What's the deal with old Ge transistors?

[Jim Thompson]

Then there are no "good" zeners, at all.

That's right ;-)

The best reference is an active circuit, such as a TL431 (or a modern
equivalent), where the "breakdown" is sharp and the TC is low.

...Jim Thompson

 

[Ben Bradley]

dated Sun, 20 Jul 2003 09:02:14 GMT,


There is no such thing as a KNEE in a silicon diode !

Since both Si and Ge diodes have exponential (forward) I-vs-V
curves, I presume this statement also applies to a germanium diode.

...

There is no such thing as a KNEE in a silicon diode !

There is no such thing as a KNEE in a silicon diode !

In followups to this post there is discussion about Zener diodes
(and others used for regulation, apparently using 'avalanche' mode),
and of a 'knee.' Can someome give a good, perhaps even mathematical,
definition of a 'knee?' I'm not afraid of derivatives and such...

 

[James Meyer]

In followups to this post there is discussion about Zener diodes
(and others used for regulation, apparently using 'avalanche' mode),
and of a 'knee.' Can someome give a good, perhaps even mathematical,
definition of a 'knee?' I'm not afraid of derivatives and such...

One lawmaker said, "I don't have a good legal definition of pornography,
but I can recognize it when I see it."

I think diode knees are a lot like pornography.

Jim

 

[Watson A.Name - 'Watt Sun']

I was at Motorola SPD in the '60s so my view is biased... I built
audio amps using RF grade power transistors ;-)

He was in the Sewage Processing Dept. One flush should do it.

(Hey, if you throw obscure TLAs around, expect people to misinterpret
them.)

But I think Ge power held its own against Si until the early '70s.

I was still using Ge diodes in the mid '80s for low drop situations,
instead of (then) expensive Schottkys.

...Jim Thompson

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[Watson A.Name - 'Watt Sun']

As others have said, there are very pronounced thermal dependencies. At
constant temperatures we get kinda these curves. Same with transistors. that
is why a Si-semiconductor makes a bad switch, because you always have an
offset.
^
Log(I) | /Ge /Si
| / /
| / /
|/ /
100nA/ /
| /
= =
0--/------------>
35-200mV Vf(25°)

Speaking of knees.. He doesn't know is ass from his elbow!

right, but there must be a knee in the curve in the 3rd quadrant, where
avalanche/zener effects are occurring.

ciao Ban

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[Watson A.Name - 'Watt Sun']

Excuse me, but a capacitor *cannot* change DC gain!
Furthermore the capacitor would *increase* the AC gain above a certain
frequency.

I think he should have said kind of the inverse that you said. The
capacitor reduces the gain at DC, not the DC gain. Yes, I agree it's
not really expressing it correctly but one could think of it as the
glass is half empty instead of half full. Crude analogy..


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[Kevin McMurtrie]

One lawmaker said, "I don't have a good legal definition of pornography,
but I can recognize it when I see it."

I think diode knees are a lot like pornography.

Jim

Do you have any photos to back up your claim

 

[Robert Baer]

Then there are no good zeners for voltages below about 4.8 volts.
Some other technology is needed for the characteristic you describe at
those voltages.

The 1.25V, 2.5V and 5V voltage references use the "band gap" scheme.
The LM385-2.5 curve looks resistive to the knee at about 25uA and then
rapidly curves up into voltage regulation at about 30uA and again is
linear, stable, and quiet up to the 10mA spec limit.
Many companies make that part or the equivalent and Maximum advertises
a rather decent spec equivalent - but i dare anyone to *FIND* any to
even sample; you gotta hav e lotza bucks to order them by the thousands
and then wait until they actually *make* them, if you can find a place
that will take your order.
Maxim aka Dallas is a company to steer clear of, as their "support"
and claims are worse than Micro$uck's infamous "Not There" operating
system (remember the years and years of empty promises?).
And if someone from Maxim takes umbrage, tough shit - many dealers and
distributors will not touch your vaporware because, in effect, it does
not exist for them to sell.

 

[Robert Baer]

Since both Si and Ge diodes have exponential (forward) I-vs-V
curves, I presume this statement also applies to a germanium diode.



In followups to this post there is discussion about Zener diodes
(and others used for regulation, apparently using 'avalanche' mode),
and of a 'knee.' Can someome give a good, perhaps even mathematical,
definition of a 'knee?' I'm not afraid of derivatives and such...

I do not think a mathematical definition was applied to the "knee"
concept.
Basically, a curve trace of the V-I function has a relatively rapid
change of slope at some point, and was called a "knee" maybe due to the
similarity of slope change of a leg hanging down from a chair.

 

[Tim Shoppa]

Your figures above are total fabrications. You cannot get a milliamp
thru any silicon diode with only .4V. Period. (non-junction types
like Schottkys are an exception). I have a PS and meter connected to
a 1N4148 and I see the following:

.01 mA = .380 V
.1 mA = .487 V
1 mA = .590 V
10 mA = .710 v
100 mA = .895 V
200 mA = 1.00 V

I agree the original guy's numbers are fabrications, but even with
your figures we see that each time the current goes up by a factor
of ten from .01mA to 10mA, the voltage went up by 0.10 to 0.12 Volts.

Maybe I'm not as picky as you... after all, when I was growing up
(not that long ago!) 20% tolerance resistors were the norm and anything
else was high precision.

