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Vertical Amplifier
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Circuits
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Circuit Concepts Series
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OSCILLOSCOPE
VERTICAL
AMPLIFIERS
BY
BOB ORWllER
CIRCUIT CONCEPTS
FIRST EDITION, FIRST PRINTING, DECEMBER,1969
062-1145-00
© TEKTRONIX, INC.; 1969
BEAVERTON, OREGON 97005
ALL RIGHTS RESERVED
CONTENTS
INTRODUCTION 1
1 VERTICAL AMPLIFIERS 5
2 INPUT CIRCUITS AND
COMPENSATED ATTENUATORS 39
3 FOLLOWERS 59
4 TRANSMISSION LINES 173
5 SINGLE-ENDED AMPLIFIERS 193
6 DIFFERENTIAL AMPLIFIER FAMILY 253
7 FEDBACK AMPLIFIERS 393
INDEX 459
VERTICAL
AMPLIFIER CRT
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POWER
SUPPLY
TRIGGER SWEEP HORIZONTAL
GENERATOR AMPLIFIER
I
INTRODUCTION
In a study of conventional oscilloscopes, the
various circuits of the instrument fall into
general groupings: the power supply, cathode-ray
tube, trigger circuit, sweep generator, horizontal
amplifier and the vertical amplifier.
Each performs an important function. Combinations
of individual contributions determine instrument
performance.
power The powe~ suppZy converts some form of available
supply power (depending on the type and location of the
instrument) to DC operating potentials for all the
active circuits of the oscilloscope. Regulation
holds most of these potentials to narrow tolerances,
guaranteeing precise circuit performance. A power
supply might also contain filament power for
vacuum-tube circuits, overload protection and
facilities for adapting the instrument to various
local power conditions.
cathode-ray The cathode-~ay tube (CRT) displays light on a
tube two-dimensional phosphor screen. It conveys
intelligence in the form of alphanumerics, picture
images or graphs. Graphical presentations offer
an analytical approach: Actual measurements taken
with a graticule along the "X" and "Y" screen axis.
The CRT eZect~on gun is sealed inside an envelope.
A vacuum minimizes collisions between free gas
particles and the electron beam. High-voltage power
supplies create controllable electrostatic fields
which accelerate free electrons from the heated
cathode to form an electron beam. The beam of
electrons then transit an electron lens which
converges or focuses the beam on a phosphor screen.
When the high-velocity electrons collide with
phosphor atoms at the focal point, photons of light
emit towards the viewer.
2
Varying 'the electrostatic fields between a set of
"X" and "Y" deflection plates positions the light
source on the screen. Thus the electron beam
creates a display anywhere within the viewing screen
area. Since electrons have an extremely small mass,
they can be deflected or scanned over the entire
screen area millions of times per second. This
permits the viewer to observe changing phenomena in
real time.
The CRT must not unduly load either the vertical or
horizontal amplifier nor require a greater dynamic
deflection range of voltage than the amplifiers can
supply. In practice, the CRT and deflection
amplifiers are designed together for maximum
performance and efficiency.
Input signals take a wide variety of shapes and
amplitudes, many unsuitable as sweep-initiating
trigger triggers. For this reason a trigger oirouit
circuit converts these signals to pulses of uniform
amplitude and shape. This trigger circuit makes it
possible to start the sweep with a pulse that has a
constant size, eliminating variations of the sweep
circuit operation caused by changing input signals.
The operator now uses either slope of the waveform
to start the sweep, selects any voltage level on
the rising or falling slope of the waveform, and,
in some instances, eliminates selected frequencies
of the input signal with ease and repeatability.
sweep The sweep generator produces a sawtooth waveform
generator for processing by the horizontal amplifier which
then deflects the CRT beam. The sweep generator
produces a sawtooth waveform, with the proper rate
of-rise, amplitude and linearity, suitable as a
time-measuring reference.
3
horizontal Primarily, the horizontal amplifier converts the
ampl ifier time-base ramp, developed in the sweep generator,
to deflection voltage for the horizontal CRT
deflection plates. The resulting trace is the
reference for Y-T displays (voltage plotted as a
function of time). In those instruments offering
X-Y capabilities (where both X and Y inputs are
dependent variables) the horizontal amplifier reacts
to the external (X) input as a linear amplifier.
It exhibits a frequency response comparable to that
of the vertical amplifier. Additionally, the
horizontal amplifier provides DC-level and amplifier
gain controls which permit positioning of the
horizontal trace and sweep magnification (expansion)
respectively. The latter facility extends
instrument sweep speed without imposing additional
sweep-rate requirements on the sweep generator.
vertical The vertical amplifier determines the useful
ampl ifier bandwidth and gain of the instrument. Vertical
amplifiers take three general forms: a fixed
vertical, a complete vertical in a plug-in form or
a fixed main vertical amplifier preceded by a
plug-in preamplifier.o Selection of plug-ins
allows a range of characteristics. An additional
type of instrument takes the drive directly to the
CRT plates without passing through any type of
amplifier.
The general-purpose oscilloscope provides a faithful
display of an input voltage. For meaningful results,
displayed waveforms contain few aberrations and these
but a few percent of the total waveform amplitude.
4
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TIME!DIV
Fig. 1-1. Oscilloscope display.
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VERT YAXIS DRIVE,.
PROBE AMPLIFIER
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POWER XAXIS DRIVE
SUPPLY
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1
...,.. TRIGGER r- SWEEP f---- HaRtZ
~ GENERATOR AMPLIFIER
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Fig. 1-2. Basic oscilloscope block diagram.
5
VERTICAL AMPLIFIERS
Vertical amplifiers meet requirements briefly
summarized below:
1. Buffer the signal source (probe) and the
CRT.
2. Provide various modes of operation such as:
direct or AC coupling, multiple trace, and,
perhaps, selectable differential modes.
3. Finally, the amplifier faithfully
reproduces voltage waveforms within
specified risetime-bandwidth-amplitude
limits.
Why these items are important and how they are
accomplished by Tektronix is what this book is about.
An oscilloscope graphically displays signals of
interest, as shown in Fig. 1-1. Here periodic
rectangular pulses appear superimposed on a grid
graticule called the graticule. Equal divisions divide the
graticule X and Y axes. Fig. 1-1 shows ten major
horizontal (x) divisions and six major vertical (y)
divisions. Small markings along the center lines
describe minor division dimensions at 20% of a major
division.
The horizontal base is calibrated in units of time
per division; the vertical in units of voltage per
division. For example, if each vertical increment
is 0.5 volts and horizontal is 0.5 milliseconds
per division, then Fig. 1-1 depicts a l-volt pulse
train with pulses recurring each millisecond.
Accurate information from both horizontal and
vertical circuits creates faithful reproductions of
the input waveforms. To accomplish this an
oscilloscope generally requires the basic blocks
shown in Fig. 1-2.
