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Remembering Dr. Middlebrook Part 3
(Power Electronics Technology Via Acquire Media NewsEdge) Sum: Back in the 60’s, I think, you wrote a book on
differential amplifiers, any comments?
RDM: The book on Differential Amplifiers, this was not a
textbook, it was a monograph, and it was a very narrow and
specialized topic and it was really not even about differential
amplifiers. A better title really was “Analysis of Unbalanced
Symmetrical Circuits” because a differential amplifier is a
symmetric circuit and there’s a well-known way of analyzing
such a circuit, the half and half method, the common mode and the
differential mode, and if you split the signals into those two
components, each of them can be solved for with a half-circuit. The
half-circuit is different for common mode and differential.
That’s well-known, although you don’t see it much
because it’s one of those old topics that’s been very
thoroughly dealt with and of course forgotten. That’s the
problem, like so many things, but anyway, that was the point where
I picked it up because what I wanted to deal with was where those
two symmetric halves were symmetric topologically but were
imbalanced numerically, because there’s always unbalance with
symmetric components, especially the betas of the transistors, but
also there’s always a tolerance on the resistors or whatever.
What I came up with was simply an analytic technique of how to
incorporate quantitative unbalances yet still retain the simplicity
of dealing with only half the circuit, and that’s all that
monograph was about, so it was highly specialized. It was simply an
analytic technique, but it was Low Entropy. I hadn’t thought
of that word in those days and I hadn’t even formulated the
idea, and yet in retrospect, that’s what the overall
objective was to keep the simplicity in dealing with only half the
circuit. That, in my present terminology, that’s looking for
a Low Entropy answer.
Sum: Would you think that this is a good time to write a
book on the subject?
RDM: My students at Caltech periodically press me to
write a book because there is no text for that course, there never
has been, and in fact, I’ve always said it’s my policy,
if I ever do write a text for this course, I won’t give the
course anymore, because one reason they don’t have any text,
I don’t even give them notes, at Caltech they don’t
even get the notes the outside people get, because I think it
undermines the course if you have a text, especially if the text is
written by the instructor. The students always say “well, I
am busy and tired, I’ll skip the lecture and I can always
read it in the book later” and I want to try to forestall
that because 90% of the value, if any, of what I am trying to do is
in the philosophy and the approach, not in the factual material. I
tell the guys in class at Caltech at the very beginning of the
course “if you are going to get anything out of this course,
you have to come to the lectures, there is no other way.
That’s where all the value is if there is any at all.”
So that is why I don’t even give the Caltech people those
notes.
Sum: What about putting it all on video tape? It is, of
course, not as good as being at the lecture, but I think this is
the closest thing to being in the lecture, what do you think?
RDM: But anyway that pressure to write a book has been
there for a long time and then the people out in industry say the
same thing “why don’t you put all this in a book, why
don’t you videotape it?” and I keep saying “maybe
I will some day” and then I say “well, I’ve
written two books before and I don’t want that millstone
around my neck again”, because to write a book, it has to be
top priority because it takes a huge amount of time and a text is
even more.
Text is very difficult to write because you have to have
balanced subject material, you have to anticipate what a lot of
different universities want to have in their course, and you have
to be careful to have a balance, it’s a lot more work as
well. So I have resisted doing that but finally I did videotape the
whole course and then I began asking the class (this is the outside
people), “would the videotape be useful?” Yeah.
“How much would you pay for it?” Not much. And
it’s clear that it would not be marketable as a standalone
for anything like a reasonable price. There’s pros and cons
on having the thing on a video tape, but it’s not a viable
market place to replace the courses, so the videotape still sits
there.
Sum: So, are we going to get a book or not?
RDM: So then I decided “well, I’m going to
write a book, but it’s not going to be a text” and the
least amount of work, which is the only way I can make the
commitment to do it is it’s going to be the book of the
course, so I had the videotape audio track transcribed and all I am
going to do is do minimal editing of that transcription, put the
pictures and equations in it, which is basically the course notes
and it’s going to be called “Middlebrook’s
Structured Analog Design Course: The Book”. It’s not
going to pretend to be anything else. It’s the book of the
course, as it is, so I don’t have to deal with any questions
of balance and structure. It’s not a text for students. It
may be a text for professors. At least that gets it down on paper,
because the feedback I got from the people at the course was that
the book is preferable to the videotape. A real book with words in
it, they would like better than the videotape anyway.
Sum: You have mentioned about giving these magic
techniques names, how are you going to do that?
RDM: Last year sometime, well, two years ago I guess, I
came to that conclusion and so pretty soon, having reached that
conclusion, I decided that the whole point about this approach is
the philosophy of Design-Oriented Analysis in terms of Low Entropy
expressions. The only analysis worth doing is analysis that can be
worked backwards for design, otherwise what’s the point.
