This explains a lot about what the feedback is, including emitter resistors:
http://www.allaboutcircuits.com/vol_3/chpt_4/10.html
http://www.allaboutcircuits.com/vol_3/chpt_4/10.html
I know I don't have to tell you guys this, so just for the record,
R5, R6 and R7 form a triangle (or delta) circuit which can be
transformed into a star circuit in which case we end up with
two emitter degeneration resistors and a tail resistors as
is usually seen with diff pairs. Then, if emitter degeneration is
feedback seems to be a question with many answers on
this forum.
R5, R6 and R7 form a triangle (or delta) circuit which can be
transformed into a star circuit in which case we end up with
two emitter degeneration resistors and a tail resistors as
is usually seen with diff pairs. Then, if emitter degeneration is
feedback seems to be a question with many answers on
this forum.
Christer,
please have a look at the link shown above and repeated by Jan Didden. Very simple for primary school
. The feedback is clearly defined in circuits theory and every electrical engineer must be aware of it, regardless marketing tricks of those who from some hardly understandable reason restrict feedback to the global one from output to input. This is more or less visual approach than the engineering one.
please have a look at the link shown above and repeated by Jan Didden. Very simple for primary school
. The feedback is clearly defined in circuits theory and every electrical engineer must be aware of it, regardless marketing tricks of those who from some hardly understandable reason restrict feedback to the global one from output to input. This is more or less visual approach than the engineering one.Pavel,
I was not arguing against you, rather the opposite. Emitter
degeneration is called feedback in my textbook too, but on
the forum there seems to be different views on this. What
I meant with my comment was just that if you do a triangle-
to-star transformation it becomes more apparent that the
circuit actually uses emitter degeneration, which at least
some of us consider as a kind of feedback.
I was not arguing against you, rather the opposite. Emitter
degeneration is called feedback in my textbook too, but on
the forum there seems to be different views on this. What
I meant with my comment was just that if you do a triangle-
to-star transformation it becomes more apparent that the
circuit actually uses emitter degeneration, which at least
some of us consider as a kind of feedback.
Nelson Pass said:
Just to clarify for the people that like to argue.
Nelson is referring to a complete amplifier circuit in his above statement.
On the other hand when referring to a single stage, it is common to refer to emitter degeneration as "negative feedback". This is easily verified by referring to any electronics textbook.
The difference in usage when referring to a complete amplifier circuit versus a single amplifier stage has been beat to death in this thread:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=28448
And if you didn't find enough arguing there, you can also feel free to argue on this current thread:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=35686
But please keep your silly arguments out of this thread. This thread is about circuits and not semantics.
When it comes to complete amplifiers, we have such luminaries as Nelson Pass, John Curl, and Scott Wurcer (designer of the AD797, AD711/12, and many more) agreeing that referring to an amplifier circuit as "no feedback" or "zero feedback" is the correct terminology to use when only local degeneration is employed. And that's good enough for me. I don't give a rat's *ss what Steve Eddy or Pavel say.
Charles is correct. We designers know that there are two kinds of feedback: local, and loop (or global) feedback.
To save time, we usually refer to loop (or global) feedback as applying feedback to an amplifier.
This was the principle of negative feedback developed by Black, at Bell labs, in the 1930's. For many years, we thought that global feedback was OK or even the greatest thing since canned milk! However, Otala and others showed us that negative loop feedback caused as many problems as it eliminated. Therefore, many designers, including Otala, Hansen, and Pass, and even me, on occasion, have relied much more or even completely on local feedback, which still presents SOME problems, but not nearly as many sonic problems as loop feedback.
This definition of 'feedback' is much like what we have settled on for 'transistors'. Apparently, 'fets' are "unipolar transistors", and what we call 'transistors' are really" bipolar transistors". We just say fet or transistor, today and we know what we mean. It saves time and trouble.
To save time, we usually refer to loop (or global) feedback as applying feedback to an amplifier.
This was the principle of negative feedback developed by Black, at Bell labs, in the 1930's. For many years, we thought that global feedback was OK or even the greatest thing since canned milk! However, Otala and others showed us that negative loop feedback caused as many problems as it eliminated. Therefore, many designers, including Otala, Hansen, and Pass, and even me, on occasion, have relied much more or even completely on local feedback, which still presents SOME problems, but not nearly as many sonic problems as loop feedback.
