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by: Meredith Wolf

ElectronicsforScientists PHYS645

Marketplace > University of Delaware > Astronomy > PHYS645 > ElectronicsforScientists
Meredith Wolf
GPA 3.57


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Class Notes
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This 5 page Class Notes was uploaded by Meredith Wolf on Saturday September 19, 2015. The Class Notes belongs to PHYS645 at University of Delaware taught by Staff in Fall. Since its upload, it has received 28 views. For similar materials see /class/207154/phys645-university-of-delaware in Astronomy at University of Delaware.


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Date Created: 09/19/15
Nonideal opamps 0 InTroducTion In our analysis of circuiTs based on The operaTional amplifier we made a number of imporTanT assumpTions a The inpuT currenTs are zero b The gain of The opamp is infiniTe for all frequencies c There are no inpuT offseT volTages ie in a properly working circuiT V V39 0 d The opamp oquuT impedance is zero and The oquuT volTage can change aT any raTe eg in response To a sTeep sTep of an inpuT volTage dVoufdT gt oo e There is no noise AlThough modern opamps approximaTe some of These ideal properTies quiTe well a single device usually does noT saTisfy Them all simulTaneously For example an opamp may be quiTe fasT buT have a significanT offseT volTage or a device may have exTremely low volTage noise buT large inpuT currenTs The following exercises are inTended To give you some inTuiTion for cerTain Types of nonideal behavior as well as for whaT can be achieved wiTh generally available op am ps 1 InpuT offseT volTage and noise Assemble a noninverTing amplifier wiTh a gain of 1000 Fig 1 and ground The noninverTing inpuT The amplifier now amplifies iTs own noise Vnoise and inpuT Non ideal op amps ottset vottage vu Measure vu usmg your mm and thh the hetp ot an osctHoscope esttmate Do thts tor three dtttereht opiamps the LM741 OP27 and LT1792 Note not onty the dtffer ence m magmtude of the hotse but atso tts spectrat charactertsttcs 4 tun k Ohm Vuut tun Ohm thuret ottset and hutse 2 Input currents To measure the thput current you can connect a reststor Rh between the Ho rmvzr tmg thput and ground The current thto the thput causes avottage drop across Rm whtch ts amphtted m the same way as V The mput current INS ts gtven by INS AVDMIOOOXRM where AVou ts the dtfference m the output vottage thh Rm shorted or hot shorted Opramps thh FET mputs have an Iot s that ts so smaH that thts method becomes tess practtcat Nun39tdeat upramps InsTead of measuring The voITage drop across a resisTor one can moniTor The change in voITage across a capaciTor as iT is charged by The bias currenT In This case Ibms CgtltAVDU 1000AT wiTh AVE The change in The oquuT voITage over The measuring Time AT DeTerrnine The inpuT bias currenTs for The LM741 OP27 and LT1792 4 i kohm Vuut mu Ohm mom 4 i kohm Vuut mu Ohm iii t Figure 2a Determining the bia Figure 2b Determining the bias current for the LT1792 Nonrideal opramps iii 5 current for the LM741 and OP27 3 GainbandwidTh producT A convenienT number To characTerize an opamp is iTs gainbandwidTh producT IT Tells you direchy which bandwidTh can be realized for which closed loop gain To look aT This deTermine The small signal frequency response for noninverTing amplifiers wiTh gains of 10 100 and 1000 based on The OP27 WhaT do you obTain for The gainbandwidTh producT 4 TransienT response and slew raTe The TransienT response ie The response To a sTep or pulse of an amplifier is deTermined by The frequency response of The opamp as well as ThaT of The feedback loop WiTh amplifiers configured for a high closed loop gain one will primarily see The effecT of The limiTed over all bandwidTh The response To a sTep will have an exponenTial shape wiTh a rise Time 17 127ch fc is The 3dB bandwidTh WiTh low gain amplifiers The deTails of The feedback circuiT become imporTanT as well as The behavior of The opamp aT high frequency STray capaciTance can cause phase shifTs ThaT make The circuiT unsTable FirsT consTrucT an amplifier wiTh a gain of 2 Fig 3 Apply a square wave 100 kHz 100 mV signal To The inpuT and compare inpuT and oquuT signals Do This for The LM741 and The LT1792 The oscillaTions observed wiTh The LT1792 can be eliminaTed wiTh a small capaciTor parallel To The feedback resisTor Non ideal op amps Fw ure 3 A mrcmuu determme me transwent respunse Secmdw ncrease me gem of me amph er m 10 and 393er compare me behavwor of me cwrcuwt W h an LM741 and an LT1792 It shou d be dear mat Wh e me LT1792 can fOHCW me mput swgna W h ease me LM741 S HOW too s ow FmaHy shH Wnn cm amph er gum an en uuk m Wnev neppens Wnen vne sngnex amphmde n3 mcreased Fur smaH amphmde yuu shm d ubserve a new de ned amphmde mdependem N32 nnne fur Me szu Wnen Me mpuf quage exceeds 200 M We eneneeven uf Me uuvput quage mm m enenge I nuw appears Mm Me quage can un y enenge m a xed nave mdependem an M2 amphmde m We mpuf Tnns rme n3 caHed Me s ew rme Try m nepeev vnns Wnn Me m792 On y very Muse m We pmnv Wnene M2 BMW 9239s eunnpxevexy semnevee Wm yuu see devmhun frum ween benewun Ntmdaa mamas


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