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Old 30th August 2012, 04:28 PM   #541
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Quote:
Originally Posted by hitsware View Post
> Except it's not an op-amp and not an easy drop in in lots of places.
> Does not scale easily to phono use either.

Isn't it a ' current feedback ' op-amp ?
Sort of, the performance of the non-inverting input would be problematic in many places. Those PFET's are pretty noisey too sort of like the J177's I looked at.
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Last edited by scott wurcer; 30th August 2012 at 04:35 PM.
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Old 30th August 2012, 04:35 PM   #542
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Quote:
Originally Posted by scott wurcer View Post
Sort of, the performance of the non-inverting input would be problematic in many places.
Opamp is like universal wrench: seems flexible, but can't fit everywhere.

Click the image to open in full size.
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Old 30th August 2012, 04:35 PM   #543
bcarso is offline bcarso  United States
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Default 2145 gate leakage measurement

Quote:
Originally Posted by bcarso View Post

EDIT: I'll try to do a better measurement of Ig after some more sleep. I could infer it by guessing at what the 10M-loaded follower gain is and hence what the effective laod R was, but a more direct measurement with a high-meg R is also possible with some attention given to shielding. I'll bet it is a good deal less than a nA (which is the max spec for the conventional hookup).
OK: with a much-improved setup, chopping the input gate-to-common resistor between 2.7kohm and 2Gohm, I see output delta V of only 800uV, indicating Ig of only 400fA. Much bettah! This is with +9V on the upper drain, -9V on the lower Idss-connected current sink. Drain current is 2.73mA for this setup.

Knew that Victoreen resistor would come in handy someday.

With this result I'm leaning still further toward the theory that these are separate chips inside.

Last edited by bcarso; 30th August 2012 at 04:40 PM. Reason: afterthought
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Old 30th August 2012, 04:37 PM   #544
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by john curl View Post
Well Richard's example gives us a lot of leeway to use a simple differential pair, so long as we can trim. For example:
Of course, trimming can get you zero offset at the output. but, is it thermally stable - no drift with time/temp? That is the subject brought up and being discussed by others here.... getting both..... without a dc servo circuit added. :-)
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Old 30th August 2012, 04:39 PM   #545
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Quote:
Originally Posted by scott wurcer View Post
Except it's not an op-amp and not an easy drop in in lots of places. Does not scale easily to phono use either.
Scott
I'm not sure that Dick is saying that it's an op amp, I guess this is the RNM headphone amp.
It looks like an early current feedback amp, but with comp jfets at the input.
As I see it it is more like a buffer with some gain and cfb.
With local compensation and used close to unity gain it could be designed as a OK ops for the "No Global NFB folks", (use the available gain to lower THD and output Z).
Then the "frontend" wich is what most here are discussing could do the voltage amplification and the current is left to the "buffer".
Or it could be used as a part in a "nested" fb design.

(a bit Krill)

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Old 30th August 2012, 04:46 PM   #546
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Richard, with the proper monolithic dual jfet pair, the drift would be negligible after manual DC trimming.
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Old 30th August 2012, 04:48 PM   #547
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by scott wurcer View Post
Except it's not an op-amp and not an easy drop in in lots of places. Does not scale easily to phono use either.
All true. Just met my goals for audio line stage or headphone amplifier.
Main point -- no servo and thermally stable. And, cheap and simple to make. Why is it thermally so stable? Does answere have utility in opamp?
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Old 30th August 2012, 04:51 PM   #548
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by john curl View Post
Richard, with the proper monolithic dual jfet pair, the drift would be negligible after manual DC trimming.
John, could you get same results with your toplogy in current use w/o dc servo? You indicated you wanted the external dc servo [to allow optimum performance without compromise of circuit?.] That isnt clear to me.

I know trimmer contacts causes trouble but ignoring that for a second -- if you unbalance diff stage in some topologies, the thd goes up.... at least in low-modest gnfb designs. Need dc trim that doesnt do that and still be thermally stable.

With dc trim shown in my simple circuit... it wasnt needed in practice as offset was so close without it due to matching devices. It is thermally stable into 30 Ohm loads. Better performance into higher Z loads, of course.

Is it possible that many topologies dont need dc servo if properly balanced?

What about opamp circuit of conventional appl as Scott points out. Can it be done there? We shall see. -RNM

PS - I just put up the example that dc servo isnt always needed and to answere the reader about my whinning on the matter and not showing anything myself. It makes a point. Gets the job done. (BTW - other popular jFEt were also tried with similar results).

Last edited by RNMarsh; 30th August 2012 at 05:14 PM.
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Old 30th August 2012, 04:56 PM   #549
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Quote:
Originally Posted by john curl View Post
Richard, with the proper monolithic dual jfet pair, the drift would be negligible after manual DC trimming.
N and P together! Where?
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Old 30th August 2012, 05:00 PM   #550
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Look before you criticize, Scott. The discrete 'op amp' that I recently put up was designed in 1977, before I started using servos. That is WHY it has a DC offset adjust.
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