pleasure of meeting Barrie, though.           Jim Ford Sent from my Verizon, Samsung Galaxy smartphone

-------- Original message --------From: Dennis Tillman W7pF <dennis@ridesoft.com> Date: 2/22/21 3:06 PM (GMT-08:00) To: TekScopes@groups.io Subject: Re: [TekScopes] 2N3904 B-E junction as fast diode substitute? Hi Morris,So far I think there have been at least 7 or 8 things the humble B-E junction can be used for. I'm certain none of them were ever mentioned in any of my EE courses, and I would guess no-one else was taught about their many uses in their EE courses. These are the kinds of things you learn on the job from talented fellow engineers who think outside the box or who, like Barrie Gilbert, was always wondering "What would happen if I hooked this to that? I miss his impish smile and his modesty. The first thing he would say when I would ask how many patents he had would be something like "Anyone could have thought of the things I did". Dennis Tillman W7pF -----Original Message-----From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Morris OdellSent: Monday, February 22, 2021 1:14 PMTo: TekScopes@groups.ioSubject: Re: [TekScopes] 2N3904 B-E junction as fast diode substitute?The other useful property of a reverse biased BE "zener" is that it's usually noisier than a real zener. I have used them very successfully that way in random number generator applications.-- Dennis Tillman W7pFTekScopes Moderator


 

Thanks for all the interesting side discussion... Prof. Lee did answer my question, noting that there would probably be too much blow-by.

Too much to be adjusted out by the blow-by compensation circuitry in these heads?

How about a different fast switching transistor (say a 2N5769? I have scads of those).

I remember seeing those transistor B-E (or should I say E-B) clamps as a kid, back in the 70's transistorized designs from Heath. I always wondered why they did that... now I know :)