EE 215A Fundamentals of Electrical Engineering Lecture Notes Operational Amplifiers (Op Amps) 8/6/01 Reviewed 10/04

Transcription

1 EE 5 Fundamental Electrcal Engneerng Lecture Nte Operatnal mpler (Op mp) 8/6/ eewed /4 ch Chrte Oerew: The peratnal ampler, r p amp r hrt, a undamental buldng blck n crcut degn. Stued nde a chp are a bunch trantr and ther element that make up a neardeal ltage cntrlled ltage urce (VCVS) wth nearnnte gan. We'll tart by aumng t deal. Crcut Symbl: The p amp ha a pecal crcut ymbl, a trangle wth tw nput and ne utput The and cnnectn are the nnnertng nput and the nertng nput. Thee are uually, but nt alway, drawn wth the mnu () at the tp. Ideal Op mp elatnhp: n deal p amp an deal ltage cntrlled ltage urce. We can thnk the p amp ymbl beng replaced a llw: ( )

2 Ntce what th mple: I nte, then " and Thee are n act the deal p amp relatnhp, and nce we get ued t them we can ue them t le p amp crcut. (t th pnt yu mght be wnderng why the p amp ha t wn pecal ymbl when t' really jut a VCVS. The anwer ha t d wth packagng. The p amp cme n a ngle package whch nclude the nertng and nnnertng nput. The p amp ymbl ndcate that the package cmbne all the cnnectn n ne place, where the VCVS ymbl let the nput be anywhere.) Op mp Pwer Supple: But wat, the nput current and are zer, where de the utput current cme rm? The dependent ltage urce, n the mdel, but n real le, we hae t upply the p amp wth an external pwer upply. Many real wrld p amp (the LM74, r example) requre bth pte and negate ltage (a pped t pte and grund, called ngle urce.) I yu really want t d KCL r the p amp (r a upernde cntanng the p amp, r r the utput nde), yu hae t nclude the pwer upply. Uually t' nt neceary and the pwer upply cnnectn are mtted we all undertand that they are there, why clutter up the crcut wth them?

3 Op mp Crcut naly: Let' try a crcut wth an p amp. What' the relatnhp between and? nce they are cnnected. rm the deal p amp relatnhp. Snce, and thu L Th crcut called a ltage llwer r a buer ampler. t rt glance we hae nt dne much. But let' ak: cnder the urce (cnnected t ) bere t cnnected t the p amp. What' the Theenn equalent retance? It' pretty bu that t Nw cnder the Theenn equalent retance een lkng n t the whle thng. c jut, and c well, t' nnte ( mut be becaue zer, then mut equal whch mean n matter what the current but t hrted > nnte current.) L c S t and the c Theenn equalent an deal ltage urce alue : 3

4 Saturatn: I we actually bult ne thee ltage llwer and ared the nput ltage, we wuld ee an nteretng relatnhp between nput and utput. I we kept cntant, and ared L the utput current ared, we wuld al ee methng unexpected. at at What' happenng that the p amp aturatng, that, mng ut the lnear regn and ceang t perate a an ampler. We can nly mdel the p amp a deal when t nt aturated. We can check aturatn, thugh: at at at and at cme rm the peccatn heet. at uually a lt r tw le than the pwer upply ltage. Fr example, the pwer upply / V and at m, then the maxmum utput ltage V I at V, what' the maxmum L? (nnte, haha. Maybe a mre nteretng quetn ) What' the mnmum? $ L L L Slew ate: at $ m $ m # 5k % 3 " 4

5 t th pnt we huld prbably nte that anther departure rm dealty n the p amp d that the lew rate,, al lmted. Th aect hw at the p amp repnd t a dt tep change n nput, and hw hgh a requency t can perate at. We wn't pay much attentn t lew rate n th cure, but yu wll run nt t n later cure, and n the real wrld Mre naly: Let' lk at anther crcut (yu can d the analy) bere, nd a a unctn. Frt, what'?. & $ % # " S what'? Nw wrte KCL at : ' $ % " & # Th called a nnnertng ampler. Ntce that the gan the ampler can be changed by changng the rat the retr, r example wth a ptentmeter r. Degn wth Op mp: Smewhere n yur textbk (r me textbk mewhere) a catalg p amp crcut that perrm derent unctn. There are lt beynd what' n the bk. The crcut are generc, that, they d nt hae pecc alue r the retance. The catalg a tartng pnt r crcut degn. We nw knw enugh t d me almt real wrld degn. The prce : Gen a unctn t perrm Che a crcut rm the catalg that perrm the unctn. (Or nent a new ne. I yu d, patent t.) Fgure ut alue r the crcut cmpnent that the pecc unctn we want mplemented (e.g. pecc gan). Check the crcut agant lmt. 5

