Re: NFB 101 Part Deux

Patrick Turner wrote:
Good work Ian.


RDH4 has all this spelled out of course, and uses µ , "mu" for amplification factor and ß "beta" for fraction of output
voltage either in series with input voltage for series voltage NFB or in shunt
with applied NFB
as in shunt voltage NFB.


Yes I wanted to use the proper symbols but I took me so long to get my head around the word processors formula editor that I never got round to working out how to do special characters. I will revise that aspect though.

RDH4 lists all the many varieties of feedback, both positive and
negative,
and current and voltage types and whether it is shunt or series.
RDH4 also has a table to show what the effects are of all the different
types
of NFB.

But not every single fascinating aspect of feedback is explored in RHD4.

For example, did you know that positive "series" current feedback will
reduce the output resistance of any amplifier, but at the cost of reducing bandwidth and increasing distortions?

If we consider a "normal" amp response of having a reducing output
voltage as the RL becomes lower
as having a POSITIVE output resistance, then the positive CFB can reduce
this amount of Rout
to a lesser value; a typical ultralinear amp with say Ro = 7 ohms with
PCFB can easily have its Rout reduced to say 1 ohm.
We assume Rout has been reduced from +7 ohms to +1 ohm.
Then the application of the global series voltage "normal" NFB will
reduce it even more to a lower value along with all distortions of the
amp and those
produced by the PCFB applied internally.

We can even apply so much PFFB that the positive Ro becomes a NEGATIVE
output resistance,
and the result is that output voltage rises when RL value is reduced.
So +1 ohm can be reduced further so Rout = ZERO ohms, then even less
than zero ohms, ie maybe -1ohm.
Obviously, such negative Ro tempts fate and credulity because its
impossible to get an amp which makes say 16Vrms at clipping into 8 ohms to make
20Vrms into 1 ohm.
But it is possible to make the same amp which makes 1.6V into 8 ohms
deliver 2V into 1 ohms.
But we would find this difficult to live with; once you examine how its
done, and experiment
with it, there are definate stability issues, and the open loop gain (
OLG ) must be reduced or phase tailored
to prevent oscilations, and the application of the PCFB be prevented at
extremities of LF and HF bandwidth.
Very interesting, but don't say I have not warned you.

The maths involved around each different form of feedback will take you
another 20 pages to explain.

I do suggest that all your terms for RL, Ra, and all others be made the
same as in RDH4,
because it was a good standard and everyone should know it, and that
equations be written in the same way.


Clearly RDH4 has covered this material in some detail and I did include a link to chapter 7 saying I was not going to repeat that chapter. The question is what is the right balance for RAT readers, especially noobies as this is supposed to be NFB101 and it is already bogged down in maths. Personally I like the analysis to be clear without such things as 'this can be expressed as' without any attendant explanation. Obviously it needs to be heavily tube related so I thought after the CF I would do he unbypassed CC followed by shunt derived and applied FB around a triode of pentode stage mainly because they are easier to understand and there are no stability problems. Next I thought I would cover the classic two stage shunt fed series applied circuit which leads nicely into stability issues.

In your theoretical workings for NFB application, how about showing some
typical tube power amplifier schematics with NFB applied with all working
voltages with their polarities
so ppl can measure their own amps and understand it all a lot better?


That sounds like a good idea.

Using a triangle pointing to the right to represent an amp with two
inputs on the left vertical side and one output at the right point is the text book
way to represent an amp
so people do not have to keep in mind all the complex inner amp topology which distracts them from the basic idea.
The same model can then be used for a tube amp or an opamp.


I purposely avoided that because its common usage is to represent an amplifier with infinite gain, zero output resistance and infinite input impedance and as we both know, tubes only meet one of those criteria. I was trying to emphasize that tubes fall far short of this ideal which is why many of the op-amp simplifications just don't apply to tubes.

However, to include all possible phase shift peculiarities of the open
loop
character of an amp and the equivalent networks in the amp which produce
them and their interaction when FB is used takes rather a lot of work.

NFB theory and application has already covered in many old books, and
many should be found then read,
and the messages in each will overlap each other books's shortcomings,
and you end up wize while you remember it all, then dumb again when you forget it all.
Unless of course all you do all day everyday is design and stabilise new
and old amplifiers.
I probably do enough to keep me wize.

I have never seen an online calculator for NFB, where one dials in the details of the open loop gain and all its phase shift rates and bothers,
and then ask for 20dB of NFB, and click "calculate", and have the program come up
with the FB network and including all the phase tweaking networks needed
for unconditional stability into any possible type of reactive loading, R
load, or no load at all.
Such a program could possibly be a boon for the dumbos to whom feedback
is a terrible mystery, and always will be,
and hence hated fiercely, and avoided.
But with a programmed or synthesized solution,
one must ensure it is still a viable solution which works in practice.
Since garbage in = garbage out with simulation programs, expect many simulated solutions using tubes and OPTs to still be good oscillators
when nobody expected it.
That's because its difficult for anyone to correctly define all the open
loop gain and phase shift character. Too fucking hard. Just bulid it, and learn to stabilise it by empirical methods of network
applications and trial and error
and by observation with a CRO. This is a far quicker way than all the
calculations in the world sitting
down at a table when you should be in the workshop achieving something
real.

Models of the single tube amplifier stage should include an extremely
low voltage generator producing output of µ x Vg with series resistance between the gene output and the anode terminal
should be explained
as equivalent models of the triode or pentode ot any other tube.
Newbies NEED to know the very boring basics before thay can have any
chance of understanding.
Most don't have a clue what a voltage generator is, or what the dynamic
anode resistance is at all!!!



An interesting point. Do you think NFB101 should start with a statement of what knowledge is assumed along with pointers to references for those who don't have it?

I welcome you to borrow whatever you need from

http://www.turneraudio.com.au/tube-operation1.html

and

http://www.turneraudio.com.au/tube-operation3.html

I don't have all the possible various feedback applications mentiuoned
at my site.


Thanks for the permission to use ingfo from your site.

Cheers

Ian
.

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