Analog Applications Journal Analog and MixedSignal Products www - PDF document

24Analog Applications Journal www.ti.com/aaj3Q 2005amplifiers seem to do identical jobs. Various amplifier Noninverting ampGroundInput signalDetermined by gainDetermined by gainZ Inverting integratorInput signalGroundROpenZ BufferGroundInput signalOpenShortShortOpen SubtractorInput signal –Input signal +R Table 1. Changing component values yields many op amp circuits aZ1Z2VOUTZG many different circuits Texas Instruments Incorporated 25Analog Applications Journal 3Q 2005www.ti.com/aajThis discussion is limited to voltage bufferloops. Voltage buffers have a gain of one,the op amp. This is the first limitation of thesignal, and this situation is detrimental to good perform-The key to solving the input impedance problem is to useusually an emitter-follower type configuration that has low. The emitter-follower output impedance increasesand errors at high frequencies. Worse yet, the outputstage of a rail-to-rail op amp is common-collector; and theance, lowering the overall output impedance dramatically.poles. The best solution for high load currents is to buy antures the output transistors require. In summary, outputimpedance may cause you to migrate to a buffer, select acapacitors. We see from Table 1 that when all the externaltractor. Equation 1 is the general subtractor equation.discrete resistor approach is that the resistors don’t matchwell. A designer using 1% tolerance resistors hopes for aare laser trimmed to the final accuracy.Subtractors have resistors; thus they don’t present the VVVOUT OUT2 VoltageVoltageVoltageVoltage G=+1 + Texas Instruments Incorporated 26Analog Applications Journal www.ti.com/aaj3Q 2005and A3. The subtractor stillcapability, but A1 and A2 buffer the subtractor,error. Another advantage of the instrumentationproblem goes away. The down side of theextra signal delay, and a reduced common-contains a subtractor to obtain high common-circuit inputs, thus it has the high input imped-amplifier. This instrumentation amplifier has noninverting input has one stage delay. Unequalwithin the IA’s operating range.(VFAs). VFAs have an open-loop gain that startsof frequency. This gain decrease causes poorgain on into very high frequencies. VFAs must IN– 2 –++RG Gain=1+R+R with fewer parts IN– GR2 –––++1=R46 Gain=1+R+R DirectGainVFAGainCFAGainClosed-LoopGain(1+A)(1+A) DirectGainClosed-LoopGain Texas Instruments Incorporated 27Analog Applications Journal 3Q 2005www.ti.com/aajloop gain begins to fall off so quickly. CFAsdon’t work under this constraint, so theyamplifiers; it looks worse for a CFA becauseCFA uses transimpedance rather than gain.stage and buffered by the output buffer.CFAs are usually limited to bipolar transis-communications and video, don’t requireinput lead looks into the output of a buffer; so it has lowemitter-follower stage.The noninverting lead looks into a buffer input, so it is aamplifier look into the base-emitter junctions of a long-voltage feedback amplifier derives its accuracy. It is impos-sible to match the input and output stages of a buffer, sothe CFA is not a high-precision circuit. We design the CFAfor speed, and while the VFA usually tops out at about400-MHz GBW, the CFA often reaches a GBW of severalspecial class of VFAs called wideband fixed-gain amplifiers(WFGAs). The CFA covers the frequency range fromA caution about using CFAs is mandatory. Many designersquency attribute of the VFA for stability because a circuitwith a gain less than one is unconditionally stable. ThisCFA keeps its gain as frequency increases, thus it doesn’twith VFAs can become unstable when implemented withCFAs. Furthermore, the input lead and feedback resistorof a CFA is sensitive to stray capacitance, so the designermust be much more aware of the layout. Vendor evalua- +–IZBZNoninvertingInputInvertingInput BG(I) 47pF50-SourceFB=FerriteBead 47pF 100k1M10M100M1G10GFrequency,f(Hz)SmallSignalFrequencyResponse =100=100mVV=5VSmallSignalGain(dB) Texas Instruments Incorporated 28Analog