Transistor current-switching circuits
In recent years, the transistor has replaced the vacuum tube
for most switching applications and there are various types of high-speed
switching circuits.1 A very useful class of switching circuits known as
current-switching circuits or current-mode logic (c.m.l.) circuits has been
devised by Yourke.2 This type of switching circuit, which avoids the satura-
tion region and hence the hole storage phenomenon associated with it, uses low
voltage swings and low impedance levels. It fully utilizes the inherent band-
width of the triode and attains very high switching speeds approaching to the
theoretical minimum rise and fall times for a given transistor. A number of
modi- fications3 ' 4*5j 6 have been suggested to simplify the logical operation
and circuit techniques and a num- ber of advantages have been described in the
literature for this type of circuitry. Investigations have shown that the
current-switching circuit possesses the follow- ing disadvantages: (1)
Relatively large number of transistors required. (2) Large number of power
supplies required. (3) Stringent tolerance requirements of the com- ponents and
the power supplies used in the circuits. (4) Large power dissipation in
transistors. The purpose of this paper is to point out the limitations of the circuits
proposed previously to show that most of the above-mentioned disadvantages can
be over- come by suitable circuit modifications; further, a more detailed
analysis and design philosophy and the experimental results of the c.m.l.
circuits is presented.
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