IEEE Circuits and Systems Magazine - Q3 2021 - 89

Pioneers in CAS
Bernard Tellegen
B
ernard Tellegen (June 1900-August 1990) was
a Dutch electrical engineer and inventor of the
pentode and the gyrator. He is also known for
a theorem in circuit theory, Tellegen's theorem. He attended
Delft University of Technology (TU Delft), where
he obtained his Master's degree in electrical engineering
in 1923. In 1924 he entered into the service of the Philips
Research Laboratories in Eindhoven, The Netherlands.
These laboratories had been founded in 1914 by Holst
and Oosterhuis. Tellegen belonged to a fairly small nucleus
(van der Pol, de Groot, Penning, Druyvestein, Bouwers)
around which one of the largest research centers
in the world would grow. As a scientific adviser, he later
became a member of its board of directors.
Tellegen's first studies concerned vacuum tubes. He
became interested in electron motions in triodes and
multigrid tubes. In 1926 he invented the pentode, which
was patented in a number of countries. It was the first
in a series of about 57 patents, which he received either
alone or in cooperation with others. In the following
years, Tellegen became interested in electrical circuits
on which he published in 1928, 1933 and 1934. In 1932
it was noticed, that the programs of some transmitters
from Beromünster, Switzerland, when received in the
Netherlands, seemed to carry also the program of Radio
Luxemburg and cross-modulation in the receiver tubes
was suspected. Tellegen showed that this was really a
nonlinear effect in the ionosphere, caused by the powerful
Luxemburg transmitter.
During his further studies in electrical networks, he
became more interested in fundamental problems such
as duality and geometric configurations and network
synthesis in particular of lossless two port (4-pole) networks.
During his basic study of the classical passive
network elements, Tellegen arrived at the conclusion
that a further element " the gyrator " could complete
the series in an elegant way. This new element does not
comply with the reciprocity relations, and is anti-symmetrical.
Tellegen studied the properties of circuits with
Digital Object Identifier 10.1109/MCAS.2021.3092587
Date of current version: 12 August 2021
the " gyrator. " Its first realization came in the microwave
field with the use of pre-magnetized ferrites. The circulator
was a further result of this idea. When the miniaturization
of electronic circuits led to new possibilities,
the gyrator soon became an important building block
for selective circuits at low frequencies.
In 1952 Tellegen published an important paper on
a general network theorem with applications. Fundamentally
" Tellegen's theorem " gives a simple relation
between magnitudes that satisfy Kirchhoff's laws.
Many treatises and a book on the application of this
theorem were published. A feature of this theorem is
that it applies to any system which can be represented
by a directed linear graph, and for which the throughvariables
obey the equivalent of Kirchhoff's current law
across any boundary (cut set), and the across-variables
obey the equivalent of Kirchhoff's voltage law around
every closed loop. Even more remarkable is that the
'currents' may relate to one network and the 'voltages'
to another network provided only that they have the
same directed linear graph, This makes possible many
applications to otherwise difficult problems including
calculating the sensitivity of electrical networks to
component tolerances.
A paper on " Synthesis of 2n-poles by networks consisting
of a minimum number of elements " proved his interest
in economy. He also discussed the ideal amplifier
or " nullor " (a represent an ideal amplifier, having infinite
current, voltage, transconductance and transimpedance
gain) although he did not use that name which
arose much later, and after many others had also proposed
similar ideal elements to enable active circuits to
be discussed in a similar style to passive circuits. For
examples, A.W. Keen proposed the 'unitor' in the early
1950s, later H.J. Carlin introduced the " nullator " (a theoretical
linear, time-invariant one-port defined as having
zero current and voltage across its terminals) and
" norator. " It was somewhat later than that there was the
realization that the nullor was a useful idealization of a
transistor operating as an amplifier and then of the Operational
Amplifier which became the cheap and widespread
amplifying element in many practical circuits.
THIRD QUARTER 2021
IEEE CIRCUITS AND SYSTEMS MAGAZINE
89
IEEE Circuits and Systems Magazine - Q3 2021

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