Vertical Amplifier Circuits

OSCILLOSCOPE VERTICAL

AMPLIFIERS BY

BOB ORWILER

CIRCUIT CONCEPTS

FIRST EDITION, FIRST PRINTING, DECEMBER 1969 062-1145-00 PRICE $1.00

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

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POWER SUPPLY

1 SWEEP

GENERATOR HORIZONTAL AMPLIFIER

power supply

cathode-ray tube

1

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.

The poweP supply converts some form of available 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.

The aathode-Pay tube (CRT) displays light on a 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 eZeatPon 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

trigger circuit

sweep generator

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 triggers. For this reason a trigger 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.

The sweep generator produces a sawtooth waveform 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.

horizontal amp I ifier

vertical amplifier

Primarily, the horizontal amplifier converts the time-base ramp, developed in the sweep generator,

3

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.

The vertiaal amplifier determines the useful 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. 0 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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Fig. 1-1. Oscilloscope display.

VERT Y AXIS DRIV PROBE AMPLIFIER

TRIGGER

X AXIS DRIVE

HORIZ AMPLIFIER

Fig. 1-2. Basic oscilloscope block diagram.

graticule

VERTICAL AMPLIFIERS

Vertical amplifiers meet requirements briefly summarized below:

5

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-ampZitude 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 called the graticule. Equal divisions divide the graticule X and Y axes. Fig. 1-1 shows ten major horizontal (X} divisions and six major vertiaaZ (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 1-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.

6

CRT

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ACCELERATION OR DRIFT REGION SCREEN

Fig. 1-3. CRT sections.

Cathode-ray tubes present a reactive load to deflection amplifiers. CRT construction and applied voltages cause a beam of the electrons, emitted by the CRT cathode, to form. The beam forms during electron travel from cathode to phosphor-coated faceplate. Energy contained in this concentrated mass of electrons striking the faceplate or screen is partially converted to light. Deflecting the beam vertically (Y) and horizontally (X) graphically reproduces waveforms. Deflection may be either electrostatic or electromagnetic. Since very few oscilloscopes use magnetic deflection, this book covers electrostatic deflection only.

Two pairs of plates mounted at right angles, as shown in Fig. 1-3, deflect the beam when energized. Leads from each plate penetrate the CRT envelope for connection to external excitation sources. The beam in the CRT shown passes first between the vertical deflection plates, then the horizontal.

A CRT and power supply alone function as a crude oscilloscope. Fig. 1-4 shows such an arrangement.

horizontal plates connect to Dl-D2 and vertical plates to D3-D4. Dl, D2 and D4 connect to a common voltage, shown as ground. D3 connects to a signal source represented by the three-position switch. Applying zero volts to D3 centers the beam. Throwing the switch to +V attracts the beam toward D3. Repelling occurs with -v applied to D3.

Connecting either Dl or D2 to a signal source results in horizontal deflection.

7

A vertical deflection system like this one has one advantage -- simplicity. Unfortunately, there are disadvantages. Some are:

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8

deflection sensitivity

signal source isolated

sensitivity improved

deflection factor

One needs a means of measuring deflection. Assume a vertical scale inscribed on the CRT faceplate. Scale increments begin at the center extending three divisions up and three divisions down. One needs to know the defleation sensitivity to calibrate the scale; i.e., how many scale divisions each volt deflects the electron beam. If divisions were scaled one cen