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low capacitance zener

Discussion in 'Electronic Design' started by John Larkin, Apr 19, 2007.

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  1. John Larkin

    John Larkin Guest

    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.

  2. Jamie

    Jamie Guest

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

    Joerg Guest

    Yeah but are you sure next month's shipment will be close enough to that
  4. Mike Monett

    Mike Monett Guest

    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.


    Mike Monett
  5. John Larkin

    John Larkin Guest

    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.

  6. Joerg

    Joerg Guest

    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.

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

  7. Mike Monett

    Mike Monett Guest

    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 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.


    Mike Monett
  8. Joerg

    Joerg Guest

    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:

    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.
  9. Mike Monett

    Mike Monett Guest

    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.

    Mike Monett
  10. Joerg

    Joerg Guest

    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.

    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 ;-)

    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

  11. Mike Monett

    Mike Monett Guest

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

    Mike Monett
  12. 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.
  13. Fred Bloggs

    Fred Bloggs Guest

    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.
  14. Mike Monett

    Mike Monett Guest

  15. 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.
  16. 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
    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
  17. John  Larkin

    John Larkin Guest

    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.

  18. John  Larkin

    John Larkin Guest

  19. John  Larkin

    John Larkin Guest

    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.

  20. Joerg

    Joerg Guest

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