Re: NFB101 part 3
- From: Patrick Turner <info@xxxxxxxxxxxxxxxxxx>
- Date: Sun, 01 Jun 2008 13:12:10 GMT
flipper wrote:
On Sat, 31 May 2008 02:20:05 GMT, Patrick Turner
<info@xxxxxxxxxxxxxxxxxx> wrote:
Ian Thompson-Bell wrote:
Patrick Turner wrote:
A cathpode follower is a case of local series voltage NFB.That is shunt (voltage) derived, series (voltage applied)applied. Again
you need to differentiate between how NFB is derived and applied.
A cathode follwer is NOT shunt NFB.
This is because the output voltage at the cathode is all fed back and is
the SAME
phase as the grid input.
You cannot just say shunt NFB or series NFB, you *must* make the
distinction between how the NFB is derived and applied.
In a CF the feedback is taken from directly across the load and is
therefore shunt derived. Remember shunt derived feedback lowers output
impedance which is why a CF has a low output impedance.
RDH4 does not agree with what you are saying.
That's because RDH4 is old, apparently , but modern texts will specify
both ends just as he says.
The feedback in a standard Williamson or a cathode follower
is known as series voltage negative feedback.
RDH4 is apparently speaking of the input side only but look back at my
headphone amp thread. There are two ways to get a signal from the
output end. One is what most would likely consider 'normal': across
the load. That's shunt derived
Indeed all resistance networks across a load, or usually from live end
of the
load to 0V are shunting the load.
That does NOT ever make it shunt NFB; its just using resistors to
develop
a voltage that is applied to one of two inputs to the amp, one being a
non-inverting grid,
the other being an inverting second grid of a diff pair
or a cathode of single tube.
The two inputs have extremely high impedance between each of them.
So sure we use output current to get the voltage NFB, so current is
invloved.
But shunt NFB is very different from series NFB but the effect on Rout
can be equal.
The effect on Rin is that the series voltage NFB raises Rin, while shunt
FB lowers Rin.
And the shunt FB also reacts to the source R of the signal.
Hence in tube power amps there is not a single one I have ever
seen with shunt FB; its all series voltage NFB.
The other would be to put a current sense R under the load and feed
that signal back just as the other was. That's series derived.
But then you have current FB which if positive lowers Rout, but
increases THD/IMD and shortens bandwidth.
If negative FB, Rout is raised, but BW widened and THD/IMD reduced.
Nodody except Bogen ever used positive current FB.
And nobody ever uses global negative current FB.
RDH4 doesn't provide any means to describe the two but they will
behave quite differently.
I don't see a high need to spend the time re-reading it right now.
You have not backed up your statement with a quote.
The word 'shunt' does not appear, because the NFB does not
try to shunt the input signal.
The word shunt doesn't appear because they ignore how the signal is
derived.
The derivation of the FB signal is nearly always derived from current
flow
in a resistance network.
Its such a basic premise RDH4 doesn't need to spell it out for you.
Your supposed to know by now.
100% of the output voltage is fed bacl back so ß is 1.
The signal input is connected between g and ground, the amp input is
between g and k so the feedback is connected to the amp input in series
with the signal input in such a way that the feedback subtracts from the
input. The feedback is therefore negative and series applied. Remember
series applied feedback can increase input impedance which does happens
in some CF versions.
True, so why bicker about the series FB being shunt derived?
Say what you mean and mean what you say.
RHD4 is not a book of riddles.
All follower circuits are like this even opamp followers.
Agreed, and emitter followers too.
Shunt FB is where the Vo is the opposite phase to the input signal.
Again, you cannot just say shunt feedback, you need to distinguish
between how the feedback is derived and applied.
Its taken as being derived from a resistance network where the input
is applied to one end of two R and output of opposite phase at the other
far end
of two R, and the junction of the R pair is the input to grid.
THIS is standard shunt NFB, because the opposite phased output
is trying to reduce the input voltage, which is does,
but leaving a residue of THD/IMD at the R join which is applied and
amplified
A0 times to cancel its own production.
That's true of any NFB, shunt or series, otherwise it would be
positive feedback.
Given enough phase shift, NFB does become PFB.
The difference is whether voltages or currents are
summed at the input and, the 'missing link' in RDH4, whether you sense
voltage or current at the output.
Its all perfectly clear in many textbooks you apparently have not read
very closely.
If you want to build on what has aleady been said in many textbooks,
then you should familiarize yourself with the conventional
engineering terminology used in such books.
http://www.ece.mtu.edu/faculty/goel/EE-4232/Feedback.pdf
Maybe you find it clear, but I find it confusing.
Starting on page 2 of the pdf has a similar basic FB layout like Ian
has addopted, but it looks nothing like a real world circuit.
So I prefer the triangle to represent an amp with +/- input and assumed
+ output;
These have been used to depict topologies in all reputable
engineering tomes, and are for the beginner very easily understood.
I just think that whatever is used as a block diagram should be
EXACTLY as a bloke solders up in his workshop, but without
all the distractional detail of biasing and internal amp circuitry.
Patrick Turner.
.
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