Zener Vs PN diode difference in forward mode

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Apologies for the on-topic post.

I have a class B amplifier with complementary Darlingtons for output,  
and an op-amp driving the tied-together bases with -ve feedback from the  
output.  It's simple and works pretty well.

I was tinkering with LTspice to reduce the crossover distortion. I don't  
want class AB quiescent current, but putting three diodes between the  
bases to make the voltage 'gap' one junction rather than four and using  
pull up/down resistors improves things - less gap for the op-amp to  
cover, I guess.

However, using Zeners instead of ordinary diodes improves things still  
further as far as the simulation goes.

What are the fundamental differences between a PN diode and a Zener in  
forward conduction, or is this likely to be an artefact of the Zener models?

Cheers
--  
Clive

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

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Probably the zeners have smaller junctions than whatever regular
diodes you've selected. You could compare the forward drops.

Another way to reduce distortion is to add a base-emitter resistor. At
low levels, the driver stage gets to the load through the resistor; at
higher levels, the transistors help. This needs lots of loop gain to
work well.

This is fun too:

https://www.dropbox.com/s/2xx07q0muytdvpv/Opamp_Boosted.JPG?raw=1






--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Zener Vs PN diode difference in forward mode
On Wednesday, 12 July 2017 17:06:41 UTC+1, John Larkin  wrote:
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I'm surprised you use that. It's thermally unstable - ok if you keep the output trs cool, but that's inefficient use of parts for a linear amp.


NT

Re: Zener Vs PN diode difference in forward mode
On Wed, 12 Jul 2017 10:06:26 -0700 (PDT), snipped-for-privacy@gmail.com wrote:

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What's thermally unstable about it?


--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: Zener Vs PN diode difference in forward mode
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1. Assume class AB conditions (and all that implies).
2. Consider Vbe(T) dependence.

Indeed, because of the particular form of #2, such a circuit cannot be  
stable; it'll always have a runaway point.  The best you can hope for is  
making that temperature out in the destruction region (150C+), but that  
doesn't prevent runaway, that only avoids it.  Besides, you can't do that  
/and/ have class AB operation.

If there exist op-amps with supply bias IPTAT, you'd be set, but I haven't  
seen any that behave anywhere near that.

Another fundamental view: there's simply no feedback between output current  
and op-amp drive.  If they were at least mirrors rather than CE BJTs, there  
would be a weak claim of that, if not a properly closed loop.  As shown,  
output current is unbounded and hFE dependent, a huge no-no.

It's an old circuit, but that doesn't make it a good circuit.  The 555 and  
741 are old circuits, too. :^)

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 Wed, 12 Jul 2017 17:52:31 -0500, "Tim Williams"

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Vbe on the transistors should be small, maybe a couple of tenths of a
volt, at no output. The transistors are quiescently OFF. The opamp
drives small swings all by itself.


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It has no possibility of runaway, if it's done right.

 The best you can hope for is  
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I don't know what "class AB" means in this context, except that it
sure sounds like I don't want it.

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The transistors turn on exactly as hard as they need to, when they
need to, to help the opamp drive the load. The load feeds back,
through the opamp output back to the base drives.


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I sold a couple thousand NMR gradient drivers with a very similar
circuit, power fets driven from opamp supply pins. Worked great.


--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: Zener Vs PN diode difference in forward mode
On Wed, 12 Jul 2017 16:54:19 -0700, John Larkin

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Tim totally misses the 'feedback' loop.  I won't define it... let's
see if Tim can get it >:-}

(I used a one-sided version of this, with uA741's no less, to make
linear regulators for the TOW missile, 1970-73)

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                                        ...Jim Thompson
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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Re: Zener Vs PN diode difference in forward mode
On Wed, 12 Jul 2017 17:02:48 -0700, Jim Thompson

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AC stability needs to be considered, of course. Minor detail.


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It works sort of like a very precise Sziklai pair, or two of them
actually.

The helpers could be Darlingtons, too. Or, in the 21st century,
mosfets. Or cascodes. Lots of variants.


--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: Zener Vs PN diode difference in forward mode
On Wed, 12 Jul 2017 17:41:31 -0700, John Larkin

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[snip]
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[snip].
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Yep.

[snip]
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Exactly.


My TOW version used a PNP Darlington.
        
                                        ...Jim Thompson
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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Re: Zener Vs PN diode difference in forward mode
On 07/12/2017 07:54 PM, John Larkin wrote:
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Yeah. If it _were_ biased class AB it would be thermally unstable,  
because even with DC voltage feedback around the whole thing there's no  
way to bias the booster stage quiescent current reliably as drawn; the  
collector current could be anything. You'd have to add diodes in the  
supply lines and emitter resistors - which would compromise the output  
swing.

