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Old 9th March 2013, 05:02 AM   #2801
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Quote:
Originally Posted by kgrlee View Post

I don't think you will gain anything with 1) 2) 3) or 6)
...
But you can't do 7) Omitting your D6 gives really nasty overload. BAV199 won't do much to help. It's not leakage but modulation of the junction capacitance that introduces the distortion. Unfortunately, ANY diode in that position will give THD only about that of the original 990.
According to LTSpice, both 2) and 3) improve matters - not much, maybe a few dB each, but mostly orthogonal, so they add up. The caveat: my THD20 sims are at unity gain, so the effect of the Vds modulation on a JFET LTP CCS is more pronounced (Vds swings are of the same amplitude as the output), so the LTP CCS cascode definitely helps. I'm not so sure about the mechanism of the distortion reduction with Wilson CM, but it probably reduces H2 a bit. You're welcome to check both in LTSpice.

Regarding 7), almost all the Spice diode models on the net seem to be broken with respect to reverse leakage and reverse-biased junction capacitance. However, I did find a Zetex BAV99 (not BAV199) model which does give order-of-magnitude lower reverse-leakage, both DC and incremental, than a 1N4148 or similar, but YMMV - it may be broken too. I got significantly lower THD20 with this model.

Another possibility that occurred to me is to use two high reverse-breakdown voltage diodes in series - e.g. MMBD7000 or similar, for the anti-latching diode, thus halving the junction capacitance as well as the reverse voltage across each diode. Not sure if it will work correctly for its primary purpose, i.e. anti-latching.
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File Type: txt bav99.txt (465 Bytes, 18 views)
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Old 9th March 2013, 05:14 AM   #2802
jcx is offline jcx  United States
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Bob Pease related a story about anti-latching diodes in that position killing gain due to their zero-bias conductance - recommended small signal bjt B-E junction as superior to most diodes

in my composite op amp circuits I have used series diodes with bootstrapped center node
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Old 9th March 2013, 09:52 AM   #2803
dadod is offline dadod  Croatia
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Quote:
Originally Posted by RNMarsh View Post
Do you have a circuit that will be -140 THD+N ? Or greater than -130?
Need it for an ultra low distortion signal generator.

Thx-RNMarsh
You can buy this one.
Damir
Ultra Low Distortion 0 00002 1kHz Sine Generator Assembled and Tested PCB | eBay
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Old 9th March 2013, 10:07 AM   #2804
bcarso is offline bcarso  United States
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Quote:
Originally Posted by jcx View Post
in my composite op amp circuits I have used series diodes with bootstrapped center node
Now we're talking. Or some other active bias, to remove the capacitance for the most part.
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Old 9th March 2013, 05:50 PM   #2805
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by dadod View Post
I bought Viktor's first unit. Fine THD but noise level is higher than I'd like and very limited in frequency selection. Its the noise level now which gets in the way of seeing the harmonics most of the time. The THD+N number (without FFT averaging to remove noise) is what i am looking at. I did a simple mod on a KH4402B ultra low distortion generator and it has selectable freqs.... now typical 1KHz thd+N is .00015%. others are better but noise is limiting them. Looking for an opamp-like circuit that can be used in other ultra-low thd+n oscillators. So far the ShibaSoku generator has the lowest noise by 10 dB and as good as Victors THD. I am up against an analyzer noise floor around -150-160 re 1v. So there is room to go lower in thd plus noise (THD+N) from generators/sources.

