High speed amplifier.pdf

Application Note 47

August 1991

High Speed Amplifier Techniques

A Designer’s Companion for Wideband Circuitry

Jim Williams

PREFACE

This publication represents the largest LTC commitment

to an application note to date. No other application note

absorbed as much effort, took so long or cost so much.

This level of activity is justified by our belief that high speed

monolithic amplifiers greatly interest users.

Historically, monolithic amplifiers have represented pack-

ets of inexpensive, precise and controllable gain. They

have partially freed engineers from the constraints and

frustrations of device level design. Monolithic operational

amplifiers have been the key to practical implementation

of high level analog functions. As good as they are, one

missing element in these devices has been speed.

Devices presently coming to market are addressing mono-

lithic amplifiers’ lack of speed. They bring with them the

ease of use and inherent flexibility of op amps. When

Philbrick Researches introduced the first mass produced

op amp in the 1950’s (K2-W) they knew it would be used.

What they couldn’t possibly know was just how widely,

and how many different types of applications there were.

As good a deal as the K2-W was (I paid $24.00 for mine -

or rather, my father did), monolithic devices are far better.

The combination of ease of use, economy, precision and

versatility makes modern op amps just too good to be

believed.

Considering all this, adding speed to op amps’ attractions

seems almost certain to open up new application areas.

We intend to supply useful high speed products and the

level of support necessary for their successful application

(such high minded community spirit is, of course,

capitalism’s deputy). We hope you are pleased with our

initial efforts and look forward to working together.

AN47-1

Application Note 47

TABLE OF CONTENTS

PREFACE .......................................................................................................................................................... AN47-1

INTRODUCTION ................................................................................................................................................ AN47-5

PERSPECTIVES ON HIGH SPEED DESIGN ....................................................................................................... AN47-5

MR. MURPHY’S GALLERY OF HIGH SPEED AMPLIFIER PROBLEMS

Unterminated Pulse Generator ..................................................................................................................... AN47-7

Poorly Terminated Line ................................................................................................................................ AN47-8

Poor Probe Grounding ................................................................................................................................. AN47-8

Undercompensated Probe ............................................................................................................................ AN47-8

Overcompensated Probe .............................................................................................................................. AN47-9

Mismatched Delay in Probes ........................................................................................................................ AN47-9

Overdriven FET Probe ................................................................................................................................... AN47-9

Probe at Amplifier Summing Point ............................................................................................................. AN47-10

Poor Quality Probe ..................................................................................................................................... AN47-10

Oscilloscope Overdriven ............................................................................................................................. AN47-10

Poor or No Ground Plane ........................................................................................................................... AN47-11

No Bypass Capacitors, Heavy Load ............................................................................................................ AN47-11

No Bypass Capacitors, No Load ................................................................................................................. AN47-11

Poor Quality Bypass Capacitors .................................................................................................................. AN47-12

Paralleled Bypass Capacitors Ring ............................................................................................................. AN47-12

Almost Good Enough Bypass Capacitors ................................................................................................... AN47-12

2pF at Amplifier Summing Junction ........................................................................................................... AN47-12

Noise Due to Coupling Into Critical Nodes .................................................................................................. AN47-13

1pF Coupling Path’s Effects ........................................................................................................................ AN47-13

Decompensated Amplifier at Too Low a Gain ............................................................................................. AN47-13

Excessive Capacitive Load .......................................................................................................................... AN47-13

Common Mode Overdrive ........................................................................................................................... AN47-14

Booster Stage with Local Oscillations......................................................................................................... AN47-14

Booster Stage with Loop Oscillations ......................................................................................................... AN47-14

Excessive Source Impedance ..................................................................................................................... AN47-14

TUTORIAL SECTION

About Cables, Connectors and Terminations .............................................................................................. AN47-15

About Probes and Probing Techniques ...................................................................................................... AN47-16

About Oscilloscopes ................................................................................................................................... AN47-20

About Ground Planes ................................................................................................................................. AN47-24

About Bypass Capacitors ............................................................................................................................ AN47-25

Breadboarding Techniques ......................................................................................................................... AN47-26

Oscillation .................................................................................................................................................. AN47-29

