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low capacitance zener

J

John Larkin

Jan 1, 1970
0
Most 5-volt zeners have tons of capacitance, 100 pF maybe. The b-e
junction of a BFS17 is about 1.7 pF at zero bias and seems to be a
very nice 5.5 volt zener.

John
 
J

Jamie

Jan 1, 1970
0
John said:
Most 5-volt zeners have tons of capacitance, 100 pF maybe. The b-e
junction of a BFS17 is about 1.7 pF at zero bias and seems to be a
very nice 5.5 volt zener.

John
down around there it's between the zener and impact mode. so I guess
it should be some what stable as a voltage reference.
 
J

Joerg

Jan 1, 1970
0
John said:
Most 5-volt zeners have tons of capacitance, 100 pF maybe. The b-e
junction of a BFS17 is about 1.7 pF at zero bias and seems to be a
very nice 5.5 volt zener.

Yeah but are you sure next month's shipment will be close enough to that
5.5V?
 
M

Mike Monett

Jan 1, 1970
0
Joerg said:
John Larkin wrote:
Yeah but are you sure next month's shipment will be close enough
to that 5.5V?

Doesn't matter these days with software calibration.

But does the junction capacitance stay at 1.7 pF when it goes into
Zener mode? I thought that's what caused the huge increase in
capacitance. I believe some typical ones may even reach 1nF.

There have been several discussions on using the base-emitter
reverse breakdown. Here's some info from 1999:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
some app notes say operating in avalanche mode, however briefly,
will degrade the noise figure in low-level RF amplifiers.

The 2N2222 base-emitter junction has a very sharp breakdown - about
7.2V from < 1uA to > 10mA. Very repeatable on a number of units from
the same purchase.

The 2N2369 (or 2N3227) is not so sharp but the knee is still under
10uA. About 6.4V.

The 2N5139 is sharp. About 7.8V.

The 2N4209 is sharp. About 6.0V.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Regards,

Mike Monett
 
J

John Larkin

Jan 1, 1970
0
down around there it's between the zener and impact mode. so I guess
it should be some what stable as a voltage reference.

Many NPN transistors will act like "reference zeners" of you use the
c-e terminals. The b-e junction acts like a zener, in series with the
forward-biased c-b junction, giving a net 6.2 volts with near zero TC.

Interestingly, the BFS17 doesn't do this. The b-e junction zeners at
5.5, but you only get about 3.5 volts in the "reference zener" mode.
This is worth investigating some rainy day.

John
 
J

Joerg

Jan 1, 1970
0
Mike said:
Doesn't matter these days with software calibration.

But then you might as well use a diode plus tempco adjust. Most uC have
onboard temp sensors these days. Or take a BAV70 and compare the other
half against a reference.

But does the junction capacitance stay at 1.7 pF when it goes into
Zener mode? I thought that's what caused the huge increase in
capacitance. I believe some typical ones may even reach 1nF.

There have been several discussions on using the base-emitter
reverse breakdown. Here's some info from 1999:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
some app notes say operating in avalanche mode, however briefly,
will degrade the noise figure in low-level RF amplifiers.

Yeah, it wouldn't be the ethical thing to sell them off via EBay
afterwards ;-)

The 2N2222 base-emitter junction has a very sharp breakdown - about
7.2V from < 1uA to > 10mA. Very repeatable on a number of units from
the same purchase.

The 2N2369 (or 2N3227) is not so sharp but the knee is still under
10uA. About 6.4V.

The 2N5139 is sharp. About 7.8V.

The 2N4209 is sharp. About 6.0V.

Interesting.
 
M

Mike Monett

Jan 1, 1970
0
Joerg said:
But then you might as well use a diode plus tempco adjust.

John just mentioned using a forward-biased b-c junction plus the b-e
junction to give zero tempco. I guess that needs a transistor with a
repeatable b-e breakdown voltage at the correct tempco. Which gets
us back to the start of the thread:)
Most uC have onboard temp sensors these days. Or take a BAV70 and
compare the other half against a reference.

Scratch that. I just looked up the capacitance for the 1N751 series.

The maximum capacitance of around 100pf for a 5V Zener is at zero
bias, as John said.

Increasing the bias reduces the capacitance, but there is no data
for the capacitance at the Zener voltage.

I had the distinct impression it was quite high in Zener mode. Tony
Williams might have much more info.

