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Common Emitter OPS?
Common Emitter OPS?
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Old 17th November 2016, 03:37 AM   #1
Dave Zan is offline Dave Zan  Australia
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Default Common Emitter OPS?

There looks to be some theoretical reasons to prefer a Common Emitter Output Section.
Dr Ed Cherry has advocated this for many years, in contradiction to Self's rather poor opinion of the idea, and usual practice.
I have already reinvented some of Cherry's work, which reassures me a bit but is not a very productive use of time.
I would prefer not to reinvent any more, so does anyone have any references to any other analysis of the CE OPS?
Not sure yet if there is some sort of optimum bias point, similar to the Oliver bias point for an emitter follower.

David
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Old 17th November 2016, 09:52 AM   #2
sgrossklass is offline sgrossklass  Germany
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I am only aware of one major advantage of CE output stages: They are able to run extremely close to the rails (even more so with MOSFETs), which is why they tend to be standard fare in rail/rail opamps. Literature on those may prove enlightening.

Other than that, there's plenty of reasons why people have traditionally preferred followers. The output stage tends to be the slowest component as-is, and going CE makes it even slower and more prone to capacitive loading. (Keep in mind an emitter follower may have a bandwidth of >fT if driven hard.) I don't think there is any optimum biasing per se, it generally is the more the merrier. It stays a current source output (with output impedance then knocked down by feedback) until way into Class A territory.
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Old 17th November 2016, 11:10 AM   #3
AndrewT is offline AndrewT  Scotland
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A very few Threads have discussed this in the past.
I wonder if the Cordell interviews gave any info?
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Old 17th November 2016, 02:08 PM   #4
kasey197 is offline kasey197
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Common Emitter OPS?
Dave is correct of course. It suboptimal to wrap gnfb around the local nfb introduced by cc/cd ops. But the practice remains the mainstay of amplifier designs for many of the reasons mentioned above. Plus scaling up a design by simply adding more followers is substantially preferred to redesigning the amp for larger output power. Nelson Pass has been using them in his designs for decades and so assertions that they do not work well in practice are misguided.
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Old 17th November 2016, 09:12 PM   #5
Dave Zan is offline Dave Zan  Australia
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Quote:
Originally Posted by sgrossklass View Post
They are able to run extremely close to the rails (even more so with MOSFETs), which is why they tend to be standard fare in rail/rail opamps. Literature on those...
My principle reason is for feedback optimization, but the rail-to-rail is a nice bonus, as you say, especially for mosfets.
I hoped that someone would have some references to the op-amp literature, manufacturers don't seem to release as much information as they used to.

Quote:
Other than that, there's plenty of reasons why people have traditionally preferred followers... CE makes it even slower and more prone to capacitive load...
Simple theory says the increased global feedback should compensate for the reduced local feedback.
So either the popular conception is incorrect or my analysis is too simplistic.
Hence my curiousity.

Quote:
Originally Posted by AndrewT View Post
I wonder if the Cordell interviews..
Not that I know of, appreciate a reference if it exists.

Quote:
Originally Posted by kasey197 View Post
Dave is correct of course. It suboptimal to wrap gnfb around the local nfb introduced by cc/cd ops...
That's my main motivation.

Quote:
Nelson Pass has been...
I suspect Nelson does this partly to keep the rails volts down, makes it more DIY friendly, and partly just for the interest in an unconventional solution.
He seems reasonably unconcerned about the lower distortion possible and I haven't seen any analysis from him.

Best wishes
David
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Old 18th November 2016, 07:14 PM   #6
soundbloke is offline soundbloke  United Kingdom
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You might also be interested in this thread... ATC Active style amps. These long tried and tested amps employ common source outputs stages similar to a more recent design of John Vanderkooy published in Linear Audio I believe (?).

Most interesting in the ATC implementation is the provision of adjustable local feedback around the output stage that nulls distortion by balancing the transconductances of the output devices. It is a very effective technique that I would recommend trying...

Also of note is that the ATC implementation adopts a low voltage (op amp) driver stage and necessarily separate power supplies for each of the three output stages (for their 3-way active speakers). IMHO this circuit topology has a number of advantages and deserves more attention than conventionality (if that is a word) affords.
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Old 19th November 2016, 01:04 AM   #7
Dave Zan is offline Dave Zan  Australia
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Quote:
Originally Posted by soundbloke View Post
... be interested in this thread... ATC Active style amps.
The concept (floated power supply CE) seems similar to the QSC power amps but I haven't looked too closely at either yet so that's just a first impression.
It would be nice to see the circuits, can you post the ATC's in that thread if they are public domain?
Cherry discusses some of these options in an article in Wireless World (or whatever it was called at the time).
I would expect that power supply stray capacitance would load the amp, which seems undesirable, but haven't checked if it's a problem, or even if it happens.
Exactly the sort of issues that I want to explore here, so thanks.

Best wishes
David
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Old 19th November 2016, 02:57 AM   #8
Dave Zan is offline Dave Zan  Australia
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Just noticed that coincidentally there is another thread on the QSC version of CE OPS >HERE<.
Will take this chance to mention that the "School of Delft" also advocates CE OPS.
Their approach looks similar to Cherry's, as far as I have checked.

David
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Old 20th November 2016, 11:22 AM   #9
Dave Zan is offline Dave Zan  Australia
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Quote:
Originally Posted by Dave Zan View Post
Just noticed that coincidentally there is another thread on the QSC version of CE OPS...
A correction. The QSC in that thread is indeed a "Grounded Collector".
So it's an emitter follower, except with the collector grounded and the supplies float.
I had a recollection that QSC did some kind of Common Emitter circuit in some amps.
A "Grounded Source" would make sense for the FETs that have the Source connected to the case (some laterals I think).
Or possibly a CFP OPS if the Drain/Collector is connected to the case.
It seems QSC was more interested in the practicality benefits of Grounded case rather than the CE circuit as such.
Many of the picture links in earlier discussions in DIY have disappeared, any QSC experts are welcome to be pedantic here.

David
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Old 20th November 2016, 02:07 PM   #10
soundbloke is offline soundbloke  United Kingdom
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Quote:
A "Grounded Source" would make sense for the FETs that have the Source
As discussed in the thread I referenced, this is the case in the ATC implementation where the devices are sandwiched between the heat sinks and a ground plane. It does indeed have its benefits although the MOSFET pin inductances can require some attention...
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