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Old 1st November 2012, 05:41 PM   #1941
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Quote:
Originally Posted by kgrlee View Post
bobaruni, please don't be discouraged by Guru Wurcer's comments. He's really much nicer than his avatar.

I for one would be very interested in an LTSpice file of your upside down SK99b as it touches upon some Jurassic experience of mine on low feedback amps.
I'm not hurt by his comments as I did not design the SK99B or the Jensen Op Amp that it's based on, if there is something better (as I'm sure there is), I definitely want to learn how it works and build it

Anyhow, here is an attached LTspice ASC file using only the default models.

The THD is extremely low, not sure if this sim would truly reflect what is possible in real life but will find out next week when the parts arrive. Then there's the problem of how to test it!

1Khz at 1V, gain = 1, inverting mode.

Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 1.000e+03 1.000e+00 1.000e+00 180.00 0.00
2 2.000e+03 1.569e-08 1.570e-08 88.88 -91.12
3 3.000e+03 2.316e-08 2.316e-08 97.31 -82.69
4 4.000e+03 2.150e-08 2.150e-08 100.50 -79.50
5 5.000e+03 2.205e-08 2.205e-08 103.50 -76.50
6 6.000e+03 2.596e-08 2.596e-08 105.22 -74.78
7 7.000e+03 2.472e-08 2.472e-08 110.66 -69.34
8 8.000e+03 2.414e-08 2.414e-08 108.93 -71.07
9 9.000e+03 2.657e-08 2.657e-08 110.71 -69.29
Total Harmonic Distortion: 0.000007%

Bob.
Attached Images
File Type: png fft.png (36.4 KB, 342 views)
File Type: png schem.png (87.7 KB, 328 views)
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File Type: asc SK99B compliment.ASC (6.9 KB, 25 views)
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Old 1st November 2012, 06:06 PM   #1942
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Quote:
Originally Posted by bobaruni View Post
I'm not hurt by his comments as I did not design the SK99B or the Jensen Op Amp that it's based on, if there is something better (as I'm sure there is), I definitely want to learn how it works and build it


Bob.
Did I miss something? Nothing wrong with those circuits never said there was. We will show you how to measure THD.
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Old 1st November 2012, 06:23 PM   #1943
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I was planning on setting the gain to 100x or more then using a 24Bit 192k Soundcard to sample and perform an FFT, but I'm more worried about generating such a pure sine wave to inject into the thing.
Also, as I'm new to the world of LTspice, I'm not sure if the way I'm obtaining THD information is correct?
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Old 2nd November 2012, 01:23 AM   #1944
fas42 is offline fas42  Australia
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Quote:
Originally Posted by bobaruni View Post
Also, as I'm new to the world of LTspice, I'm not sure if the way I'm obtaining THD information is correct?
If you're getting numbers out of LTspice, and measuring the right node then there shouldn't be a problem. The main thing to think about is if the distortion figures seem to be too poor, which typically means that compression has been applied to the measurement results. The latter is set in the Control Panel or via the plotwinsize directive, in case you're not aware of this.

The reason for compression is that LTspice can create massively sized results files on the hard drive very easily, and this tames that capability if it makes no sense to record a high level of accuracy.

Frank
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Old 2nd November 2012, 01:34 AM   #1945
fas42 is offline fas42  Australia
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Quote:
Originally Posted by kgrlee View Post
Frank, we're in luck. Guru Wurcer likes us Ozzies. You can be Russell C and I'll be Mel G.
As in, 30 Odd Feet of Grunts ...

Quote:
Frank, how did you get on with your improved BF862 model?

Presently, I'm moving my *.asc to a970/2240, BF862 & J305 but its slow for this LTspice newbie.
Sorry, I'm a bit behind the 8 ball at the moment, will try and get that happening ASAP ...

