Sixty years ago this month the PROCEEDINGS OF THE INSTITUTE OF RADIO ENGINEERS (IRE) included a paper by Frederick B. Llewellyn concerning the theory of vacuum tubes at ultrahigh frequencies. At the time he was employed as a research engineer at the Bell Telephone Laboratories. He began the paper by reviewing the history of the analytical theory of vacuum tubes at lower frequencies. He stated that designers of electronic circuits had relied on “cut and try” methods until the introduction of the theory of equivalent circuits by van der Bijl and others who showed that, for purposes of analysis, a vacuum-tube amplifier could be treated as a fictitious generator with an internal impedance determined from the static volt-ampere characteristics. Llewellyn continued that the key to the success of this method was in the separation of alternating and direct current components, with the equivalent circuit applying only to alternating currents.
In his November 1933 paper, Llewellyn undertook to extend the equivalent circuit approach to cases where the transit time of electrons from cathode to plate was comparable to the period of a signal to be amplified. He cited earlier work by Irving Langmuir, Karl T. Compton, and W. E. Benham as being important in setting the stage for his own work. Beginning with the “classical equations” of electromagnetic theory, Llewellyn derived equations for a modified equivalent circuit for vacuum tubes used at ultrahigh frequencies. He expressed the hope that his analysis would help designers better to visualize cases where the “time of flight” of electrons had to be taken into account.
Llewellyn was born in 1897 in New Orleans, LA. As an early radio enthusiast, he worked as a radio operator for the United States Navy and the Merchant Marine intermittently from 1915 to 1921. He received an engineering degree from Stevens Institute of Technology, Hoboken, NJ, in 1922 and then worked as a laboratory assistant to Frederick K. Vreeland during 1922-1923. In 1923 he was hired as a research engineer at a laboratory of the Western Electric Company and he continued at the laboratory when it became part of the Bell Telephone Laboratories in 1925. He was author or coauthor of seven IRE papers published between 1930 and 1935 dealing with topics such as noise, modulation, oscillators, and high-frequency vacuum-tube amplifiers. He received the Morris N. Liegmann Memorial Award of the IRE in 1935 in recognition or his studies of vacuum tubes at ultra-high frequencies. His book Electron Inertia Effects was published in 1941.
During World War II, Llewellyn served as a consultant to the Office of Secretary of War. He and Edwin H. Armstrong designed a sensitive receiver which was used to detect a radar signal reflected from the moon. Llewellyn served on the IRE Board of Directors from 1939 to 1945 and was IRE President in 1946. He held various consulting positions, mostly concerned with military electronics systems, in the postwar years and was assistant to the President of Bell Telephone Laboratories from 1956 to 1961. He was affiliated with the Institute of Science and Technology at the University of Michigan from 1961 to 1965 and then was Research Director of the Polytechnic Institute of Brooklyn from 1965 until his retirement in 1967. During his career he received around 39 patents. He died at the age of 74 in 1971.
James E. Brittain
School of History, Technology, and Society
Georgia Institute of Technology
Submitted by Dick Reiman, Historian
Copyright 1993 IEEE. Reprinted with permission from the IEEE publication, “Scanning the Past” which covers a reprint of an article appearing in the Proceedings of the IEEE Vol. 81, No.11, November 1993.