Whew. Good thing it isn't!



Re: Zener Vs PN diode difference in forward mode
On 07/12/2017 08:32 PM, bitrex wrote:

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AND compromise the PSRR of the driver unless you used some kind of  
boostrapping back to both bases

Re: Zener Vs PN diode difference in forward mode
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Derp, there's two variants of this, one with the output _grounded_, that I  
had in my head for some reason!

In any case, I correctly answered "why it might be unstable", and you  
correctly noted that might be undesirable. ;-)

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 Wed, 12 Jul 2017 20:05:01 -0500, "Tim Williams"

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That's one. Or the output can go to a resistor to ground. Or to a
voltage divider from the output, which provides higher voltage out but
some feedback. But none of those allow the opamp to drive the load
directly at low level, which brings back the class-AB biasing dilemma.

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Here's the output stage from one of my old gradient drivers. The load
is a coil, so we want to be a current source.

https://www.dropbox.com/s/a2m9wfab8hh3rrc/Chimera_Out.jpg?raw=1

Vaguely the same idea, the signal path being from an opamp's supply
rails.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Zener Vs PN diode difference in forward mode
On Thursday, 13 July 2017 04:04:53 UTC+1, John Larkin  wrote:
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Your comment about be being way below 0.65v quiescent tells me you're running class B. Like that it can be stable enough. If you ran it AB as I thought you were it would be very unstable. Why...
1. Trs idle with 0.65v Vbe, much of opamp's Iq going through the Rs, some to tr bases.
2. As Trs warm up under exercise, their Vbe drops
3. You then have more Ib and less I through the psu/base R
4. So the trs turn on more, get hotter and turn on more. Vicious cycle with no way to reduce quiescent current or overlap of drive to the 2 trs.


NT

Re: Zener Vs PN diode difference in forward mode
On Thu, 13 Jul 2017 01:28:33 -0700 (PDT), snipped-for-privacy@gmail.com wrote:

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If you really want to do it wrong, you can.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Zener Vs PN diode difference in forward mode
On Wednesday, July 12, 2017 at 11:04:53 PM UTC-4, John Larkin wrote:
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That's interesting, thanks!  Can you tell me a little about  
what Q6 and Q7 are doing.  

(The feedback with Q3 and Q9 are confusing too.)

George H.  
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Re: Zener Vs PN diode difference in forward mode
On Thu, 13 Jul 2017 07:54:14 -0700 (PDT), George Herold

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They allow the opamp to run within its voltage rating, but still shoot
its supply currents up to the +-16 volt rails.  

  
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Those just let us shut down the output stages when the customer wants
zero magnetic field, like when he's shimming the superconductive
magnet.

D6 and D7 add an output deadband that further reduces current and
noise when we want zero field. NMR has parts-per-billion sensitivity,
so tiny zero offsets matter.

We don't do NMR instrumentation any more. Agilent acquired our
customer, Varian, and eventually killed off the NMR operation. I think
other analytical chemistry techniques have mostly replaced NMR; the
big magnets were really expensive to buy and operate. Agilent also
killed off the Varian FTMS products, which needed even bigger magnets.
We were developing a cool FTMS controller too.

https://www.dropbox.com/s/5y21u1wjh4sg66d/IMG_0304.JPG?raw=1

https://www.dropbox.com/s/sgoiuyuv3o6gbmt/DSC03578.JPG?raw=1

Science is fun, as long as you don't have to do the
grad-postgrad-PhD-postdoc-publish thing yourself.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Zener Vs PN diode difference in forward mode
On Thursday, July 13, 2017 at 11:41:23 AM UTC-4, John Larkin wrote:
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Thanks...those diodes confused me too.    
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I met a few people laid off from Varian's NMR group.
  
I know about quadrupole* mass spectroscopy.. kinda cool  
how it all works.   I'm not sure about the Fourier transform part.    
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I suck at writing, but like to make pretty pictures for publication.  
The worst part of research is grubbing after the money.  

George H.  
*I want to spell that quadrapole, quadrupole sounds like  
you've multiplied your mass spectrometers by four.  
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Re: Zener Vs PN diode difference in forward mode
"George Herold"  wrote in message  
On Thursday, July 13, 2017 at 11:41:23 AM UTC-4, John Larkin wrote:
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I met a few people laid off from Varian's NMR group.

