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Old 14th September 2012, 11:32 PM   #951
bcarso is offline bcarso  United States
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Originally Posted by coluke View Post
Uhm... maybe, but I'm afraid OLG would drop too much.
If you use a stiffer feedback arrangement you could lose J2 and J4, and feedback directly to the input sources. Not that I would bother though --- I'm not seeing the benefits of such parts count restrictions for this activity to begin with.
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Old 15th September 2012, 12:45 AM   #952
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Then it is a current feedback amp. Could help the speed but then the negative input is low impedance and the feedback resistors can not be chosen at will any more. This restricts the use in filters.
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Old 15th September 2012, 12:56 AM   #953
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The Push-Pull version of the buffer could look something like this. Very experimental at the moment. I am not sure how much voltage it will swing.
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Old 15th September 2012, 02:18 AM   #954
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Originally Posted by bcarso View Post
Not that I would bother though --- I'm not seeing the benefits of such parts count restrictions for this activity to begin with.
Why would you want to pay for parts you dont have to have?

Cost and space/size.... to fit onto/into a IC socket, perhaps? The thd will be fine.
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Old 15th September 2012, 04:14 AM   #955
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Quote:
Originally Posted by bcarso View Post
Here is a version with a bit more voltage across the input JFET...

R2 minimizes thermal shifts due to dissipation changes in Q10.
Thanks - impressive optimizations! The RC shunt in series with the collector of the NPN is well outside my ken. That's what is great about this forum, it quickly brings people with significant experience and intuition to improve on ideas.
I'm on a flaky smartphone browser - I'll try out the mods in simulation and respond later.
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Old 15th September 2012, 03:44 PM   #956
bcarso is offline bcarso  United States
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Originally Posted by linuxguru View Post
The RC shunt in series with the collector of the NPN is well outside my ken.
If you look at the dissipation shifts with signal, there is a series resistance which minimizes them (although doesn't eliminate them). The cap is there to make the effect of the resistor go away at high frequencies.

In some differential and other configurations, one can thermally balance the active devices with selected resistances so that their dissipation shifts with signal match. Thus the differential drifts are cancelled. This is an old scope amplifier trick which has been rediscovered by some and applied to audio, but goes back to vacuum tube days. Dennis Feucht discusses a number of these things in his book, Handbook of Analog Circuit Design.

There is also a thermal shift with signal for the input PNP transistor, but it's a good deal smaller and its effect on the output is small, since the JFET is still the dominant determinant. There is a slight effect due to the PNP base current, both due to its flowing in the source impedance and in terms of the deduction from the emitter current.

I'm looking at ways to generalize this topology to considerably higher currents, without simply paralleling a whole bunch of parts. It also has appeal as a buffer to be enclosed in an overall loop, given the rather high bandwidth, high input impedance, and low distortion.

I've worked with load compensation a lot, but this particular arrangement has appeal. Thanks for presenting it.
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Old 15th September 2012, 03:55 PM   #957
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Originally Posted by RNMarsh View Post
Why would you want to pay for parts you dont have to have?

Cost and space/size.... to fit onto/into a IC socket, perhaps? The thd will be fine.
Constraints that we may or may not wish to accept.

With surface mount we can fit a bunch of parts onto such a footprint, but power will be restricted without complex heat dissipators. The argument for minimalism from cost considerations might have made sense in 1968, but except for exotic parts, transistors are cheap. As far as those who believe minimalism translates into better sound, I have no arguments that will dissuade.

I had a supervisor in those early years who used to restrict the number of transistors I was allowed to use for a particular design. These were usually TO-5 devices His concern was not cost, particularly, as these were one-off circuits for use in research. When he got his degree I lost the constraints, but much later did plenty of cost-sensitive designs.
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Old 15th September 2012, 05:07 PM   #958
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Default I understand

Quote:
Originally Posted by bcarso View Post
Constraints that we may or may not wish to accept.

With surface mount we can fit a bunch of parts onto such a footprint, but power will be restricted without complex heat dissipators. The argument for minimalism from cost considerations might have made sense in 1968, but except for exotic parts, transistors are cheap. As far as those who believe minimalism translates into better sound, I have no arguments that will dissuade.

I had a supervisor in those early years who used to restrict the number of transistors I was allowed to use for a particular design. These were usually TO-5 devices His concern was not cost, particularly, as these were one-off circuits for use in research. When he got his degree I lost the constraints, but much later did plenty of cost-sensitive designs.
I understand that. always did. However, the comments werent for you alone. Many IC designs are full of added parts for various compensation circuits for lower input currents and offsets and the like.... it can all get pretty involved if you want it to be. But, there is a good reason - same as I have - why your supervisor asked for a design with minimal parts. Same as if he was teaching you computer programming.... to be both effective and efficient is harder. Sometimes it pushes you to think outside the box. Creativity is fostered that way.
I would point to Scott's opamp design here as an example of getting a lot with as little as possible. Or, Linuxguru's idea. -RNM

Last edited by RNMarsh; 15th September 2012 at 05:16 PM.
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Old 15th September 2012, 06:46 PM   #959
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Originally Posted by RNMarsh View Post
I understand that. always did. However, the comments werent for you alone. Many IC designs are full of added parts for various compensation circuits for lower input currents and offsets and the like.... it can all get pretty involved if you want it to be. But, there is a good reason - same as I have - why your supervisor asked for a design with minimal parts. Same as if he was teaching you computer programming.... to be both effective and efficient is harder. Sometimes it pushes you to think outside the box. Creativity is fostered that way.
I would point to Scott's opamp design here as an example of getting a lot with as little as possible. Or, Linuxguru's idea. -RNM
I don't think his motivation was to inspire me, or to think outside the proverbial box. He was just peeved by complexity, and also wanted to exert control. Overall though, we got along fine. I was hired, in fact, because he was playing with electronics (including developing some good instruments) instead of finishing his dissertation.

The sad thing was to see the departure from physics and astronomy guys like John O., who knew their academic material as well as being handy around electronics and other instruments, to single-minded grad students who just focused on their narrow fields. This was very evident by the mid-70's and after, although there were the rare ones who could do hardware and software. Sadly, only a few got jobs in the field, I think primarily due to a deficiency of political skills.
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Old 15th September 2012, 07:09 PM   #960
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I don't think his motivation was to inspire me, or to think outside the proverbial box. He was just peeved by complexity, .
Well, though, that is my intent.
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