AN47-2

Application Note 47

APPLICATIONS SECTION I - AMPLIFIERS

Fast 12-Bit DAC Amplifier ........................................................................................................................... AN47-32

2-Channel Video Amplifier .......................................................................................................................... AN47-32

Simple Video Amplifier ............................................................................................................................... AN47-32

Loop Through Cable Receivers ................................................................................................................... AN47-32

DC Stabilization – Summing Point Technique ............................................................................................ AN47-33

DC Stabilization – Differentially Sensed Technique ..................................................................................... AN47-34

DC Stabilization – Servo Controlled FET Input Stage .................................................................................. AN47-34

DC Stabilization – Full Differential Inputs with Parallel Paths ..................................................................... AN47-35

DC Stabilization – Full Differential Inputs, Gain-of-1000 with Parallel Paths ............................................... AN47-35

High Speed Differential Line Receiver ......................................................................................................... AN47-37

Transformer Coupled Amplifier .................................................................................................................. AN47-38

Differential Comparator Amplifier with Adjustable Offset ............................................................................ AN47-39

Differential Comparator Amplifier with Settable Automatic Limiting and Offset .......................................... AN47-40

Photodiode Amplifier .................................................................................................................................. AN47-41

Fast Photo Integrator .................................................................................................................................. AN47-41

Fiber Optic Receiver ................................................................................................................................... AN47-43

40MHz Fiber Optic Receiver with Adaptive Trigger ..................................................................................... AN47-43

50MHz High Accuracy Analog Multiplier .................................................................................................... AN47-44

Power Booster Stage .................................................................................................................................. AN47-45

High Power Booster Stage ......................................................................................................................... AN47-47

Ceramic Bandpass Filters ........................................................................................................................... AN47-48

Crystal Filter ............................................................................................................................................... AN47-48

APPLICATIONS SECTION II - OSCILLATORS

Sine Wave Output Quartz Stabilized Oscillator ............................................................................................ AN47-48

Sine Wave Output Quartz Stabilized Oscillator with Electronic Gain Control ............................................... AN47-49

DC Tuned 1MHz-10MHz Wien Bridge Oscillator ......................................................................................... AN47-49

Complete AM Radio Station ....................................................................................................................... AN47-50

F Converter ..................................................................................................................... AN47-54

8-Bit, 100ns Sample-Hold .......................................................................................................................... AN47-56

15ns Current Summing Comparator .......................................................................................................... AN47-57

50MHz Adaptive Threshold Trigger Circuit ................................................................................................. AN47-58

Fast Time-to-Height (Pulsewidth-to-Voltage) Converter ............................................................................. AN47-58

True RMS Wideband Voltmeter .................................................................................................................. AN47-61

APPLICATIONS SECTION IV - MISCELLANEOUS CIRCUITS

RF Leveling Loop ........................................................................................................................................ AN47-63

Voltage Controlled Current Source ............................................................................................................. AN47-63

High Power Voltage Controlled Current Source .......................................................................................... AN47-63

18ns Circuit Breaker ................................................................................................................................... AN47-63

AN47-3

APPLICATIONS SECTION III - DATA CONVERSION

1Hz-1MHz Voltage-Controlled Sine Wave Oscillator ................................................................................... AN47-51

1Hz-10MHz V

Application Note 47

REFERENCES .................................................................................................................................................. AN47-67

APPENDICES

A. ABC’s of Probes – Contributed by Tektronix, Inc. ................................................................................. AN47-69

B. Measuring Amplifier Settling Time ........................................................................................................ AN47-82

C. The Oscillation Problem – Frequency Compensation Without Tears ..................................................... AN47-86

D. Measuring Probe-Oscilloscope Response ............................................................................................. AN47-93

E. An Ultra Fast High Impedance Probe .................................................................................................... AN47-96

F. Additional Comments on Breadboarding ............................................................................................... AN47-98

G. FCC Licensing and Construction Permit Applications for Commercial AM Broadcasting Stations ...... AN47-123

H. About Current Mode Feedback ............................................................................................................ AN47-124

I. High Frequency Amplifier Evaluation Board ........................................................................................ AN47-127

J. The Contributions of Edsel Murphy to the Understanding of the Behavior of Inanimate Objects,

D.L. Klipstein (with permission of Cahners Publishing Co.) ................................................................ AN47-130

AN47-4

Application Note 47

INTRODUCTION

schematic” (this descriptive was originated by Charly

Gullett of Intel Corporation) usually dominates the circuit’s

form, particularly at high speed.

