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4 Interfacing

May 12, 2017

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  • Interface Circuits:

    Hooking Up To TheOutside World

    Prof. Greg Kovacs

    Department of Electrical Engineering

    Stanford University

  • EE122, Stanford University, Prof. Greg Kovacs 2

    Design Note: The Design Process

    Definition of function - what you want.

    Block diagram - translate into circuit functions.

    First Design Review.

    Circuit design - the details of how functions areaccomplished.

    Component selection Schematic Simulation Prototyping of critical sections

    Second Design Review.

    Fabrication and Testing.

  • EE122, Stanford University, Prof. Greg Kovacs 3

    Interface Circuits

    Interface circuits connect between conventionalelectronic circuits (op-amps, logic, etc.) to theoutside world.

    They include circuits to buffer, amplify, andprocess sensor signals - INPUT of information.

    Also, they can include circuits to drive actuators,relays, etc. - OUTPUT of information.

    In general, they translate between the volts andmilliamps of conventional circuits and theirequivalents within several orders of magnitude.

  • EE122, Stanford University, Prof. Greg Kovacs 4

    Power Driver Circuits

    There are a variety of devices that one might wantto drive that require more current or highervoltages than inexpensive op-amps can produce.

    Of course, one solution is to purchase specialtyop-amps with high current or high voltageoutputs.

    However, it is very useful to know how to extendthe capabilities of op-amp (and logic circuit)outputs to avoid this, particularly when the moreexpensive approach is not warranted.

  • EE122, Stanford University, Prof. Greg Kovacs 5

    Power Transistors/Heatsinks

  • EE122, Stanford University, Prof. Greg Kovacs 6

    Unipolar Power Switches

    For many output devices, one simply needs toswitch a drive voltage on and off.

    In this case, one can use a bipolar powertransistor with sufficient current gain (or aDarlington configuration) or a power MOSFET.

    Today, the most efficient choice is usually theMOSFET.

  • EE122, Stanford University, Prof. Greg Kovacs 7

    Basic Switch Can use BJT or

    MOSFET.

    If loads areinductive, needflyback protectdiode.

    Can drive directlyfrom TTL/CMOSlogic instead(want logic-driveMOSFET or BJT).

    Use current-limitresistor for BJT.

    V1

    V

    V-

    +

    V2

    FlybackProtectDiode

    Rg

  • EE122, Stanford University, Prof. Greg Kovacs 8

    Example Flyback CircuitV+

    High voltagepulses out!

    2N34401 k

    30 mH

    Pulse Source

  • EE122, Stanford University, Prof. Greg Kovacs 9

    IRLZ-34 Logic-Level MOSFET60V, 30A, 0.05

    5V VGS

  • EE122, Stanford University, Prof. Greg Kovacs 10

    Relay

  • EE122, Stanford University, Prof. Greg Kovacs 11

    Pulse-Width Modulation Pulse-width modulation, or

    PWM, offers a simple,DIGITAL output way ofmodulating power.

    The idea is to vary the dutycycle of pulses from zeroto 100% and if the timeconstants of the devicebeing driven are muchlonger than the pulsetimes, a low-pass filteredequivalent power isobtained.

    10%

    50%

    90%

  • EE122, Stanford University, Prof. Greg Kovacs 12

    Random PWM Ideas

    SG3525A/SG3527A

    LM3524

    Dedicated PWM Chips

    Source:NationalSemiconductorLinear 3Databook

    TriangleWave

    GeneratorV

    V-

    +

    VMOD

    VOUT

  • EE122, Stanford University, Prof. Greg Kovacs 13

    DC Motors

  • EE122, Stanford University, Prof. Greg Kovacs 14

    Bipolar Power Switches

    V+

    LoadDrive Drive

  • EE122, Stanford University, Prof. Greg Kovacs 15

    Modern Vacuum Tube Audio

  • EE122, Stanford University, Prof. Greg Kovacs 16

    Power Inverters

    Digital drive totransformer togenerate higher orlower voltage.

    Can use to powerfluorescent lights, ACappliances, or togenerate higher DCvoltages (needrectifier and filter).

    Can make negativesupply rail.

    V+

  • EE122, Stanford University, Prof. Greg Kovacs 17

    MOSFET Power Driver

  • EE122, Stanford University, Prof. Greg Kovacs 18

    HV Inverter

  • EE122, Stanford University, Prof. Greg Kovacs 19

    FashionStatement

  • EE122, Stanford University, Prof. Greg Kovacs 20

    Power Voltage Sources

    In some cases, a beefy and variable voltagesource is needed (e.g., audio power amplifiers,signal generator outputs, power supplies, etc.)

    In this case, one can either purchase power op-amps and use them in the standardconfigurations, or use power booster circuits withconventional, low-cost op-amps.

  • EE122, Stanford University, Prof. Greg Kovacs 21

    THE COMPLEMENTARY EMITTERFOLLOWER AMPLIFIER

    ("PUSH-PULL") A COMPLEMENTARY PAIR OF

    TRANSISTORS ARRANGED ASTWO EMITTER FOLLOWERS CANPROVIDE LOTS OF POWER WITHINEXPENSIVE PARTS!