OTOH, the LM431 is neat. Adjustable, with 50 ppm/°C.

[...]
Regards, Joerg

Regards,

Mike Monett
 
J

Joerg

Jan 1, 1970
0
Mike said:
John just mentioned using a forward-biased b-c junction plus the b-e
junction to give zero tempco. I guess that needs a transistor with a
repeatable b-e breakdown voltage at the correct tempco. Which gets
us back to the start of the thread:)


Scratch that. I just looked up the capacitance for the 1N751 series.

The maximum capacitance of around 100pf for a 5V Zener is at zero
bias, as John said.

Increasing the bias reduces the capacitance, but there is no data
for the capacitance at the Zener voltage.

I guess it usually doesn't matter because the zener acts like a pretty
conductive path up there. Maybe figure 5 in this datasheet helps a bit
in estimating the trend:

http://www.onsemi.com/pub/Collateral/BZX84C2V4ET1-D.PDF

I had the distinct impression it was quite high in Zener mode. Tony
Williams might have much more info.

OTOH, the LM431 is neat. Adjustable, with 50 ppm/°C.

It sure is. However, these days I find myself more and more in
situations where those don't cut it anymore. Even when using them in a
feedback path you normally have to make sure it sees the minimum cathode
current when driven at the REF input. For the LMV431 that should be at
least around 100uA. Too much for many apps. The old TL431 needs 1mA,
with some species dropped to 400uA.

There are alternative versions but that gets expensive.
 
M

Mike Monett

Jan 1, 1970
0
Joerg said:
I guess it usually doesn't matter because the zener acts like a pretty
conductive path up there. Maybe figure 5 in this datasheet helps a bit
in estimating the trend:

http://www.onsemi.com/pub/Collateral/BZX84C2V4ET1-D.PDF

Yes, that's what I was looking at. The bias curves stop at 50% of VZ. Can
we assume the capacitance keeps getting lower, or does it turn around and
increase in Zener mode? I don't know the answer to that.

There was a lot of discussion about using Zener noise as a wideband noise
source. Since the noise is negative-going this takes two zeners driving a
differential input to get symmetrical noise on the output.

The bandwidth depends on the junction capacitance in Zener mode. It looks
like the regular Zener might have quite high capacitance, so John's comment
says the reverse biased b-e junction would be much better than a plain
zener diode.
It sure is. However, these days I find myself more and more in
situations where those don't cut it anymore. Even when using them in a
feedback path you normally have to make sure it sees the minimum cathode
current when driven at the REF input. For the LMV431 that should be at
least around 100uA. Too much for many apps. The old TL431 needs 1mA,
with some species dropped to 400uA.
There are alternative versions but that gets expensive.
Regards, Joerg

Regards,

Mike Monett
 
J

Joerg

Jan 1, 1970
0
Mike said:
Yes, that's what I was looking at. The bias curves stop at 50% of VZ. Can
we assume the capacitance keeps getting lower, or does it turn around and
increase in Zener mode? I don't know the answer to that.

Probably does go down but once you are up the knee the impedance drops
so far that it'll dominate for most applications except RF where zeners
aren't used. If I (hopefully) find a quiet time some day I'll put one on
the impedance analyzer and have it scan the capacitance from zero to the
rated zener current. That should be easier than hooking up an inductor
and measure resonances with a dip meter.

There was a lot of discussion about using Zener noise as a wideband noise
source. Since the noise is negative-going this takes two zeners driving a
differential input to get symmetrical noise on the output.

I was never too fond of using zeners as noise sources. Those little
bulbs for watches are much nicer. It's just a matter of finding one that
presents a nice 50ohm source while glowing hot enough. But not too
bright, of course ;-)

The bandwidth depends on the junction capacitance in Zener mode. It looks
like the regular Zener might have quite high capacitance, so John's comment
says the reverse biased b-e junction would be much better than a plain
zener diode.

John has the luxury of small production runs with high margins and
really high per unit prices. So he could probably buy a reel or two,
have that all measured, stash it somewhere and it'll last him until he
retires.


[...]
 
