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Old 29th October 2004, 03:19 PM   #631
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Aha. I'm not sure whether there is any connection though between stability with cap load and the ability of the output to absorb impressed junk like RF and EMI. Is this with an open-loop buffer?

Jan Didden
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Old 29th October 2004, 03:35 PM   #632
PMA is offline PMA  Europe
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Closed loop (visit my web by clicking the button downstairs). I only mentioned the stability just not to suspect oscillations with capacitive load.
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Old 29th October 2004, 04:12 PM   #633
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Pavel,

what kind of cable do you use from your buffer output?

Have you (or anybody else for that matter) tried anything else than an output resistor component with good result?

Michael
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Old 29th October 2004, 05:16 PM   #634
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Quote:
Originally posted by lumanauw
But NE5532 have an interesting property. Once I series 8 of these opamps in buffer mode. The final result, the array of these 8 buffers with NE5532 gives no (little) difference with the real input sound. With other opamps, the difference is bigger, so other opamps are"making" their own sound, instead of reproducing what is fed to them.
Hi Lumanauw

You nail it...output equal to the input...this is the essence of the null test.

Try to do a null test between a NE 5532 and the OPA 2604...you will be surprised.

The sound of a series of amps ,tell us much about the linearity of each one .As the no linearity adds and become much more audible.

After all , because of the problem of the added distortion in a series of amps ,that was a problem in the repeaters amps in the telephone lines, that Harold S. Black bring us the Negative Feedback.
Great minds think alike!!...

More about op amps here:

http://fnt-www.ss.titech.ac.jp/~haji...rtion/opa.html

Cheers
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Old 29th October 2004, 05:27 PM   #635
jcx is online now jcx  United States
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Default are we losing track of our reasoning?

returning to input diff pair distortions (really, really consider starting new threads, this thread is way unwieldy with just input stage distortion discussion going on)

To illustrate the some points about the relative gain/distortion tradeoffs of bjt vs fet diff pair inputs I’ve simulated a perfectly linear model amp with a un/degenerated bjt diff pair vs a fet diff pair input

In this sim I am trying to eliminate everything except diff pair input stage gm nonlinearity, the current mirror (V2, F1) nearly eliminates 2nd order nonlinearity, to exclude dynamic nonlinearity from bjt/fet nonlinear junction C I scaled frequency down, feedback impedance is low to eliminate base current effects

With the current mirror the gain intercept for this sim is 436 Hz for the bjt diff pair (closed loop gain = +14, GBW = 14 * 432 Hz = 6.1 KHz, just scale up frequencies by ~1-2K and C1 down by the same ratio to get back to typical audio amp numbers)

The test signal is a 10 Hz sine, amplitude modulated with a 1 Hz sine (“unbalanced” to preserve the carrier this time ‘round) – BTW, I believe this would be a good sim approach to Gilbert’s analysis, similar GBW ratio, slightly more gain and the amplitude of the test signal is “automatically” swept by the AM modulation – no need for andy_c’s math sw skills, anyone competent to seriously modify or design their own audio amps should be able to use spice at this level, feel free to modify and examine whatever phase/IMD ratio you want in (free) Lt Spice
Click the image to open in full size.
The first fft is an “unfair” comparison, just dropping in fets in place of the bjts isn’t a clear distortion win (the unfairness comes from the loop gain reduction with lower fet gm)
Click the image to open in full size.

The second fft evens up the loop gain/GBW by decreasing C1 for the fet input circuit, now the fet looks pretty good – but I maintain this is still an unfair comparison, we just added nearly 20 dB loop gain to make up for the fet’s low gm
Click the image to open in full size.

