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Re: Zener Vs PN diode difference in forward mode
On Wed, 12 Jul 2017 16:56:13 +0100, Clive Arthur

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Try some manufacturers' models for the zener instead of the
hokey/idealized LTspice model.
        
                                        ...Jim Thompson
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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Re: Zener Vs PN diode difference in forward mode
Clive Arthur wrote:
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   Easier to have tracking temperature compensation with diodes; zeners  
go the opposite way.


Re: Zener Vs PN diode difference in forward mode
On Wednesday, July 12, 2017 at 8:56:31 AM UTC-7, Clive Arthur wrote:
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A bit of emitter resistance is usual, also.

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A few important differences: the low-voltage Zener mechanism has a similar
temperature coefficient to your Darlingtons' base bias,  but 5V and up
Zeners operate on avalanche (wrong sign of tempco).   There's lots of diodes
available, fewer choices for Zeners, fewer package options.  Best performance requires your
compensation diodes to be heatsinked right next to the Darlingtons.

Most important, the spec sheets of a typical diode don't cover forward voltage
(it's not a controlled parameter) and you really WANT control down below
a tenth of a volt (or an order of magnitude in bias current).  You'll want to
have a trim resistor and a procedure for setting the bias.




Re: Zener Vs PN diode difference in forward mode
wrote:

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Another trick is to use big emitter resistors and some moderate
quiescent current, and put big diodes across the emitter resistors. No
need to tweak Iq and no chance of thermal runaway.


--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: Zener Vs PN diode difference in forward mode
On Wednesday, July 12, 2017 at 10:26:33 PM UTC+2, whit3rd wrote:
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https://en.wikipedia.org/wiki/Rubber_diode

shows you how to get the forward voltage you want - R1 and R2 should have a pot in between them. Obviously the Vbe drift with temperature gets multiplied by the same ratio as the Vbe drop, so you don't want too much multiplication.

Transistors do tend to have tighter Vbe specs than most diodes.

--  
Bill Sloman, Sydney

Re: Zener Vs PN diode difference in forward mode
On 13/07/2017 08:55, snipped-for-privacy@ieee.org wrote:
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That works very well, at least in simulation, thanks.

Cheers
--  
Clive

Re: Zener Vs PN diode difference in forward mode
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Models.  Zeners are usually much /larger/, so have lower Vf.  They also have  
thinner junctions (well, depending on which Vz you've got), reducing Vf even  
further.  Test a few with a DMM and compare. :)

(Large by area, that is.  Check the C vs. Vz plot in the On Semi 1N4733 etc.  
datasheet, for instance.)

Ideal is a couple diode-strapped (C to B) BJTs, running at similar current  
density as the output transistors.  That will give nearly the same Vbe, and  
can be rearranged to show that it is, in fact, a current mirror  
configuration, which should give you ideas about biasing, sizing the VAS,  
controlling thermal drift and so on. :)

Tim

--  
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
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Re: Zener Vs PN diode difference in forward mode
On Wednesday, 12 July 2017 11:56:31 UTC-4, Clive Arthur  wrote:
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The ESR of the Zener and PN Diode are related to the inverse of their power ratings and should match the power rating of the power rating of the R_be string being biased to match the string of  ESR's or Zzt's.

Also critical for thermal stability is thermally coupling to track NTC Shockley effects of bias diodes to load diodes.  

 There's more to this but, that's the main criteria, not the arbitrary comparison of a Zener vs an ordinary diode.

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