Race Cars, Oscilloscopes, and Fully Differential Amplifiers	Page 1 of 2
by Dennis Feucht

Semiconductor companies are now introducing fully differential amplifiers. These diff-amps are differential not only at their input, but also at the output, doubling their output range. These input and output circuits have closed paths not shared by other signals through a common ground node. This circuit (and hence signal) isolation improves signal integrity. By keeping both the input and the output circuits complete in themselves, the ground is only important for dc analysis and range determination. So what do these diff-amps have to do with race cars?

The "Race Track" in Electronics

State-of-the-art or "leading-edge" technology is often developed in adventurous settings. New automotive ideas, such as overhead cams and multi-port valving, are refined and characterized by applying them in demanding situations that have no significant effect on the company's present business. That setting is the race track, where technical concepts compete for performance advantages. The winning ideas go on to become integrated into products, where they then help compete for market share.

Where is the "race track" setting in electronics? Although the electronics industry lacks a glamorous event like the Indy 500, exciting developments occur in several branches of the field. For example, the Radiation Laboratory at MIT turned out some excellent results in the development of radar during WW II. Electronikers can admire, even to this day, the set of "Rad Lab" volumes from that era, containing a good, solid presentation of theory backed up by the electronics implemented. An earlier example: when Vladimir Zworykin developed television at RCA, this was an impressive breakthrough entirely unlike the incremental improvements of next-generation commodity products.

In the 1950s, the development of a laboratory-quality cathode-ray oscilloscope at Tektronix by Howard Vollum, Jack Murdock, Cliff Moulton, John Kobbe (to whom one Tek legend attributes the invention of the JK flip-flop), Bill Polits, and other highly creative engineers led to a most desirable technical environment for the motivated designer. With over 70% of the oscilloscope market—a market driven by technological advances—and with founders that were inventive engineers themselves, Tek was an engineering-driven company, an idea-advancing enterprise.

Although Tek and H-P are the outstanding examples of test and measurement (T&M) instrument companies, it is generally the case that high-performance T&M is the "race track" of the electronics industry. After all, one must be able to measure the behavior of circuits being developed, and test-equipment circuitry must be that much better to make the measurements. No wonder that interesting circuits are found in T&M equipment. And this leads to diff-amps.

Diff-Amps Found in Scopes

Oscilloscopes have used fully differential amplifiers for decades. (Why did it take so long for them to appear as commercial ICs?) Typical examples are found in vertical amplifiers. These scope subsystems amplify the probe voltage by precise gains before applying the amplified waveform to the vertical deflection plates of the CRT. And except for the first stage, which is driven by a ground-referenced probe, they are fully differential amplifiers, through and through. To demonstrate, let's look at part of the vertical amplifier of a Tek T935A 35 MHz scope—now obsolete, 1970s vintage, and low-cost. The input buffer amplifier stage, scanned from the manual, is shown below in Figure 1. (And by the way, the old Tek "instruction manuals" as they were called, contained schematics that were works of art, unparalleled by EDA CAD drawings of today—the price of progress!)

Figure 1 - Tek T935A 35 MHz Oscilloscope Input Buffer Amplifier

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