Frank
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Old 2nd November 2012, 02:19 AM   #1946
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Originally Posted by vacuphile View Post
Thanks for making us all at the most just four degrees from Obama.
I used to work for J. Edgar Hoover's nephew, I'll let your imagination run with that one. For you Ozzies, J. E. H. violated rule number 3.
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Last edited by scott wurcer; 2nd November 2012 at 02:22 AM.
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Old 2nd November 2012, 05:14 AM   #1947
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Originally Posted by fas42 View Post
If you're getting numbers out of LTspice, and measuring the right node then there shouldn't be a problem. The main thing to think about is if the distortion figures seem to be too poor, which typically means that compression has been applied to the measurement results. The latter is set in the Control Panel or via the plotwinsize directive, in case you're not aware of this.

Frank
Ok, I have turned off compression, and now all the high frequency crud in the FFT has gone (I was worried this was some sort of oscillation) and the THD in the log is even lower. Thanks for your help Frank
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Old 2nd November 2012, 06:08 AM   #1948
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Originally Posted by RNMarsh View Post
general comment -- due to mismatched components in the input stage (in practice - including the driving source Z), a small amount of CM signal usually feeds thru to the output. Thus, my suggestion of >-100dB rejection.

So what else contributes to input stage cmr? In traditional (?) diff input, increasing Zo of the diff pair's current source helps (often via cascoded current sources and other such Zo increase methods). Here...? Thx-RNM
Scott -- what would you suggest could be done to get some more cmr out of the SW-OPA ? Shooting for 10-20 db more. Could you make a cmr test with cascoded current sources or what ever you suggest. Thx-RNM -somwhere in Bangkok.
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Old 2nd November 2012, 11:57 AM   #1949
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Default Twitchy SW-OPA?

I'm at the point where I'd like to compare my poor efforts at a better FET-990 with Guru Wurcer's SW-OPA. To do this, I'm trying to use a common set of SPICE models .. to whit BF862, J111 & J305 from Scott and also 2sc2240/2sa970 or 4401/3 from Scott, Cordell, fas42 or Staccatiss.

But I've come across a feature that fas42 encountered earlier. SW-OPA seems inordinately sensitive to the transistor models used and also their Iq.

The offset voltage changes up to more than 4V with different BJTs, FETs and small changes in Iq for input & output stages.

I'm using R3 to adjust the DC offset but this is not only overly sensitive but requires huge changes in value to accomodate different BJTs. R3 goes from 37R with fas42's 4401/3s to 20R with Scott's 2240/970s.

I gather, one of the hallmarks of this design is that the 125R resistors should have exactly 2x the current of the 250Rs but this doesn't seem to be the case .. some 5-8% imbalance from this ratio which sorta correlates with the imbalance in the input device currents ...

Though this behaviour is with a gain of 100x as shown in the attached GIF, the FET990 circuit is far more tolerant and output offset changes much less with different transistors & Iq.

Am I missing something or does this mean we have to match complements?

I enclose my *.asc & also the models used which you can see are taken from various gurus.

Quote:
I used to work for J. Edgar Hoover's nephew, I'll let your imagination run with that one. For you Ozzies, J. E. H. violated rule number 3.
Scott, your evil avatar is taking over
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Old 2nd November 2012, 02:55 PM   #1950
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Quote:
Originally Posted by kgrlee View Post
I'm at the point where I'd like to compare my poor efforts at a better FET-990 with Guru Wurcer's SW-OPA. To do this, I'm trying to use a common set of SPICE models .. to whit BF862, J111 & J305 from Scott and also 2sc2240/2sa970 or 4401/3 from Scott, Cordell, fas42 or Staccatiss.

Scott, your evil avatar is taking over
I'll help you out this weekend, and get you going (I have not had these problems). I found an old laptop to put LTSPICE on so I won't irritate our lawyers. The OS resistors depend on absolute Vbe a little I'll show you how to compute a close value (you only need the diode function on your handheld VOM).

BTW took two strips of 100 4401/4403 from ebay and find most matched better than 1-2mV Vbe.

Hopefully Lord Shiva will look favrably on our efforts.
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Last edited by scott wurcer; 2nd November 2012 at 03:06 PM.
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