The Evolution and the History of Current Conveyors - Current Conveyors: Variants, Applications and Hardware Implementations - page 3

However, the entire literature on CCs, which has been scattered over 1,000
journal papers, has still not been made available at a single source by any academic
author/publisher so far. Although a few topic chapters/topics have been written on
some limited aspects of CCs, any comprehensive treatise on CCs has still not been
brought out.
This monograph aims to fill this void by bringing out a repertoire of important
hardware implementations and applications of CCs and their numerous variants in
modern analog circuit design and thereby bringing out a single-source reference on
almost everything related to CCs at one place.
To put the contents of this topic in the right perspective, in the following
sections, we present a brief historical account of the story of the invention of the
CCs as well as successive evolution of a large number of their variants which have
been proposed by a number of researchers from time to time.

1.2 The Origin of the First Generation Current Conveyor

Although the story of how the concept of current conveying and its embodiment the
Current Conveyor were originated has been mentioned by Sedra in [ 23 ] it is still
interesting and inspirational to reiterate the same. It is told that when Sedra was
working for his Master

s thesis he was assigned the task of designing a good
temperature-insensitive constant current source.
Sedra perceived that the common base arrangement shown in Fig. 1.1a was the
natural choice to achieve this.
However, the output collector current in such a case is temperature-sensitive due
to the presence of V EB1 which is equal to V T 1n

'

I co wherein V T is directly
proportional to the absolute temperature (being equal to KT/q where K is Boltzman
constant, T is absolute temperature and q is electron charge). To get rid of this
unwanted term, he thought of cancelling out V BE1 by ungrounding the base of
transistor Q 1 and bringing another matched PNP transistor Q 2 in the loop, with the
collector current of Q 2 made same as I 0 . This modification clearly makes the
voltage

I 0

V EB2 ). Since Q1 and Q2 carry essentially the same
amount of collector current (assuming the

e to be e ¼ (V EB1

' '

of the transistors to be very high
thereby considering the base currents to be negligibly small), it is clear that the
corresponding V EBs of the two transistors will cancel out thereby rendering the
voltage e

β

0 (a sort of virtual ground). Therefore, the resulting output current is
now given by I 0 ¼

¼

+V/R 0 which is, as required, temperature- insensitive!
Now in order to ensure that Q 2 carries the same current as Q 1 , a current copier
comprised of three matched NPN transistors was added as shown in simplified
version of Fig. 1.1c (the circuit devised by Smith and Sedra [ 1 ] contains three
matched resistors added between the emitters of the transistors Q 3 , Q4 and Q 5 and

V). This completed the design of the required temperature-insensitive current
source.