6 Iterate a requred. Example: We want t amply an nput gnal by a actr (gan ). The nput gnal ha a range t mv, but we want a pte ltage utput. Pretty clearly we want the nertng ampler crcut rm the catalg: Let' check the analy: Fr ur degn we want but that leae a lt chce. What ther cncern are there? Surce current: Suppe the urce lmted t m utput current. Then < m mv < m > a K I th a gd degn? What abut at? max " ( mv ) V We'll need a pwer upply arund / V. / V a cmmn tandard (yu can nd th ltage nde many dektp PC) r / 5V, bth are OK but NOT / 5V, whch the mt cmmn dgtal electrnc pwer upply ltage, althugh lwer ltage (3.3 V,.6 V) are ganng. What abut at? 6

7 K m() Whp, the lmt we talked abut earler wa m. (Th lmt wll ary rm p amp type t p amp type, maybe we jut need t buy a derent p amp. But let' ue the ne n the part kt wth a m utput current lmt.) I we ncreae, and ncreae t keep the rat, then wll al g dwn, and we wll meet bth cntrant. < m < m > 5k m 5 Nte th al meet the general gudelne t ue 5 kω kω retr arund an p amp. We huld prbably al wrry abut retr pwer dpatn, but wll leae that up t yu. What culd we d the urce current lmt wa much, much maller? We culd cacade p amp, ung a ltage llwer then nertng amp, r example. 7

8 eal Op mp: The deal p amp ha already been brught cler t realty by addng aturatn and lew rate lmt, but the cre cmpnent wa tll an deal VCVS. In real p amp the VCVS nt deal. In partcular, real p amp hae: a nte (but large) gan,, eru nnte n the deal mdel a nte (but large) nput retance, MΩ eru nnte a mall utput retance, 75 Ω eru zer Tny ba current, 8 n eru zer Small et ltage, mv eru zer In many, dare I ay mt cae, thee mperectn are mply gnred and the deal p amp analy (wth aturatn lmt) gd enugh r mt wrk. It' certanly gd r thnkng up new p amp crcut. Hweer, there are applcatn where the mperectn aect perrmance. They are uually accunted r wth mre detaled mdel. Fr example, the nte gan and retance mdel : ( ) Let' apply th mdel t the nertng ampler we degned and ee hw much errr we get. ( ) We want t get the relatnhp between and and cmpare t t the deal ern. Let' wrte KCL at : 8

9 9 and at, Frm the ecnd equatn Subbng n t the rt equatn " " # $ % % & ' Whew Frt let' ee th reduce t the deal relatnhp wth, and.

10 ( ) ( ) ( ) ( ) nd th matche. Nw r me number. In ur deal degn 5Ω, 5 kω and. In the mre detaled crcut,,, MΩ, 75 Ω (LM 74 number) ( ) ( )( ) ( ) 6 # ( 75," 5,) 6 ( 75 5)( # 5) # 6 " 5", we can ee t' nt a majr accuracy ht, abut.5%. Ba Current and Oet Vltage: The ther way that p amp deate rm dealty the preence ba current and et ltage. Ba current are due t aymmetre n the p amp degn and ba ltage t the nherent electrphyc. They jut reult n mre cmplex mdel: O cure, thee addtn can be cmbned wth the nte gan and nternal retance mdel dered. Fr a ltage llwer, the typcal eect ba and et ne thuandth ne percent. Thu they dn't get a lt attentn b b

11 Cmmn Mde Gan: Een n the nte gan mdel the p amp, the gan cndered cntant. In act, t depend n the aerage the nput ltage and al n the requency the nput. The dependence n aerage called cmmn mde gan. The VCVS ltage equatn really & $ % # ( ' ) " cm The cmmn mde gan cm hdden n the pec a the cmmn mde rejectn rat (CM). (N engneer cal le jke, pleae). CM cm Wth CM 3,6 (a the LOW alue) th can ten be gnred. Varatn gan wth requency mlarly dgued n the pec a ganbandwdth prduct. What really tay cntant gan tme requency (dc and neardc a pecal cae) B ( ) ( ) B nd that' pretty much t r p amp untl we knw methng abut energy trage dece.