So I said to myself “there’s one thing that
I’m going to do before writing that book and that is
I’ve got to get those two key phrases into the archival
literature, so there’s a reference, because what I am trying
to do all follows from those two phrases, so I said well first what
I’ll do is I’ll submit a paper, I’ll join the
IEEE Education Society, which I’ve never been associated with
before. I did that and then I submitted a paper to one of their
conferences called “Frontiers in Education”, and I gave
one with those two key phrases in the title and to my surprise they
accepted it, and I went and gave the paper at Purdue University in
November 1991.
The time came for the session and I got up with considerable
trepidation because this is the first time I’ve talked to an
academic audience. All the stuff I talk about is either to my own
class at Caltech, or to outside working engineers. This was the
first time that I talked about this topic to academic people
because here’s the whole audience of professors and
instructors, and so I made sure there was a wide path to the exit
in that meeting room, because here am I someone unknown to them
getting up and saying “you’re doing it all wrong and
here’s the way to do it”. To my relief and surprise it
went over very well. So well in fact that one of them got up at the
end and said “I was a working engineer for 11 years and now I
am teaching at an academic place and I can really see the value of
this and I am going to try to adopt this in my own
course.”
As Val, said afterward to some people, we didn’t plant him
in the audience, but he was a perfect shill. Anyway it went over
very well, but of course lot of things go over well, but
that’s the last you hear of them, but anyway I dropped those
phrases into the archival literature but I was already interested
in a new development.
Sum: What is the nature of the analog course you are
teaching in Caltech now?
RDM: My course at Caltech is a second level course and so
is the outside one for industry, and it’s really a course in
“technical therapy” because I have to take the guys
through a Freudian regression, back to high school, because
that’s where the quadratic equation comes in. That’s
the lowest level material. A lot of the other stuff was in
electronics college courses, but you have to take them back through
all of that to the point that they recognize and accept the fact
that a lot of the stuff that we were told is not useful. It’s
not wrong of course, but it’s not in the best form, not in
the most useful form, in other words, it is not Low Entropy.
I gave a couple more papers at these education-type conferences.
In fact, I went to the same conference again last year, Frontiers
in Education, the location was Nashville this time. They move
around, the same conference was in November 1992.
The second paper I gave was called “The Quadratic Equation
Revisited” and that went over well too. But of course
vanished, I mean, it’s in the proceedings but no one will
ever notice. The quadratic is the first formula we learn and
it’s ingrained in our minds. We’ll never forget that
and what I tried to show in that paper is there’s a lot
better formats, depending on the purpose, that is you can get more
useful information out of it than is apparent.
Sum: What are some of the problems you find in the
quadratic equation?
RDM: It has two defects: it’s High Entropy because
you can’t interpret the meaning of the a ,b, c in that
combination. You might as well not write that formula in terms of
circuit elements, you might as well put numbers in right away
because that’s all you can do after you put symbols in the
formula, is put numbers in.
The second defect is the computational inaccuracy, in the case
of real roots, and so what I refer to as the preferred version of
that function overcomes both of those disadvantages. In other
words, you stand back and you get a much broader perspective of
what that equation is telling you, therefore on what you can do
with it. The rest of the techniques are not as low a level as that,
they are college level, but similar.
Sum: Perhaps you could enlighten us on the impracticality
of the old methods.
RDM: We were taught loop and node analysis. We were
taught how to find input and output impedances and it’s OK,
it’s not wrong, but it’s very hard to do anything with
the answers that you get that way. We’re taught to get the
equation as a huge numerator divided by a huge denominator. But
what if you don’t like the number that comes out as the
answer? What if it doesn’t meet the specifications? Now what
do you do? Well, that’s where I pick up the story and try to
show putting it in Low Entropy form enables you to work back, and
the second thing is, how do you get it to come out in Low Entropy
form in the first place? And that’s where all those other
techniques come in, doing the algebra on the circuit diagram, get
rid of loops and nodes by Thevenin reduction .
We all learn Thevenin and Norton, we all do umpteen examples of
it and get A’s in the course, but how often do you use it
since? Hardly ever, because no one ever told us why it was useful
and the reason it’s useful is every time you use one of those
theorems, you get rid of one loop or one node, so you can gradually
reduce the complicated network into a single loop and write the
answer in one line. That’s why they’re useful, but we
don’t get told that. It’s simply one of these theorems
that we have to learn. It’s not going to be any use in the
real world. Well, my whole philosophy is all of those things are
useful if you go about it the right way. That’s what the
whole approach is about; that’s why I referred to these
courses as second level.
References
K. Kit Sum, “Power Factor and Its Effect on Power
Quality”, presented at the PCIM/Power Quality Conference,
Hotel Queen Mary, Long Beach, California, October 16-19, 1989.
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