A final, more even-handed comparison adds 210 Ohm degeneration Rs to the bjt emitters which lowers the diff pair stage gm to the same level as the fet input while providing local linearizing feedback, and adds the same loop gain to both by setting C1=C2= 110n
Click the image to open in full size.
Note that to even see any higher order distortion in the degenerated bjt stage I’ve extended the dB scale down to -160 dB (a tiny spot of green shows @ -160 dB, 50Hz - the fifth harmonic)

I think this shows you need to consider device gain, local and global feedback together to evaluate the roles of nonlinearity vs gain, bjts have more gain (when gm is the important parameter) than fets, enough more gain that local feedback linearization can give better distortion results with the bjt devices at equivalent gm
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Old 29th October 2004, 06:21 PM   #636
MikeB is offline MikeB  Germany
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Hi jcx !

Nice demonstration, but maybe you should have picked a more
representive jfet. Why did you use p-channel-jfets ?
I suggest you turn the circuit upside-down, and use a nchanneljfet
like the sk170. It still has lower gm than most bjts, but only most.
(And of course use the complementary of the 2n5087.)
If i remember correct, you need RE's < 10ohm for bjts to get same
gm. Now you could say, ok let's use one of best bjts for this test,
for example the mpsa18. Hmpf... would be interesting, sk170 against mpsa18 ?
I'm at work now and can't simulate.

Mike
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Old 29th October 2004, 08:07 PM   #637
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Quote:
Originally posted by MikeB
Why did you use p-channel-jfets ?
MikeB, it's N-channel. You just need to turn your head upside down

Quote:
Originally posted by jcx
The second fft evens up the loop gain/GBW by decreasing C1 for the fet input circuit, now the fet looks pretty good – but I maintain this is still an unfair comparison, we just added nearly 20 dB loop gain to make up for the fet’s low gm
That was the case I was thinking of in my earlier conversation with Jan. Lowest noise case, equal gain-bandwidth product. Seems fair to me. Sounds like the usual "reduce gm and reduce Ccomp for the same gain-bandwidth product" trick. Although I guess if the compensation cap is very small to begin with, this isn't possible.

As usual, another interesting and informative post from jcx.
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Old 29th October 2004, 08:40 PM   #638
ingrast is offline ingrast  Uruguay
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Default Re: are we losing track of our reasoning?

Excellent job jcx, now:

Quote:
Originally posted by jcx
I think this shows you need to consider device gain, local and global feedback together to evaluate the roles of nonlinearity vs gain, bjts have more gain (when gm is the important parameter) than fets, enough more gain that local feedback linearization can give better distortion results with the bjt devices at equivalent gm
Though this is true from a systems viewpoint, please let me suggest this test does not compare bjt vs. fet gm. We might say - borrowing from tube nomenclature - you are comparing mu (voltage gain) for you are loading collector / drain with high impedance.

A true gm test should load collector / drain with short circuits and should measure current (i.e. input voltage vs. output current). Probably a more accurate - and realistic considering popular circuit topologies - could be a cascode configuration where the low impedance loading approaches the short circuit condition.

Rodolfo
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Old 29th October 2004, 08:54 PM   #639
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Default Re: are we losing track of our reasoning?

Quote:
Originally posted by jcx
[snip]To illustrate the some points about the relative gain/distortion tradeoffs of bjt vs fet diff pair inputs I’ve simulated a perfectly linear model amp with a un/degenerated bjt diff pair vs a fet diff pair input[snip]

JCX,

This is a very revealing post! Great job, wish I could have done it...
Anyway, it does show that naked device properties are interesting, but the real issue is how they behave in the context of the circuit topology.

Jan Didden
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Old 29th October 2004, 09:36 PM   #640
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Quote:
No, if we remember the Taylor series for e^x, it is:

e^x = 1 + x + (x^2)/2! + (x^3)/3! + ...,

or the summation of (x^n)/n!, n = 0 to infinity. Both even and odd terms are present.
Oops! You are absolutely right, I was totally wrong. Mixed it all up with the Taylor series for arctan.

Nevertheless, the BJT produces "nastier" distortion components than the FET from an human hearing point of view. Unless minimized by feedback...

/M
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