M

Mike Monett

Jan 1, 1970
0
[...]
I was never too fond of using zeners as noise sources. Those
little bulbs for watches are much nicer. It's just a matter of
finding one that presents a nice 50ohm source while glowing hot
enough. But not too bright, of course ;-)

I've never heard of that before. What kind of light bulb makes a
noise source?
Regards, Joerg

Regards,

Mike Monett
 
T

Tony Williams

Jan 1, 1970
0
Mike Monett said:
Increasing the bias reduces the capacitance, but there is no
data for the capacitance at the Zener voltage.
I had the distinct impression it was quite high in Zener mode.
Tony Williams might have much more info.

Not me Mike. When using zeners to clamp AC signals
I've only ever biassed-up the zener and used diodes
to isolate any below-clamp AC voltage from the zener.
 
F

Fred Bloggs

Jan 1, 1970
0
John said:
Most 5-volt zeners have tons of capacitance, 100 pF maybe. The b-e
junction of a BFS17 is about 1.7 pF at zero bias and seems to be a
very nice 5.5 volt zener.

John

Check out the various signal line surge clamp products. These have been
designed for low capacitance so as not to disrupt reception of signals
in the tens of MHz region and work at logic compatible voltage clamping
levels. The clamp level is probably not a precision specification.
 
M

Mike Monett

Jan 1, 1970
0
Tony Williams said:
Not me Mike. When using zeners to clamp AC signals I've only ever
biassed-up the zener and used diodes to isolate any below-clamp AC
voltage from the zener.

Good Morning, Tony. Thanks for the reply. I was thinking of some of
the marvellous posts you and Win supplied long ago. A little
searching found some of them.

Here's one from Win's post of May 7, 2002:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
from my post of 2Aug1997
/| 1N5245B at 50uA
200mV FS / |
/| / | /|
/| / | /| / | /| / | /|
/ | /| / | / | / | /| / | / |/ | /| _____
| / |/ | /| / |/| / |/ | / |/ | / |/ |
|/ | / |/ |/|/ |/ | /|/ |
|/ |/ |
/
scope trace, continuous events, 10us FS 14.41 V
average value


The trace above is from a zener diode that was a bit more noisy than
most. A type of relaxation oscillation is clearly seen, with the
diode's 285pF self capacitance charging from the 50uA current, until
a discharge event is triggered, which happens randomly with an
increasing probability as the average voltage is reached and
exceeded. The discharge step looks nearly instantaneous, stopping at
random voltages at or below the average voltage.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Win calcuates a diode capacitance of 285pF from the charging slope.

It was difficult to find a capacitance plot for the 1N5245B, which
is a 15V zener. Vishay has one at

http://www.ortodoxism.ro/datasheets/vishay/85588.pdf

Figure 6 shows the diode capacitance vs Zener voltage at 2V reverse
bias. Reading from the graph, the capacitance for the 1N5245B is
about 25pF. From Jeorge's post, the capacitance decreases with
increasing reverse bias, as one would expect.

However, Win's post seems to indicate the capacitance is much higher
in Zener mode.

The thread is at

http://groups.google.ca/group/sci.electronics.design/browse_thread/thread
/9f7d8166d41f94bb/94a90c1a0c4d1938

Very interesting and well worth reading.

Another thread that Win refers to is the 1997 thread on Zener Diode
Oscillation. I can't seem to figure out how to get the start of the
thread, but google provided this link that enters partway through:

http://groups.google.ca/group/sci.electronics.design/browse_thread/thread
/399b6a0bf6cfa4f5/99f63edb6894429f

Very well worth reading.

Regards,

Mike Monett
 
T

Tony Williams

Jan 1, 1970
0
Mike Monett said:
Here's one from Win's post of May 7, 2002:

The trace above is from a zener diode that was a bit more noisy than
most. A type of relaxation oscillation is clearly seen, with the
diode's 285pF self capacitance charging from the 50uA current, until
a discharge event is triggered, which happens randomly with an
increasing probability as the average voltage is reached and
exceeded. The discharge step looks nearly instantaneous, stopping at
random voltages at or below the average voltage.

Win calcuates a diode capacitance of 285pF from the charging slope.

Good grief. You have a memory like an elephant Mike.
I remember those zener noise threads, but completely
forgot Win's post above, using dV/dT to measure capacitance.

Useful way of measuring the C from 0v to Vz.
Have (say) a 50uA fixed pulldown, and a clocked 1mA
constant current pullup. Look at the dV/dT over
the voltage range and see how it varies.
 
