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OP AMP HISTORY
VACUUM TUBE OP AMPS
Lionel Jofeh, within UK patent 529,044 in 1939, offered a complete catalog of eight
forms of cathode-coupled amplifiers (see Reference 7).
Harold Goldberg presented a complete multi-stage, direct-coupled differential amplifier
in 1940 (see Reference 8). Using power pentodes within a unique low voltage differential
input stage, Goldberg reported an equivalent input noise of 2µV for the circuit. This work
parallels some of the earlier work mentioned above, apparently developed independently.
In 1941, Otto Schmitt published another work on the differential amplifier topic, going
into some detail of analysis (see Reference 9). In this work he clearly outlines the
advantages of the long-tailed pair, in terms of the stage's phase-inversion properties. He
also covers the case of a degenerated long-tailed pair, where a common cathode-cathode
resistance is used for gain adjustment, and the individual cathodes are biased to a negative
voltage with resistors of values twice that of a single cathode-coupled stage.
Walther Richter wrote on cathode follower and differential circuits in 1943 (see
Reference 10). While primarily focused on single-ended cathode followers, this article
also does an analysis of the long-tailed pair.
Harold Goldberg wrote again on his multi-stage differential amplifier, in 1944 (see
Reference 11). The 1944 version still used batteries for most of the power, but did add a
pentode to supply the bias current of the first stage long-tailed pair.
Writing in 1944, G. Robert Mezger offered a differential amplifier design with a new
method of interstage level-shift coupling (see Reference 12). Previous designs had used
either a resistive level shift like Nyquist, or the more recent glow-tube technique of
Miller. Mezger's design used a 12J5 triode as the bottom level shift element, which acts
as constant current source. Working against a fixed resistance at the top, this allows a
wideband level shift. Good overall stability was reported in a design that used both
differential and CM feedback. Regulation was used for plate and critical heater circuits.
Franklin Offner wrote a letter to the editor in 1945, expressing dissatisfaction with other
differential amplifier authors (see Reference 13). In this work he comments on the work
of Toennies (Reference 5, again), …"merely an application of in-phase degeneration by
the use of a large cathode resistor,"… Offner also overlooked Blumlein's patent.
D. H. Parnum published a two-part survey of differential amplifier techniques, in 1945
(see Reference 14). This work analyzed some previously published designs, and presented
two differential-throughout amplifier examples, both DC and AC-coupled.
In a comprehensive study of differential amplifier designs from 1947, Denis L. Johnston
presented a three-part article on design techniques, with a finished design example (see
Reference 15). This article is notable not only for the wealth of detailed information, but
it also contains a bibliography of 61 references to related works.
The input stage of Johnston's design example amplifier used an input long-tailed pair
based on the 6CS7 dual triode, with the cathode current supplied by a 6J7G pentode (see
H.11