Even ohm's law breaks down somewhere... It's impressive (and important)
that the exponential V-I diode relationship is pretty good (if not perfect)
for 3 or 4 orders of magnitude in current that you observe, and it's
probably OK for two or maybe three more orders of magnitude below your
mesasurements. If we took into account heating it'd be good for a bit
longer in the high-current direction.

Tim.

 

[Watson A.Name - 'Watt Sun']

re geranium transistors... (yes they're made from geranium leaves




So what sort of temps can trannies like OC71 and AC127 take?

Half of what a Si can take; something like 85 deg C. max. And if you
get close, it's runaway time!

And what apps are they still used in? I think I've only used them twice in design.

Well, very low voltage inverters, like below 1VDC input.

Regards, NT

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Don't be ripped off by the big book dealers. Go to the URL
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Just when you thought you had all this figured out, the gov't
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[Gilbert Mouget]

dated Mon, 21 Jul 2003 19:54:30 -0700,

Your figures above are total fabrications.

I apologise for these figures. I made these measurements more than
30 years ago when I was a student. I surely made a mistake.

It seems that the real current was 100 times lower than noted.

You cannot get a milliamp
thru any silicon diode with only .4V. Period. (non-junction types
like Schottkys are an exception). I have a PS and meter connected to
a 1N4148 and I see the following:

.01 mA = .380 V
.1 mA = .487 V
1 mA = .590 V
10 mA = .710 v
100 mA = .895 V
200 mA = 1.00 V

If we take in account my factor 100 mistake (!) your figures are in
total agreement with mine. Between .01 and 10 mA there is about 110mV
change per decade. The series resistance begin showing above 25 mA as
said Jim Thompson.

As for "no knee", anyone can make a graph into a straight line with
the right kind of graph scales.

This is a statement which shows that you dont understand what
we are talking about, ie exponential variation of current with
voltage in a silicon diode.

Try to make the curve of a tunnel diode into a straight line.

 

[Robert Baer]

I agree the original guy's numbers are fabrications, but even with
your figures we see that each time the current goes up by a factor
of ten from .01mA to 10mA, the voltage went up by 0.10 to 0.12 Volts.

Maybe I'm not as picky as you... after all, when I was growing up
(not that long ago!) 20% tolerance resistors were the norm and anything
else was high precision.

Even ohm's law breaks down somewhere... It's impressive (and important)
that the exponential V-I diode relationship is pretty good (if not perfect)
for 3 or 4 orders of magnitude in current that you observe, and it's
probably OK for two or maybe three more orders of magnitude below your
mesasurements. If we took into account heating it'd be good for a bit
longer in the high-current direction.

Tim.

Actually, some decent diodes have a very well behaved log functions
for 5-7 decades, and selected DCTs up to 9+ decades.

 

[Watson A.Name - 'Watt Sun']

Since both Si and Ge diodes have exponential (forward) I-vs-V
curves, I presume this statement also applies to a germanium diode.



In followups to this post there is discussion about Zener diodes
(and others used for regulation, apparently using 'avalanche' mode),
and of a 'knee.' Can someome give a good, perhaps even mathematical,
definition of a 'knee?' I'm not afraid of derivatives and such...

The manufacturers use the term knee in their data sheets.

I rest my case.


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###Got a Question about ELECTRONICS? Check HERE First:###
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goes directly to the trash unless you add NOSPAM in the
Subject: line with other stuff. alondra101 <at> hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
@@t@h@e@@a@f@f@l@u@e@n@t@@m@e@e@t@@t@h@e@@E@f@f@l@u@e@n@t@@

 

[Gilbert Mouget]

dated Wed, 23 Jul 2003 05:46:36 -0700,

The manufacturers use the term knee in their data sheets.

They use the term "knee" only for zener diodes, for rectifiers
they use "forward voltage".

 

[Gilbert Mouget]

dated Wed, 23 Jul 2003 06:00:30 -0700,

Just plot the curve on a 1000 volt scale, with points every volt. The
negative resistance slope disappears.

Clever. Did you find it by yourself ?

The Zener manufacturers call it a knee, and that is good enough for
me.

This is a statement which shows that you dont understand what
we were talking about, ie exponential variation of forward current
with voltage in a silicon diode.

 

[Gilbert Mouget]

dated 25 Jul 2003 06:26:23 -0700,

I think in this context DCT must stand for something like "diode used in
logarithmic amp" but I'm trying to get the letters to line up with words.
I know Decade Counter Tube and Discrete Cosine Transform, but those
cannot be appropriate

Diode Connected Transistor ?

 

[John Dyson]

I think in this context DCT must stand for something like "diode used in
logarithmic amp" but I'm trying to get the letters to line up with words.
I know Decade Counter Tube and Discrete Cosine Transform, but those
cannot be appropriate

Diode Connected Transistor.

John

 

[Kevin G. Rhoads]

Zeners are used (usually) in a reverse biased manner. There is no

reasonable scale that you can chose that will convert a zener's reverse curve
into a straight line.

Sure, just use the newly defined "Zener scale". Well I suppose that isn't
a *reasonable* scale.

 

[Kevin G. Rhoads]

There is no such thing as a KNEE in a silicon diode !

Never seen a tunnel diode have ye? Not only got a knee,
it also has a tit.