Texas Instruments slod006b pdf datasheet download

slod006b

The Op Amp’s Place In The World

Ron Mancini
In 1934 Harry Black[1] commuted from his home in New York City to work at Bell Labs
in New Jersey by way of a railroad/ferry. The ferry ride relaxed Harry enabling him to do
some conceptual thinking. Harry had a tough problem to solve; when phone lines were
extended long distances, they needed amplifiers, and undependable amplifiers limited
phone service. First, initial tolerances on the gain were poor, but that problem was quickly
solved with an adjustment. Second, even when an amplifier was adjusted correctly at the
factory, the gain drifted so much during field operation that the volume was too low or the
incoming speech was distorted.
Many attempts had been made to make a stable amplifier, but temperature changes and
power supply voltage extremes experienced on phone lines caused uncontrollable gain
drift. Passive components had much better drift characteristics than active components
had, thus if an amplifier’s gain could be made dependent on passive components, the
problem would be solved. During one of his ferry trips, Harry’s fertile brain conceived a
novel solution for the amplifier problem, and he documented the solution while riding on
the ferry.
The solution was to first build an amplifier that had more gain than the application required.
Then some of the amplifier output signal was fed back to the input in a manner
that makes the circuit gain (circuit is the amplifier and feedback components) dependent
on the feedback circuit rather than the amplifier gain. Now the circuit gain is
dependent on the passive feedback components rather than the active amplifier. This is
called negative feedback, and it is the underlying operating principle for all modern day
op amps. Harry had documented the first intentional feedback circuit during a ferry ride.
I am sure unintentional feedback circuits had been built prior to that time, but the designers
ignored the effect!
I can hear the squeals of anguish coming from the managers and amplifier designers. I
imagine that they said something like this, “it is hard enough to achieve 30-kHz gain–
bandwidth (GBW), and now this fool wants me to design an amplifier with 3-MHz GBW.
But, he is still going to get a circuit gain GBW of 30 kHz”. Well, time has proven Harry right,
but there is a minor problem that Harry didn’t discuss in detail, and that is the oscillation problem. It seems that circuits designed with large open loop gains sometimes oscillate
when the loop is closed. A lot of people investigated the instability effect, and it was pretty
well understood in the 1940s, but solving stability problems involved long, tedious, and
intricate calculations. Years passed without anybody making the problem solution simpler
or more understandable.
In 1945 H. W. Bode presented a system for analyzing the stability of feedback systems
by using graphical methods. Until this time, feedback analysis was done by multiplication
and division, so calculation of transfer functions was a time consuming and laborious task.
Remember, engineers did not have calculators or computers until the ’70s. Bode presented
a log technique that transformed the intensely mathematical process of calculating a
feedback system’s stability into graphical analysis that was simple and perceptive. Feedback
system design was still complicated, but it no longer was an art dominated by a few
electrical engineers kept in a small dark room. Any electrical engineer could use Bode’s
methods to find the stability of a feedback circuit, so the application of feedback to machines
began to grow. There really wasn’t much call for electronic feedback design until
computers and transducers become of age.
The first real-time computer was the analog computer! This computer used preprogrammed
equations and input data to calculate control actions. The programming was
hard wired with a series of circuits that performed math operations on the data, and the
hard wiring limitation eventually caused the declining popularity of the analog computer.
The heart of the analog computer was a device called an operational amplifier because
it could be configured to perform many mathematical operations such as multiplication,
addition, subtraction, division, integration, and differentiation on the input signals. The
name was shortened to the familiar op amp, as we have come to know and love them.
The op amp used an amplifier with a large open loop gain, and when the loop was closed,
the amplifier performed the mathematical operations dictated by the external passive
components. This amplifier was very large because it was built with vacuum tubes and
it required a high-voltage power supply, but it was the heart of the analog computer, thus
its large size and huge power requirements were accepted as the price of doing business.
Many early op amps were designed for analog computers, and it was soon found out that
op amps had other uses and were very handy to have around the physics lab.
At this time general-purpose analog computers were found in universities and large company
laboratories because they were critical to the research work done there. There was
a parallel requirement for transducer signal conditioning in lab experiments, and op amps
found their way into signal conditioning applications. As the signal conditioning applications
expanded, the demand for op amps grew beyond the analog computer requirements,
and even when the analog computers lost favor to digital computers, the op amp
survived because of its importance in universal analog applications. Eventually digital
computers replaced the analog computers (a sad day for real-time measurements), but
the demand for op amps increased as measurement applications increased.

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