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.

2 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. 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.

3 (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. Therefore, active Voltage clampsQ3 and Q4 protect Q1 and Q2 under all input conditions in-cluding common-mode and differential overvoltage.

4 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.

5 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.

6 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. 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 .

7 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. 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.

8 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).Also, R2 may be made adjustable to vary the gain of the in-strumentation amplifier without degrading the 2.

9 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.

10 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 . 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.