LM143 Monolithic High Voltage Operational …

1 LM108,LM120, LM143 ,LM340,LM741 Application Note 127 LM143 Monolithic high Voltage Operational amplifier ApplicationsLiterature Number: SNVA516LM143 Monolithic HighVoltage OperationalAmplifier ApplicationsINTRODUCTIONThe LM143 is a general purpose, high Voltage operationalamplifier featuring 40V maximum supply Voltage operation,output swing to 37V, 38V input common-mode range, in-put overvoltage protection up to 40V and slew rate greaterthan 2V/ s*. Offset null capability plus low input bias and off-set currents (8 nA and 1 nA respectively) minimize errors inboth high and low source impedance applications. Due toisothermal symmetry of the chip layout, gain is constant forloads 2k at output levels to 37V. Because of these fea-tures, the LM143 offers advantages not found in other gen-eral purpose op amps.

2 The LM143 may, in fact, be used asan improved performance, plug-in replacement for theLM741 in most paper describes the operation of the LM143 and pre-sents applications which take advantage of its unique, highvoltage capabilities. Obviously, other applications existwhere the low input current and high slew rate of the LM143are useful. (See AN-29 on the LM108.) Application tips areincluded in the appendix to guide the user toward reliable,trouble-free DESCRIPTIONA simplified schematic of the LM143 , shown inFigure 1, illus-trates the basic circuit operation. The super- input transis-tors(1), Q1 and Q2, are used as emitter followers to achievelow input bias currents. Although these devices exhibit =2000 5000, they inherently have a low collector-base break-down Voltage of about 4V.

3 Therefore, active Voltage clampsQ3 and Q4 protect Q1 and Q2 under all input conditions in-cluding common-mode and differential overvoltage. OtherNPNs in the circuit are representative of those found in stan-dard IC op amps ( 200, LVCEO=50 70V).The input stage differential amplifier Q7 and Q8 with largebase width exhibit LVCEO=90V to 110V and high BVEBO soreadily withstand input overvoltages. The total input stagecollector current (I1=80 A) is made higher than in most opamps to improve slew rate. Emitter degeneration resistors,R10 and R11, reduce transconductance(2)to limit small sig-nal bandwidth at 1 MHz for a phase margin of 75 . Q16 andQ17 function as active collector loads for Q7 and Q8 andprovide differential to single-ended current conversion withfull differential of the highest breakdown voltages available in standardplanar NPN processing is the collector-base, BVCBO which istypically 90V to 120V.

4 To make use of this high Voltage capa-bility in the active region, the second stage consists of a cas-code (common emitter-common base pair) connection ofQ21 and Q23. The internal Voltage bias VB1, shuntsavalanche-induced leakage current away from the base ofQ21, avoiding multiplication as found in the and emitter follower Q22 are internally biased at a lowvoltage so the BVCEO mode is impossible. Frequency com-pensation is achieved with an internal, high Voltage capaci-tor, :*An externally compensated version of the LM143 , the LM144, offerseven higher slew rate in most applications. The LM144 is pin-for-pincompatible with the 1. LM143 Simplified SchematicNational SemiconductorApplication Note 127 April 1976LM143 Monolithic high Voltage Operational amplifier ApplicationsAN-127AN-127 1998 National Semiconductor PrintTime=17:59:24 44295 an007432 Rev.

5 No. 3 cmservProof1 The second stage drives a complementary class AB outputstage. A cascode connection of Q32 and Q34 is again em-ployed for high breakdown Voltage . The associated voltagebias, VB2, is internally derived. A Darlington PNP pair, Q39and Q40 with BVCEO=100V, provides the active Voltage APPLICATIONSThe following applications make use of the high Voltage ca-pabilities of the LM143 . As with most general purpose opamps, the power supplies should be adequately bypassed toground with F Vp-p Drive to a Floating LoadA circuit diagram using two LM143 s to drive up to 130 Vpeak-to-peak is given inFigure non-inverting Voltage amplifier , with a gain of AV=1 + (R2/R1), is followed by a unity gain inverter. The load is appliedacross the outputs of A1 and A2.

6 Therefore, VOUT=V1 V2=V1 ( V1)=2V1. If V1=65 Vp-p, then 2V1=130 above circuit was breadboarded and the results are asfollows:i ) Maximum output Voltage : 138 Vp-p unclipped into 10 k loadii ) Slew rate: 6V/ s 34V Common-Mode Range Instrumentation AmplifierAn instrumentation amplifier with 34V common-moderange, high input impedance and a gain of X1000 is shown inFigure a differential input signal, VIN, A1 and A2 act asnon-inverting amplifiers of gain AV1=1 + (2R1/R2), whereR1=R3. However, the gain is unity for common-mode sig-nals since voltages V1 and V2 are in phase, and no currentflow is developed through R1, R2 and R3. The second stageis simply an op amp connected as a simple differential ampli-fier of gain, AV2=(R5/R4), where R5=R7 and R4= total gain of the instrumentation amplifier isR7 may be adjusted to take up the resistance tolerances ofR4, R5 and R6 for best common-mode rejection (CMR).

