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Old 4th September 2012, 09:29 AM   #671
bcarso is offline bcarso  United States
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Originally Posted by fitzfish View Post
Good applications examples to show the other possible limitations of a discrete op amp as well. PSRR, noise, and offset in higher gain applications are interesting challenges with such simple circuits.

Dave
Now you are getting practical! These other aspects are often viewed as an afterthought, some of which that can be brushed aside by invoking a better power supply etc. Considering them at or near the outset is wise.

Another consideration is to look throughout, especially near the input stage, at how much signal swing changes power dissipation, i.e., thermal distortions, and also how the design responds to more-than-zero source impedance. And --- how does it handle overloads?
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Old 4th September 2012, 09:31 AM   #672
EUVL is offline EUVL  Europe
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My final choice of components before building, after n-rounds of Spice simulations, are, based on the Kaneda / Constellation topology :

2SK2145 or 2SK3320 (essentially same device, different package)
SMM5P15FU
DMN601DMK


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Old 4th September 2012, 12:27 PM   #673
zinsula is offline zinsula  Switzerland
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Originally Posted by fitzfish View Post
[...]Since no one has jumped in with suggestions I have a couple for the gain half–
[...]
Dave, I found your proposal very interesting.
But now, what circuit are you talking about?

Generally, are we talking about several circuits, every one heading for another direction?
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Old 4th September 2012, 01:36 PM   #674
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Please, don’t shoot yourself.
Where's the fun in that?? I was referring to that old joke..""of all the things I've lost in my life, I miss my mind the most.""
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At the end of a week with no food (only nutritious intravenous injection) I started having illusions. The mirage was black olives and slices of bread flying over my head. I was tempted to raise my hands to reach them.
For me, it would be green olives stuffed with blue cheese...

I've a friend with JC disease..expectation is less than a year left, his demeanor changes several months ago leading to the diagnosis.

Loss of mind is very bad indeed.

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With the model you are proposing, I am afraid our thoughts will never meet. There has to be a capacitor (at least one) in the model, for the element of time (rate of heat transfer) to be addressed.

George
The thermal model requires DC stability, DC identical, and a way to fast couple devices which should be tracking one another. You're two examples were incongruent as you they had different dissipation ratio's. As you described, the two example would certainly respond differently, but you ignored the power change.

The subject of thermal modelling is a very good one to have when dealing with discretes, as the IC guys cheat by putting tracking devices nanometers apart, something we have to deal with differently for an assembly.


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[I think i pithed my pants]
I believe you meant panthd.

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Old 4th September 2012, 02:15 PM   #675
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I think a few of you guys need to take the tone down a notch or two...
I agree, please resist promoting one solution over another. Let's stick with real performance issues if possible.
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Old 4th September 2012, 04:00 PM   #676
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Brad ,

Lets have a look - i 've hoarded enough J74/K170 and (J72/K147) for just such a day. Wish it was 30 year old Ardberg, but that's another cupboard .......

Last edited by ticknpop; 4th September 2012 at 04:03 PM.
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Old 4th September 2012, 04:24 PM   #677
bcarso is offline bcarso  United States
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Brad ,

Lets have a look - i 've hoarded enough J74/K170 and (J72/K147) for just such a day. Wish it was 30 year old Ardberg, but that's another cupboard .......
I know how you feel. My one lone bottle of 1971 Petrus, which has at least enjoyed continuous ideal storage, is now valued so highly by some that it is truly difficult to decide whether to drink or merely sell.

Well, to the amplifier, I'm reminded of The Hitchhiker's Guide to the Galaxy, when Deep Thought announces that it's ready to reveal the answer to life, the universe, and everything, and says "You're not going to like it" (recollection of the BBC serialized version of Doug Adams' multimorphing creation).

What I should really do is show the performance as a function of parts, as it quickly exceeds the eight of Richard's headphone buffer just getting started, as it gets steadily "better", at least in eliminating various effects like thermal distortions, and reducing noise. I also should do some sims of settling time after the renewed interest in that topic in another thread. I suspect the compensation needs to be tweaked to instill better single-pole-approximating behavior.

Soon. I actually have some paying work right now, so that's going to take priority for the moment.
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Old 4th September 2012, 04:47 PM   #678
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1971 Petrus? I'll be right over, that will remove the question as to whether to drink it or sell it so you can get on with other things. I'll leave 2SJ74s on the floor so you'll know it was me....
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Old 4th September 2012, 04:48 PM   #679
RNMarsh is offline RNMarsh  United States
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Default and the results are?

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Originally Posted by bcarso View Post
Thanks for checking that out Richard!

I did some work with a 2SK170/2SJ74 pair in sim with a somewhat different topology, including a complementary DMOS output and more cascoding, necessarily a good deal more parts and higher rails, to explore the possibilities. Of course without SJ74 it is impractical I may show it anyway. I pushed the feedback divider down to 66.5/20 ohms, and the adjustable values in the sources to a net ~5 ohms. As mentioned, the low pinchoff FETs are close to their zero tempco operating points near Idss to begin with, which helps the offset stability concerns.
What was the s/n and harmonic et al data from the sim? Finding good compliments is a problem and guess it will continue to be.

IF matched jFETs are used without source resistance, the H2, H3 is very, very low and higher harmonics are virtually non-existant. [notice John didnt use source resistors - makes matching more critical but THD and Noise is lowest - purist approach]

But as soon as source resistance is added, you might as well go to bipolars as the distortion isnt lower and H5,7.9th can be made so low as hard to measure with the best equipment available. But bipolars need extra circuitry and size, cost grows and it is harder to do without a dc servo appendage. The end result isnt any better but it is easier to make. -Dick

Last edited by RNMarsh; 4th September 2012 at 04:58 PM.
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Old 4th September 2012, 04:50 PM   #680
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Originally Posted by RNMarsh View Post
IF matched jFETs are used without source resistance, the H2, H3 is very, very low.
Distortion in JFET input stage circuits
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