    VERY EFFICIENT(APPROXIMATELY 80%)

    CAN ALSO PROVIDE ADISTORTED SIGNAL DUE TOCROSSOVER DISTORTION

    CAN DO THE SAME WITHMOSFETS

    -VCC

    +VCC

    Vin

    Vout

    THIS IS A "CLASS B" CIRCUIT

  • EE122, Stanford University, Prof. Greg Kovacs 22

    CROSSOVER DISTORTION

    -VCC

    +VCC

    Vin

    Vout

    THERE IS A +/- 0.7 V"DEADBAND"WITHIN WHICH THENEITHERTRANSISTOR ISCONDUCTING...

  • EE122, Stanford University, Prof. Greg Kovacs 23

    A CLOSE LOOK AT THE DISTORTION

  • EE122, Stanford University, Prof. Greg Kovacs 24

    OUTPUT SPECTRUM OF CLASS AAMPLIFIER WITH CROSSOVER DISTORTION

  • EE122, Stanford University, Prof. Greg Kovacs 25

    REDUCING CROSSOVER DISTORTIONWITH BIASING DIODES...

    -VCC

    +VCC

    Vin

    Vout THIS IS A "CLASS AB" CIRCUIT

  • EE122, Stanford University, Prof. Greg Kovacs 26

    BETTER PERFORMANCE WITHFEEDBACK!

    MAGIC SWITCH

    R1

    R2

    -VCC

    +VCC

    Vin

    Vout

    THE +/- 0.7V DEADBAND ISREDUCED TO

    THE SLEW-RATE LIMITATIONS OFTHE OP-AMP MEAN THAT THISDEADBAND WILL STILL BEAPPARENT AT HIGHFREQUENCIES....

    0.7AVO

  • EE122, Stanford University, Prof. Greg Kovacs 27

    HAYES & HOROWITZ SAY....

  • EE122, Stanford University, Prof. Greg Kovacs 28

    SEE ANY DISTORTION?

  • EE122, Stanford University, Prof. Greg Kovacs 29

    OUTPUT SPECTRUM OF THE SAMEPUSH-PULL AMPLIFIER WITH FEEDBACK

  • EE122, Stanford University, Prof. Greg Kovacs 30

    Peltier Devices

  • EE122, Stanford University, Prof. Greg Kovacs 31

    BeerCooler

    #2

  • EE122, Stanford University, Prof. Greg Kovacs 32

  • EE122, Stanford University, Prof. Greg Kovacs 33

    The Bridge Configuration

    Source: NationalSemiconductor LM12Application Note.

  • EE122, Stanford University, Prof. Greg Kovacs 34

    High Voltage Amplifiers For high voltage op-

    amp applications,recent pricereductions in HV op-amps make itpossible to usestandardconfigurationseasily.

    www.apexmicrotech.com is a goodsource of chips andapplication notes.

  • EE122, Stanford University, Prof. Greg Kovacs 35

    Current Sources/Sinks/Pumps

    Many transducers require current sources to drivethem (e.g., electromagnetic coils in some settings,lasers, LEDs, etc.).

    There are several simple current driver circuitsthat use op-amps to provide closed-loop control,and the high output impedances required.

    The basic principle is to sense the sourced (orsunk) current and convert it into a signal forfeedback purposes.

    If the desired currents exceed the capabilities ofthe op-amp, external pass transistors are used.

  • EE122, Stanford University, Prof. Greg Kovacs 36

    Classic Op-Amp Current Sink

    V+

    VIN

    RF

    Load

    IL

    IL =VINRF

  • EE122, Stanford University, Prof. Greg Kovacs 37

    Beer-Locked-Loop

  • EE122, Stanford University, Prof. Greg Kovacs 38

    Types of Sensors

    Electromagnetic Coils

    Strain Gauges

    Accelerometers

    Microphones

    Optical (covered elsewhere)

    Temperature Sensors

  • EE122, Stanford University, Prof. Greg Kovacs 39

    Sensor Signal Processing

    Typical sensor signal processing involves(pre)amplification, filtering and sometimes somedownstream functions.

    Downstream functions may include a comparator(decision) or A/D converter, sometimes precededby a sample-and-hold circuit.

    In some cases (not covered in EE122), the sensorsignal (before or after digitization) is transmittedto another location using telemetry.

  • EE122, Stanford University, Prof. Greg Kovacs 40

    LM334 Temperature Sensor

    Source: Linear TechnologyLM334 Datasheet.

    Note that current-output sensors allow quite longwire lengths, since they are pretty muchinsensitive to cable resistance.

  • EE122, Stanford University, Prof. Greg Kovacs 41

    Transresistance Amplifiers

    Transresistanceamplifiers simplytranslate currentfrom a sensor into anoutput voltage.

    They are justinverting amplifierswithout the inputresistor. Thetransresistance gainis given in OHMS.

    Rf

    V

    V-

    +

    V+

    Vout

  • EE122, Stanford University, Prof. Greg Kovacs 42

    Transresistance Frequency Response

    Quite often, high DCgain is desiredwithout much ACgain or controlledroll-off.

    These are twoexample approachesto achieve suchcharacteristics.

    In practice the topcircuit is most oftenused.

    RF

    C1

    VOUT

    V

    V-

    +iin

    RP

    RF

    C1

    VOUT

    V

    V-

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