M

Michael A. Terrell

Jan 1, 1970
0
Mike said:
[...]
I was never too fond of using zeners as noise sources. Those
little bulbs for watches are much nicer. It's just a matter of
finding one that presents a nice 50ohm source while glowing hot
enough. But not too bright, of course ;-)

I've never heard of that before. What kind of light bulb makes a
noise source?
Regards, Joerg

Regards,

Mike Monett

A fluorescent lamp generates noise at 4 GHz. I used to repair C-band
LNAs and LNBs in my shop at home. I didn't have a C-band signal
generator, so I would wave the repaired unit past the overhead lamp.
The noise was different than normal background noise, and the signal
level meter would show that the unit was working. Then, it was put on
the ten foot / three meter dish to see if it was OK. Most failures were
in the power supply section, so the noise figure wasn't affected by the
repairs.
--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
 
J

John Larkin

Jan 1, 1970
0
[...]
I was never too fond of using zeners as noise sources. Those
little bulbs for watches are much nicer. It's just a matter of
finding one that presents a nice 50ohm source while glowing hot
enough. But not too bright, of course ;-)

I've never heard of that before. What kind of light bulb makes a
noise source?

Any light bulb. A 100 ohm filament at 1500 C will generate about 3
nv/rthz wideband noise. Whereas a zener running at a mA will generate
about 300.

John
 
J

John Larkin

Jan 1, 1970
0
Tony Williams said:
Good Morning, Tony. Thanks for the reply. I was thinking of some of
the marvellous posts you and Win supplied long ago. A little
searching found some of them.

Here's one from Win's post of May 7, 2002:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


The trace above is from a zener diode that was a bit more noisy than
most. A type of relaxation oscillation is clearly seen, with the
diode's 285pF self capacitance charging from the 50uA current, until
a discharge event is triggered, which happens randomly with an
increasing probability as the average voltage is reached and
exceeded. The discharge step looks nearly instantaneous, stopping at
random voltages at or below the average voltage.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Win calcuates a diode capacitance of 285pF from the charging slope.

It was difficult to find a capacitance plot for the 1N5245B, which
is a 15V zener. Vishay has one at

http://www.ortodoxism.ro/datasheets/vishay/85588.pdf

Figure 6 shows the diode capacitance vs Zener voltage at 2V reverse
bias. Reading from the graph, the capacitance for the 1N5245B is
about 25pF. From Jeorge's post, the capacitance decreases with
increasing reverse bias, as one would expect.

However, Win's post seems to indicate the capacitance is much higher
in Zener mode.

The thread is at

http://groups.google.ca/group/sci.electronics.design/browse_thread/thread
/9f7d8166d41f94bb/94a90c1a0c4d1938

Very interesting and well worth reading.

Another thread that Win refers to is the 1997 thread on Zener Diode
Oscillation. I can't seem to figure out how to get the start of the
thread, but google provided this link that enters partway through:

http://groups.google.ca/group/sci.electronics.design/browse_thread/thread
/399b6a0bf6cfa4f5/99f63edb6894429f

Very well worth reading.

Regards,

Mike Monett

At higher currents the noise becomes almost gaussian.

John
 
J

John Larkin

Jan 1, 1970
0
Check out the various signal line surge clamp products. These have been
designed for low capacitance so as not to disrupt reception of signals
in the tens of MHz region and work at logic compatible voltage clamping
levels. The clamp level is probably not a precision specification.

Yeah, that would be more repeatable. Somebody (Central?) has some
multiple (quad?) esd zeners that are low pF. GE used to make a
wonderful symmetric zener in a plastic transistor can, D13T1 or some
funny number like that.

Sometimes a dual zener is just the thing to clamp an opamp. We're
playing with ideas for a frequency counter front-end circuit,
differential input over a huge voltage range, 1 MHz max maybe. We were
thinking about running each side through a nonlinear function
generator, vaguely a bidirectional log circuit (yes, I know that's
impossible) and then run them into a comparator with hysteresis.

John
 
J

Joerg

Jan 1, 1970
0
John said:
At higher currents the noise becomes almost gaussian.

That one word "almost" is what drove us to using light bulbs. Those
little watch bulbs are amazing. I thought they were designed to less
than one hour MTBF because they are only normally used for a couple of
seconds a day but they lasted forever.
 
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