Applications Journal www.ti.com/aaj3Q 2005hybrid amplifiers; i.e., the circuit is constructed from dis-substrate. This type of construction is costly; the costprocess to obtain the high GBW required to make a circuitity.These op amps are available in fixed circuit configura-tions with fixed gains, but their GBW goes up to 10 GHz, aThe cautions given for CFAs apply here, along with thetial signals with the aid of the circuit shown in Figure 8.This ends up being a difficult design job that is not cost-Besides simplicity and low cost, the FDA provides for asignals to be common to both inputs, thus the coupledrejection capability. VINVOUT+VOUT–VREF+VREF– converter circuit INRGRFRFRGVIN+VIN–ADCVOUT+VOUT–VOCM –+–THS41xx VREF signal to differential Texas Instruments Incorporated 29Analog Applications Journal 3Q 2005www.ti.com/aajmilliamps at several volts, it is time to think of using a PAbecause PAs can handle large currents and voltages. PAsare not switching-type amplifiers; rather, they are linearpower. Making a power amp is not attaching the biggestheat sink possible to an op amp with the lowest thermalin PAs, but other functions like current sense, overloadThe new PA can handle considerable current for a preci-limit set, current monitor, parallel connections, enable, aare PAs that can handle much more current at higher volt-the bells and whistles.been a special category for them since the 1950s. Mostthat ranges from simple op amps to involved switchingNaturally, you ask, “Where do I go from here?” The answeris to the Web. Any IC manufacturer worth doing businesswith has a Web site, and you can visit all the pertinentsites in a few hours. When you are on a site, look at theinteresting ICs for problem solutions, but don’t neglect thetions information while others offer little or no applicationsinformation. This information often determines how quicklyand completely you can do your job. If you don’t knowabout decoupling capacitors or thermal runaway, the hard-manufacturer is yours and shouldn’t be influenced bydon’t want to stray too far from the source of knowledge.1.Ron Mancini, Publishers, 2003). An earlier 2002 edition is available at2.Ron Mancini, “Worst-case circuit design includes compo-available online at www.edn.com/article/CA408380.htmlamplifier.ti.comwww.ti.com/sc/device/OPA569 OPA569ParallelOut1ParallelOut2I=I/475ConnectforThermalProtectionMONITOR17,1814,1512,13 does more than handle power IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reservestandard warranty. Testing and other quality control techniques areused to the extent TI deems necessary to support this warranty.applications using TI components. To minimize the risksunder the patents or other intellectual property of the third party, or aTexas Instruments products and application solutions: TIWorldwide Technical Support InternetTI Semiconductor Product Information Center Home Pagesupport.ti.comTI Semiconductor KnowledgeBase Home Pagesupport.ti.com/sc/knowledgebase Phone+1(972) 644-5580Fax+1(972) 927-6377 Internet/Emailsupport.ti.com/sc/pic/americas.htmBelgium (English)+32 (0) 27 45 5 32Netherlands (English)+31 (0) 546 87 95 45Finland (English)+358 (0) 9 25173948Russia+7 (0) 95 7850415France+33 (0) 1 30 70 11 64Spain+34 902 35 40 28Germany+49 (0) 8161 80 33 11Sweden (English)+46 (0) 8587 555 22Israel (English)1800 949 0107United Kingdom+44 (0) 1604 66 33 99Italy800 79 11 37Fax+(49) (0) 8161 80 2045 Internetsupport.ti.com/sc/pic/euro.htmInternational+81-3-3344-5317Domestic0120-81-0036Internationalsupport.ti.com/sc/pic/japan.htm Domesticwww.tij.co.jp/picInternational+886-2-23786800DomesticToll-Free NumberToll-Free NumberAustralia1-800-999-084New Zealand0800-446-934China800-820-8682Philippines1-800-765-7404Hong Kong800-96-5941Singapore800-886-1028Indonesia001-803-8861-1006Taiwan0800-006800Korea080-551-2804Thailand001-800-886-0010Malaysia1-800-80-3973Fax886-2-2378-6808Emailtiasia@ti.comInternetsupport.ti.com/sc/pic/asia.htmti-china@ti.com of similar import. 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