7 Also, R2 may be made adjustable to vary the gain of the in-strumentation amplifier without degrading the 2. 130V Drive Across a Floating LoadPrintDate=1998/08/06 PrintTime=17:59:25 44295 an007432 Rev. No. 3 evaluation of this circuit revealed noise and CMRdata as follows:i ) Frequency response with 10k load and AV=1000: at kHzCMR measurements (common-mode signal of 34Vp-p) inFigure 4 Noise measurements inFigure 5AN007432-3 All resistors are 1%,1 4W*R2 may be adjustable to trim the gain.**R7 may be adjusted to compensate for the resistance tolerance of R4 R7 for best 3. Wide Common-Mode Range Instrumentation AmplifierAN007432-4 FIGURE 4. Common-Mode Rejection PrintTime=17:59:25 44295 an007432 Rev. No. 3 cmservProof3 high Compliance Current SourceA current source with a compliance of 28V is shown inFig-ure non-inverting input of the op amp senses the currentthrough R4 to establish an output current, IOproportional tothe input Voltage .

8 The expression for IOisR3 keeps the circuit stable under any value of load resis-tance. Measured circuit performance is as follows:IOMAX= mA at EIN= 35 VROUT=2M at IOUT= mACURRENT BOOSTED APPLICATIONSB ecause of the high Voltage capability of the LM143 , somethought must be given for the selection of the minimum loadresistance. At an ambient temperature of 25 C, the LM143can dissipate 680 mW. Worst case dissipation arises whenthe load resistance RLis connected to one supply and VO=0. Then the amplifier sources IO=(38V/RL) with 38V internalvoltage drop. During this condition,Hence, load resistances less than 2k will cause excessivepower Power Boost CircuitFor loads less than 2 k , a power boost circuit should beadded. The simple booster shown inFigure 7has the advan-tage of minimal parts count, but crossover distortion is no-ticeable and there is no short circuit protection; hence, eitherthe LM143 or the boost transistors may fail under short cir-cuit mA Current Boost CircuitWith the addition of 4 diodes, a resistor and a capacitor, thebooster circuit can be short circuit protected at 100 mA asshown inFigure 5.

9 Noise MeasurementsAN007432-6 All resistors 1%metal film,1 4W unless otherwise 6. high -Compliance Current SourceAN007432-7 Heat sink is a Thermalloy No. 2230-5 or resistors are 10%, 7. Simple Power Boost CircuitAN007432-8 Heat sink is a Thermalloy No. 2230-5 or diodes are resistors are1 2W, 10%.FIGURE 8. 100 mA Current Boost CircuitPrintDate=1998/08/06 PrintTime=17:59:25 44295 an007432 Rev. No. 3 protects the LM143 by limiting the maximum drive currentto (38 ) mA, thereby keeping safely within thedevice dissipation limit of 680 mW. D1 D4 in conjuctionwith R2 and R3 protect the output transistors Q1 and Q2 byshunting the output drive current if the Voltage drop acrossR2 or R3 exceeds Data:i )Frequency Response: Limited by LM143 frequency re-sponse and slew ) Step response for unity gain, Voltage follower configura-tion: Less than 10%overshoot for step with F capacitive load, 50%overshoot with F capaci-tive load.

10 The circuit is unconditionally stable for capaci-tive ) Output Voltage : 33 Vp-p into 400 Amp Class AB Current BoosterIf crossover distortion is objectionable and currents of up are needed, the circuit inFigure 9should be output of the LM143 drives a class AB complementaryoutput stage. The quiescent current for the output stage isset by the current flow through R4, R5 and diodes D1 diodes D1 D4 are on a common heat sink with the out-put transistors Q3 and Q4 so that the Voltage drops acrossthe diodes and base-emitter junctions of the output transis-tors will track with temperature. Normally, R4 and R5 supplythe current drive for the output Darlingtons, Q1, Q3 and Q2,Q4, but if additional drive is needed, the LM143 supplies theremainder through R2 and R3. For short circuited load, thedrive current is bypassed around the output transistorsthrough D1, D5 and D6 during the positive half cycle andthrough D4, D7 and D8 during the negative half cycle.