I know about quadrupole* mass spectroscopy.. kinda cool
how it all works.   I'm not sure about the Fourier transform part.
Quoted text here. Click to load it

I suck at writing, but like to make pretty pictures for publication.
The worst part of research is grubbing after the money.

George H.
*I want to spell that quadrapole, quadrupole sounds like
you've multiplied your mass spectrometers by four.
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=============================================================

I'm bored so here's a very brief intro to FTMS.  It is based on ion  
cyclotron resonance.  Start with an ion in a vacuum in a uniform magnetic  
field.  In the direction of the field there is no force on the ion so it is  
free to keep drifting with whatever velocity it started with.  Perpendicular  
to the magnetic field the ion experiences the usual E cross B force, making  
it's path curve.  If the B field is strong and the velocity low relative to  
the mass to charge ratio m/q the path becomes a circle and you can show that  
the time to traverse that circle is 2pi m / q B or the cyclotron frequency  
in hertz is q B / 2 pi m.  Put the ion in a box and bias the ends that are  
perpendicular to the magnetic field with a few volts of the same sign as the  
charge on the ion relative to the other walls and you get a potential well  
inside the box that traps the ion along the magnetic field, and the magnetic  
field does the trapping along the other two axes so now you have a trapped  
ion cell.  Apply rf perpendicular to the magnetic field and the ion will  
absorb energy and increase its cyclotron radius.  If there is more than one  
ion of the same m/q they will each move in phase with the applied rf, and if  
you pump them up to a final cyclotron radius that is much larger than the  
initial radius before the excite they will each have approximately the same  
final cyclotron radius and the cloud will orbit coherently.  Pick a point on  
one of the cell walls perpendicular to the magnetic field and as the cloud  
approaches and then recedes it will induce an image charge that can be  
detected.  The simplest trap is a cube, with six sides.  Two end plates do  
the axial trapping, one pair of opposing side plates is used to apply the  
excitation rf, and the other pair of opposing side plates is connected to a  
differential preamp to record the signal.  For magnets in the 1-7 tesla  
range and m/q ratios in the 18 to say 10,000 range the frequencies are in  
the 1-5 kHz to 5-10 MHz range, and for ion populations of say 1000-1000000  
the raw signal is in the microvolt range with cyclotron radii of .5 to 2 cm.  
In the second picture John posted the tubular object with the ruler next to  
it is a trapped ion cell.  There are two end segments 2" long and a central  
segment 3" long.  The end segments can be simple cylinders but are often  
split into four quadrants, and the central section is split into four  
quadrants to provide the excite and detect plates.  Cells can be cubic,  
rectangular, cylindrical, and other shapes, and the length to diameter  
aspect ratio can vary also, and that discussion fills books and fuels  
arguments.  FTMS supercon magnets usually have room temperature bores of  
4-6" so by the time you stuff a vacuum chamber into one a rectangular cell  
winds up maybe 1.5 to 2.5" across and a cylindrical cell maybe 2-3"  
diameter.

Anyway, if there is just one m/q in the trap then you will get a simple sine  
wave after the excite.  The amplitude tells how many ions and the frequency  
tells the m/q.  As the ions collide with background gas the amplitude will  
exponentially decay as individual ionis are knocked out of phase coherence  
or into plates, with a time constant of about 3 seconds at 10-8 torr.  
Record the damped sine wave, FFT it, convert Hz to m/q, and there's a mass  
spectrum.  In FTNMR the excitation is a pulse of single frequency rf so the  
final phase is linear and can be easily corrected but here the frequency  
range is sufficiently large that the simplest excitation is a swept sine  
wave, which results in a quadratic phase function after the FFT so the usual  
answer is to use the magnitude spectrum.   Excite chirps are in the range of  
0-3 MHz in 1-5 msec applied differentially to the trap so maybe 50-400 volts  
pp across the trap.

It's been a while since I've given an "intro to ftms" and I tried to keep it  
short and terse, but I hope it answers the basics.  Haven't touched one  
lately but counting grad school I spent about 30 years working with them,  
building two completely from scratch, building several different vacuum  
chambers, ionization sources, and transport ion optics for others, along  
with a good bit of electronics, computer interfacing, and software to go  
along with them so if you have any questions just ask :-).

--  
Regards,
Carl Ijames



Re: Zener Vs PN diode difference in forward mode
On Thu, 13 Jul 2017 13:28:53 -0400, "Carl Ijames"

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Nice summary.

I wanted to detect a single molecule orbiting in the cell, which I
think is barely possible. Agilent killed the product line (IonSpec
originally, acquired by Varian, acquired by Agilent, killed by
Agilent) so I didn't get to try.


--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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