Thx-RNMarsh

Last edited by RNMarsh; 9th March 2013 at 06:00 PM.
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Old 9th March 2013, 06:39 PM   #2806
dimitri is offline dimitri  United States
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Richard,
You are talking noise floor around -150-160 re 1v all the time. Opamp with 1nV/sqrtHz (unity gain) will produce -137dB ref 1V in 20kHz BW. What is your BW?
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Old 9th March 2013, 06:44 PM   #2807
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by dimitri View Post
Richard,
You are talking noise floor around -150-160 re 1v all the time. Opamp with 1nV/sqrtHz (unity gain) will produce -137dB ref 1V in 20kHz BW. What is your BW?
yes, all the time. I am attempting upgrading the analyzer for lower output noise from its residual monitor output at this time to see if this helps. Another -6dB even would be useful to me. Thx-RNMarsh
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Old 9th March 2013, 08:34 PM   #2808
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RNM

You did not answer the questions asked by Dimitri.

The way I see it:

You need to be more specific.
The first thing to do is to set up a complete specification, so there is something to work out from.

It is certainly possible to design a circuit with THD + N in the-150dB to-160dB region, but it depends on your specifications.

BTW: with specifications in the area which it seems like you're talking about, you can forget about any circuit that resembles the SW-OPA, it is not good enough neither when it comes THD nor N.

S
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Old 9th March 2013, 11:35 PM   #2809
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Quote:
Originally Posted by jcx View Post
...in my composite op amp circuits I have used series diodes with bootstrapped center node
Thanks, great idea and works even in rudimentary form - bootstrapped a single diode-drop below the emitter of the emitter follower. A dual-diode bootstrapped from the emitter of the VAS may do better - I'll check it later.

The simulation schematic below gets unity-gain THD20 down to about -135 dBr at an output amplitude of 4V into 600R - within shouting distance of -140 dB. Biasing the output stage to Class-A and a few other tweaks will probably help close the gap.

Unfortunately, I didn't squeeze the LTP Wilson CM into the board I'm expecting back shortly. However, the bootstrapping mod requires just two parts, and can probably be kludged in by piggy-backing the diode and resistor with some super-glue, etc.

Edit: At an output amplitude of 1V into 600R, H2 comes in at an even -150 dB, exceeding RNM's requirement comfortably, even with the Class-AB output stage biased at 3.5 mA.
Attached Images
File Type: jpg je990_bootlatch_classab.jpg (84.0 KB, 282 views)
File Type: jpg je990_bootlatch_classab_4v600r_fft.jpg (60.5 KB, 280 views)

Last edited by linuxguru; 9th March 2013 at 11:47 PM. Reason: addendum
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Old 10th March 2013, 04:04 AM   #2810
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Quote:
Originally Posted by linuxguru View Post
Regarding 7), almost all the Spice diode models on the net seem to be broken with respect to reverse leakage and reverse-biased junction capacitance. However, I did find a Zetex BAV99 (not BAV199) model which does give order-of-magnitude lower reverse-leakage, both DC and incremental, than a 1N4148 or similar, but YMMV - it may be broken too. I got significantly lower THD20 with this model.

Another possibility that occurred to me is to use two high reverse-breakdown voltage diodes in series - e.g. MMBD7000 or similar, for the anti-latching diode, thus halving the junction capacitance as well as the reverse voltage across each diode. Not sure if it will work correctly for its primary purpose, i.e. anti-latching.
The reason I know the 990 style anti-latch diodes in degrade performance is I've tried it in real life on other stuff

I'm no SPICE guru so can't tell you if BAV99.txt correctly models junction capacitance modulation.

First guy to try a BAV99 in a similar (1ppzillion THD) circuit for real and reports the results compared to collector/base of a high voltage, low Ccb driver trans. gets to go diving on the Great Barrier Reef in Cooktown with me.

But if you want a REALLY good diode in that position, use the collector/base junction of the generic pnp or npn in LTSpice. But I can't seem to find these at Digikey, Mouser or even eBay
_______________

Quote:
Originally Posted by jcx
in my composite op amp circuits I have used series diodes with bootstrapped center node
Have you tried this for real? Bootstraps are wonky on overload ... which is why we have the anti-latch diode in the first place. Can you post a sketch or preferably a LTSpice model?

Linux, could you do the same with your latest incarnation? I'm not sure how you two have applied the bootstrapping.
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