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Karl Swartzel's Op Amp

In terms of op amp details, the Boghosian et al patent references another crucial patent document. And, many other Bell Labs M9-related patents, underscoring its seminal nature, also referenced this latter work. The patent in question here is US Patent 2,401,779, "Summing Amplifier" by Karl D. Swartzel Jr. of Bell Labs (Reference 30: K. D. Swartzel, Jr., "Summing Amplifier," US Patent 2,401,779, filed May 1, 1941, issued July 11, 1946. (The first operational amplifier, used as a summing amplifier)), and a design that could well be the genesis of op amps. Ironically, Swartzel's work was never given due publicity by Bell Labs.(Bell Labs documented virtually everything on the M9 in its Bell Laboratories Record, as can be noted by the section references. But no op amp schematics were included in this long string of articles!) Filed May 1, 1941, it languished within the system during the war, finally being issued in 1946. Of course, the same thing could be said about many wartime patents— in fact many other Bell Labs patents met similar fates.
A schematic diagram for Swartzel's op amp is shown below in Figure 3, which includes a table of values taken from the patent text. Although the context of the patent is an application as a summing amplifier, it is also obvious that this is a general purpose, high gain amplifier, externally configured for a variety of tasks by the use of suitable feedback components— the crux of the matter regarding the op amp function.
Schematic diagram and component values for "Summing Amplifier" (US Patent 2,401,779, assigned to Bell Telephone Laboratories, Inc.)
Figure 3: Schematic diagram and component values for "Summing Amplifier" (US Patent 2,401,779, assigned to Bell Telephone Laboratories, Inc.)
In the schematic of this "summing amplifier" there can be noted a number of key points. Three directly coupled tubes provided a high overall gain, with a net sign inversion with respect to the grid of input tube 4. Positive and negative power supplies are provided by the tapped battery, 25. The amplifier output swing at the load 15 is bipolar with respect to the common terminal, 26. In this instance of use, resistor R16 applies the feedback, and three signals are being summed via input resistors R1, R2, R3, with the input common to terminal 26. The input via resistor R18 was used for offset control.
The amplifier gain quoted in the patent was 60,000 (95dB), and as noted, the circuit could drive loads of 6kΩ, which is quite an achievement. It operated from supplies of ±350V, with intermediate voltages as noted. The inter-stage level shift networks are the form described by Nyquist, and there are several RC networks used for stabilization purposes.
Over time, there were changes made to this basic design, which will be described later in this section. The Swartzel op amp was truly a seminal work, as it allowed the creation of a complete, highly sophisticated analog computer system critical to WWII defense. It also spawned numerous other amplifier designs derived from its basic topology.
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