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ACTICAL A OACH
VACU BE
b y F. J. BECKETT
Tektronix, Inc.
Electronic Instrumentation Group
Display Devices Development
T\vo articles published in past issues of Service Scope contained information
that, in our experience, is of particular benefit in analyzing circuits. The first
article was "Simplifying Transistor Ihear-Amplifier Analysis" (issue #29,
December, 1964). It describes a method for doing an adequate circuit analysis
for trouble-sliooting or evaluation purposes on transistor circuits. I t employs
the "Transresistance" concept rather than the complicatecl characteristic-family
parameters. Thc second article was "Uiitlerstat~ding and Using Thevetiin's The-
orem" (issue #40, October, 1966). I t offers a step-by-step explanation on how
to apply the principles of The\-enin's Theorem to analyze and utidei-stand how
a circuit operates.
Now, in this issue of Ser\Gce Scope, \YC 131-ese~it first of three articles
the
that will offer a practical approach to transistor aiitl vacuum-tube amplifiers
based on a simple D C analysis. These articles will, by virtue of additional in-
formation and the tying together of some loose ends, combine and bring into
better focus the concepts of "trai~s~esistaiice"
and the priiiciples of Thevenin's
Theorem. W e suggest that a "refresher" reading of the t\vo previous articles
will enable our readers to more readily follo\v the inforination in this and the
two following issues of Service Scope.
The Editor
L e t us consider the general equation for One is quite justified in looking at a
current through a P.N. diode junction. OBJECTIVE transistor in terms of the two-diode con-
V cept, refer to Figure 3. Therefore, assum-
The objective of this paper is to present
(1) ing diode A to be forward biased and di-
a practical approach to Transistor and
ode R to be reverse biased, as would be the
where V = applied volts Vacuum-tube amplifiers based on a simple
case if we were to operate the transistor
D C analysis.
I, = reverse bias current as a linear amplifier, the current through
p = constant between 1 & 2 The articles will be published in the fol- diode A will conform to equation ( 1 ) .
kT lowing sequence. Let us take a closer look a t Figure 2.
and V, = - where k = Boltzmans
I
(1 1. The Transistor Amplifier. W e define conductance in the general
Const., 1.38 x 10-"' Joule/"Kelvin 2. The Vacuum-tube Amplifier. case as
T = absolute temperature in degree Kel- 3. An analysis of a typical Tektronix hy-
vin at rooin temperature, i.e., T = 300