Introduction
This is the first issue of Celtic Engineering Solutions weekly newsletter called The Celtic Engineer. We hope you enjoy it. If you have any suggestions for topics, would like to give feedback or want your email added to the distribution list please send an email to [email protected].
Op Amps
The Op Amp we use today has its origins over a hundred years ago with the invention in 1906 of the three element triode by Lee
De Forest. This invention led the way for the first feedback amplifier to be invented at Bell Labs in the late 1920’s and later to a general purpose feedback amplifier using vacuum tubes during the 1940’s. While vacuum tube amps increased in the 50’s and 60’s the invention of the transistor by Bardeen, Brattain and Shockley (1947) was about to change the form of Op Amps forever. Along with the invention of the transistor, the inventions of the integrated circuit and the development of the planer process of transistor geometry on a silicon substraight paved the way for the modern Op Amp. The first Op Amp to come out as a single integrated circuit was designed by Bob Widlar of Fairchild Semiconductor in 1963, called the µA702.
Some terminology is in order. If both inputs to an op amp are at exactly the same voltage, then the output should be at zero volts. This is not the case in reality. In order to get the output to zero volts a small differential voltage must be applied to the inputs, this is called the input offset voltage or Vos. Many amps will have pins available to adjust for the input offset voltage. It is important to note that because of an op amps offset drift it is important to use them only to null the devices offset and never the system offset. In quad packs these pins are not available and the engineer will have to rely on external circuitry to compensate for this parameter.
Another ideal behavior of an op amp is that no current flows into or out of the terminals of the device. In reality there are always input bias currents (IB). While they are small, they also travel in external impedances and produce voltages. The effects of bias current can be compensated for, when those currents are well matched, by selecting a resistor for the non-inverting input that is the parallel combination of the two resistors used in the feedback of an inverting op amp.
Op Amps are one of the most commonly used circuit devices because of their power, ease of use and versatility. The Op Amp can be configured as a gain stage, a buffer, can perform filtering functions (low pass, band pass and high pass), can be used as driver, an integrator or differentiator, and many more. There are many excellent reference books on the topic. One of my favorite is “Op Amp Applications” edited by Walter Jung and published by Analog Devices.
Final thoughts
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The Celtic Engineer Issue #1: Op Amps
