Page 79 - Analog Circuit Design Art, Science, and Personalities

True Analog Circuit Design

told that the voltage between any one of the “hots” and the “safe” was 220 V, while
the voltage between any two of the “hots” was 380 V. Being used only to “positive”
and ‘&negative” from our DC experience, this was again a mystery. If two “hots” are
each 220 V away from the “safe,“ then the two ”hots” must be either 0 V or 440 V
away from each other. Wasn’t that crystal clear? This time we found somebody who
knew the right answer, but he did not help us too much. Instead of using a simple
analog such as a phasor diagram, he started to talk sine and cosine. We accused him
of not knowing the answer either and covering up his ignorance by muttering some
mumbo-jumbo. It again took considerable time and effort before we got the picture.
Why was our progress so difficult and slow? Was it lack of intelligence? That
friend of mine is one of the world’s leading mathematicians today. At least in his
case, lack of intelligence is not a feasible explanation. I think we were slow because
our young minds needed to see the invisible electrical processes translated into easy-
to-envision analogs. And we had to develop these analogs ourselves, step-by-step.
I know that trying to “understand electricity” early in life had a lasting benefit to
me. 1 got used to “seeing electricity” in analogs and I am still seeing it that way. I
believe every electronic circuit designer could benefit from thinking in analogs, and
it is never too late to start. This belief made me write this chapter.
It is mainly during the initial, qualitative phase of designing an electronic circuit
that it is most helpful to translate the circuit’s operation into a more transparent
process. The same applies when one has to quickly comprehend the operation of a
circuit designed by somebody else during presentations, design reviews, or the like.
I am, of course, not against exact mathematical analysis or computer simulation, but
those have their justification in the next phase, if and when exact quantitative
verification of a design is required. I find that mainly circuit operation described in
the time domain is easy to synthesize or analyze this way.
My process of visualization is quite simple. Circuit diagrams are commonly
drawn with current flowing from the top to the bottom and with the individual cir-
cuit elements in the schematic positioned approximately according to their voltage.
When imagining a circuit’s operation, I am, in my mind, moving and shifting parts
of the circuit schematic up and down following the time-varying voltage they carry.
This helps me to “see” in time sequence which diodes or transistors are turning on,
which are turning off, how op-amp circuits behave, and so forth. I never draw these
distorted circuit diagrams on paper; rather, I see the moves and bends only in my
mind. (An excellent way to avoid boredom in many situations in life!) Of course,
these imagined moves and shifts are only approximate and cannot be applied con-
sistently because that would often distort the schematic beyond recognition.
To illustrate what I mean, I will describe the proccss as best 1 can in the following
few examples. Unfortunately. the static nature of printed figures in general, com-
bined with the black-only print in this book, will make it somewhat difficult to get
the message across. I wish I could attach a videotape on which I could perhaps
convey my mental images much better. Because of this difficulty, I had to reverse
the process. As examples I picked conventional circuits of well-known operation to
describe the method of visualization. Normally it would be the other way around.

Example 1. The Astable Multivibrator

The first example is an astable multivibrator shown by its conventional circuit dia-
gram in Figure 8-1. The idealized wave forms on the collectors and bases of transis-
tors Q1 and Q2 are shown in Figure 8-2. At time to transistor Q1 is in saturation,

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