In this regard, much of the text and appendices are

directed at developing awareness of, and respect for,

circuit parasitics and fundamental limitations. This ap-

proach is maintained in the applications section, where the

notion of negotiated compromises is expressed in terms

of resistor values and compensation techniques. Many of

the application circuits use the amplifier’s speed to im-

prove on a standard circuit. Some utilize the speed to

implement a traditional function in a non-traditional way,

with attendant advantages. A (very) few operate at or near

the state-of-the-art for a given circuit type, regardless of

approach. Substantial effort has been expended in devel-

oping these examples and documenting their operation.

The resultant level of detail is justified in the hope that it will

be catalytic. The circuits should stimulate new ideas to suit

particular needs, while demonstrating fast amplifiers’

capabilities in an instructive manner.

Most monolithic amplifiers have been relatively slow

devices. Wideband operation has been the province of

discrete and hybrid technologies. Some fast monolithic

amplifiers have been available, but the exotic and expensive

processing required has inflated costs, precluding

widespread acceptance. Additionally, many of the previous

monolithic designs were incapable of high precision and

prone to oscillation or untoward dynamics, making them

unattractive.

Recent processing and design advances have made inex-

pensive, precision wideband amplifiers practical. Figure 1

lists some amplifiers, along with a summary of their

characteristics. Reviewing this information reveals ex-

traordinarily wideband devices, with surprisingly good DC

characteristics. All of these amplifiers utilize standard op

amp architecture, except the LT1223 and LT1228, which

are so-called current mode feedback types (see Appendix

H, “About Current Mode Feedback”). It is clear that the raw

speed capabilities of these devices, combined with their

inherent flexibility as op amps, permit a wide range of

applications. What is required of the user is a familiarity

with the devices and respect for the requirements of high

speed circuitry.

This effort’s initial sections are devoted to familiarizing the

reader with the realities and difficulties of high speed

circuit work. The mechanics and subtleties of achieving

precision circuit operation at DC and low frequency have

been well documented. Relatively little has appeared which

discusses, in practical terms, how to get fast circuitry to

work. In developing such circuits, even veteran designers

sometimes feel that nature is conspiring against them. In

some measure this is true. Like all engineering endeavors,

high speed circuits can only work if negotiated compro-

mises with nature are arranged. Ignorance of, or contempt

for, physical law is a direct route to frustration. Mother

Nature laughs at dilettantism and crushes arrogance with-

out even knowing she did it. Even without Einstein’s

revelations, the world of high speed is full of surprises.

Working with events measured in nanoseconds requires

the greatest caution, prudence and respect for Mother

Nature. Absolutely nothing should be taken for granted,

because nothing is. Circuit design is very much the art of

compromise with parasitic effects. The “hidden

PERSPECTIVES ON HIGH SPEED DESIGN

A substantial amount of design effort has made Figure 1’s

amplifiers relatively easy to use. They are less prone to

oscillation and other vagaries than some much slower

amplifiers. Unfortunately, laws of physics dictate that the

circuit’s environment must be properly prepared. The

performance limits of high speed circuitry are often

determined by parasitics such as stray capacitance, ground

impedance and layout. Some of these considerations are

present in digital systems where designers are comfortable

describing bit patterns, delays and memory access times

in terms of nanoseconds. Figure 2’s test circuit provides

valuable perspective on just how fast these amplifiers are.

Here, the pulse generator (Trace A, Figure 3) drives a

74S04 Schottky TTL inverter (Trace B), an LT1223 op amp

connected as an inverter (Trace C), and a 74HC04 high

speed CMOS inverter (Trace D). The LT1223 doesn’t fare

too badly. Its delay and fall times are about 2ns slower than

the 74S04, but significantly faster than the 74HC04. In

fact, the LT1223 has completely finished its transition

before the 74HC04 even begins to move! Linear circuits

operating with this kind of speed make many engineers

justifiably wary. Nanosecond domain linear circuits are

widely associated with oscillations, mysterious shifts in

AN47-5

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