This definition of 'feedback' is much like what we have settled on for 'transistors'. Apparently, 'fets' are "unipolar transistors", and what we call 'transistors' are really" bipolar transistors". We just say fet or transistor, today and we know what we mean. It saves time and trouble.
Nelson Pass said:
In other words, as long as we don't call emitter resistance feedback, then companies can continue to mislead people to believe that their amps have no feedback in a market where many are of the belief that any feedback anywhere in the circuit is the kiss of death.
What's so bad about there being "no such thing as non feedback" except from a marketing point of view? Why can't manufacturers simply acknowledge the fact that there is no such thing as non feedback and simply qualify their claims when referring to feedback in their designs?
se
Am I dreaming or do suddenly everybody agree that emitter
degeneration is local feedback?? Of course there is a
distinction between local and global feedback, but I thought
we had agreed on that some hundred threads ago.
I don't mind if people refer to non-gloabl-feedback amps as
non-feedback, as long as both the designer and we understand
that it is the lack of global feedback that is intended. Which
brings us back to what Nelson said in this or some other recent
thread and which I also suggested, that feedback is about
the intention of the designer and our understanding of the
circuit rather than the circuit itself (Ok Nelson didin't say
quite that much ).
degeneration is local feedback?? Of course there is a
distinction between local and global feedback, but I thought
we had agreed on that some hundred threads ago.
I don't mind if people refer to non-gloabl-feedback amps as
non-feedback, as long as both the designer and we understand
that it is the lack of global feedback that is intended. Which
brings us back to what Nelson said in this or some other recent
thread and which I also suggested, that feedback is about
the intention of the designer and our understanding of the
circuit rather than the circuit itself (Ok Nelson didin't say
quite that much
).Christer,
There has never been an argument about whether degeneration is negative feedback when talking about a single stage.
As to your other point, referring to a "non-global-feedback" amp as "zero feedback" is completely incorrect.
Take the example of the Boulder amps. They use two high-feedback modules in series, with no "overall" feedback. Calling this "zero feedback" is like calling a Crown IC-150 preamp plus a Crown DC-300 power amp combination "zero feedback" because there is no "overall" feedback from the speaker terminal to the input of the preamp.
More detail on this can be found here:
http://www.diyaudio.com/forums/showthread.php?postid=331065#post331065
http://www.diyaudio.com/forums/showthread.php?postid=331141#post331141
But please, let's leave this discussion to another thread for anyone that actually cares about this.
There has never been an argument about whether degeneration is negative feedback when talking about a single stage.
As to your other point, referring to a "non-global-feedback" amp as "zero feedback" is completely incorrect.
Take the example of the Boulder amps. They use two high-feedback modules in series, with no "overall" feedback. Calling this "zero feedback" is like calling a Crown IC-150 preamp plus a Crown DC-300 power amp combination "zero feedback" because there is no "overall" feedback from the speaker terminal to the input of the preamp.
More detail on this can be found here:
http://www.diyaudio.com/forums/showthread.php?postid=331065#post331065
http://www.diyaudio.com/forums/showthread.php?postid=331141#post331141
But please, let's leave this discussion to another thread for anyone that actually cares about this.
Charles Hansen said:
I have actually got the impression that has been the cause
of previous heated debates, but maybe i missed the point
in those discussions.
I quite agree. It was sloppy of me and I stand corrected.
Let's say that zero feedback is when we have no feedback
loop spanning more than one amplifier stage. Do you agree
to that, or do only certain types of local feedback within a
stage qualify? This still leaves us with certain bordeline
cases like CFPs, of course.
To fit the non loop feedback idea which I think Charles uses, there should not be any feedback loop between two active devices in cascade, in any location in the amplifier- input stage, VAS, driver, output, etc.
CFP stages are not "borderline"; they are local feedback loops, and wouldn't be used in an amplifier adhering to Chas's philosophy. Same for the Hawksford "error correction" circuit.
If I got that wrong, let me know.....
~Jon
CFP stages are not "borderline"; they are local feedback loops, and wouldn't be used in an amplifier adhering to Chas's philosophy. Same for the Hawksford "error correction" circuit.
If I got that wrong, let me know.....
~Jon
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