When Electronic Design asked me to write about my
Idea for Design, first seen in this year’s August 13 issue,
it seemed a simple enough task. We’re all familiar with
design. Most of us do it in some form or another almost
every day. But, ideas? We all have them, but where do
they come from? And what is the essence of a good idea?
Simplicity? Elegance? Performance, novelty, creativity?
Thomas Edison once said that genius is 1% inspiration
and 99% perspiration. For me, inspiration is the key.
Having a good idea and turning it into a great design
requires a seed of inspiration. As a consultant engineer
specializing in analog design, most of the circuits I design
follow a customer’s specification. Creating a product
that performs to the customer’s requirements is a
challenge that requires inspiration if I am to satisfy those
requirements with a circuit that performs reliably, efficiently,
and at the right price. And that’s usually where
the good ideas come from.
But it isn’t always that way. Sometimes, an idea materializes
simply from the desire to do something differently,
or better, or cheaper. It can happen to me quite suddenly.
I tend to frown, quizzically, and look very distant.
Julie, my nearest and dearest, can always tell. “You’re getting
one of those ideas, aren’t you?” she says. I usually
nod, sheepishly, and then run off to my study. I keep lots
of paper and sharpened pencils handy. You never can
tell when an idea might strike.
So it was with the rectifier idea. I’d been going through
my burgeoning collection of electronics literature, reflecting
on the merits of different active rectifier circuits.
Some of them were half-wave, others full-wave. Most of
them used diodes inside an op amp’s feedback loop.
Others used clever arrangements of op amps and analog
switches. All very neat, all very effective. But something
nagged at me. If I were to feed an alternating signal
swinging symmetrically about ground into a single-rail
op amp, what would the output look like? Wouldn’t the
negative peaks be clamped to ground? And wouldn’t
that, effectively, be half-wave rectification?
I ran off to my study and started scribbling. Julie, bless
her, brought me tea and a slice of pizza. The first part
was easy. Op-amp IC1a, configured as a unity-gain follower
operating on a single positive rail, would, indeed,
follow the positive-going segments of the input waveform
and would level off at ground during the negative-going
portions. I added resistor R1 to protect the op-amp’s
non-inverting input from excessive current flow during
large, negative-going input swings.
So far, I had a promising idea for a half-wave rectifier
that didn’t need any diodes or analog switches. Super.
But I wanted full-wave rectification. Nothing for it but to
add a second op amp, IC1b. By combining the second
op amp in a unity-gain inverting function with the output
of the non-inverting follower, the overall function
would simply invert the negative-going input peaks when
IC1a’s output was at ground to produce a positive-going
version and would effectively allow IC1a’s output to follow
through during the positive-going peaks, producing
full-wave rectification.
Genius? Absolutely not. All I did was to take some wellestablished
techniques and combine them in a way that
suited my idea. Certainly, the resulting circuit is fairly
unique to me, but the foundations it is based on are not.
“If I have seen further it is by standing on the shoulders
of giants,” said Isaac Newton. The giants of analog design
are the likes of George Philbrick, Bob Widlar, and Barrie
Gilbert, and latter-day luminaries like Bob Pease and Jim
Williams continue to guide my way.
So, everything was looking good for big signals, but what
about smaller signals? I wanted to add gain so signals of a few hundred millivolts could be boosted
to a few volts. I knew that adding R4
and R5 to IC1a would ams needed for
the required amount of plify the positive
portion of the input signal, but the
ratio of R2 to R3 that dictates the inverting
gain of the second stage would have
to be changed so the positive and negative
peaks would be amplified equally.
Time for some math—nothing fancy,
just a little algebra. By equating expressions
for the overall gain during positive
and negative excursions, I derived
a simple equation that could be used to
determine the resistor valuegain.
Is there a route map for good design?
Possibly. But there are different ways to
turn that spark of an idea into a living,
breathing circuit. There are no hard
and fast rules. Find the way that works
best for you. I like to scribble, tweak, do
the math if necessary, then maybe run
a few PSpice simulations. Only when
things are looking good at this stage do I
move to the breadboard. I rarely rely on
simulation results to prove a circuit. For
me, the breadboard is the final arbiter.
For really simple circuits, I sometimes
use the matrix-type breadboards where
you simply plug in the components, but
they have their limitations. I much prefer
to solder components onto a piece of
copper-clad board. As well as providing a
secure anchor for the parts, it functions
as a low-impedance ground plane.
What would be my advice for good design?
Try the three R’s—read, research,
revise. Plenty of good literature is available
on electronic design, not just in
textbooks, but also in manufacturers’
application notes and magazines like
Electronic Design. Thorough research is
essential, particularly when it comes to
choosing the right components. I used
the OPA2374 dual op amp in the rectifier
circuit only after carefully assessing
characteristics such as phase-reversal
behavior and overload recovery time.
Finally, revise. There’s no such thing as
a perfect design. Circuits can always be
improved.
And remember to think “out of the
box.” It’s an overused phrase, but important
nonetheless. That’s effectively
what I did with the rectifier circuit by
using a single-rail op amp in an unusual
way. Most designs would strive to avoid
saturating an op amp’s output, whereas
the rectifier circuit actually exploits that
behaviour. I like to design circuits just
for the hell of it. It’s fun, and I continue
to learn from the process.
So, is this idea thing an addiction?
Probably. As long as I keep getting a
buzz from analog design and that sweet
rush of anticipation whenever I warm
up my soldering iron and turn on my
oscilloscope, I’ll carry on turning ideas
into designs and, with a little inspiration,
I hope to continue sharing them
with you.