Chapter 8 Operational (OP) Amplifiers

Transcription

1 hapter 8 Operatonal (OP) Amplfers Jaesung Jang Ideal Amplfers Basc Ideal OP amp rcuts Acte lters Physcal Lmtatons of Practcal OP amps ef: edra/mth, Mcroelectronc rcuts, 3rd ed., 99, hap. &

2 Amplfers - gnal Amplfcaton One of the most mportant functons n electronc nstrumentaton s that of amplfcaton. The smplest sgnal processng task s sgnal amplfcaton (gnal amplfer). The need for amplfcaton arses because sensors prode sgnals that are sad to be weak, that s, n the mcroolt or mllolt range. uch sgnals are too small for relable processng, and processng s much easer f the sgnal magntude s made larger (amplfcaton). When amplfyng a sgnal, care must be taken so that the nformaton contaned n the sgnal s not changed. The deal amplfer needs to produce an exact replca of that at the nput, except for larger magntudes.

3 Ideal (oltage) Amplfers In order not to lose a sgnfcant porton of the nput sgnal, the nput resstance that we can control must be much greater than. The deal oltage amplfer s one wth (nfnte nput mpedance) so the oltage rato at the nput sde s ndependent of the source resstance. υ υ s : oltage dder s Amplfer (shaded) or a gen L one must desgn the amplfer so that ts O that we can control here s much smaller than L. That s, an deal oltage amplfer s one wth o (zero put mpedance), so the gan s ndependent of the load resstance. If L, Aυ Aυ o, whch s the oltage gan of the unloaded amplfer, or the open-crcut oltage gan. KL : A A o υo L υo L υ υ o υ υ o L o o A o υ υo υ υ o o A υ υ o υ A υo o L 3 L

4 OP Amp Operatonal amplfer (op amp) was frst appled to the amplfers employed n analog computers to perform mathematcal operatons such as summng and ntegraton. or lnear or analog systems, the I (Integrated-rcut) op amp plays the same role as do the I logc and memory elements n dgtal systems. ombnatons of op amp and dgtal deces are wdely used n nstrumentaton and control. One of the reasons for the popularty of the op amp s ersatlty. You can do almost eerythng wth op amps! It s easy to desgn crcuts wth the I op amps. An I op amp s made up of a large number of transstors, resstors, and (sometmes) one capactor. An op amp s bascally a dfferental amplfer respondng to the dfference n the oltages of two nput (poste and negate nput) termnals. 4

5 haracterstcs of Ideal OP Amp A dfferental amplfer has two nputs, an nertng () nput and a non-nertng () nput. It has two nputs but only one put. Infnte nput mpedance (no currents gong nto the op amp) and zero put mpedance. (-> deal oltage amplfer) The op amp responds only to the dfference sgnal. ero put oltage s nduced at no dfference n the oltages at two nput termnals -> nfnte common-mode reecton: The amplfer reects or seerely attenuates sgnals that are common to both nputs. Op amps are drect-coupled deces or dc where A amplfers (low-pass amplfers). ( OL) Infnte bandwdth: an deal op amp can amplfy sgnals of any frequency wth equal gan. Infnte open-loop oltage gan -> In almost all applcatons the op amp wll not be used n an open-loop confguraton, but n a closed-loop confguraton. υ ( OL)( υ ) A υ on the order of External oltage source s called the open - loop oltage gan and typcally (n practcal op amp) 5 to 7. 5

6 Analyss of Ideal OP Amps n the Inertng onfguraton Inertng confguraton: the nertng nput termnal () s based wth oltage source s and the non-nertng termnal s usually grounded -> negate gan -> put shape s nerted wth respect to the nput sgnals. Non-nertng confguraton: the nertng nput termnal () s based wth oltage source. -> poste gan -> put shape s not nerted wth respect to the nput sgnals. An nertng amplfer has a negate closedloop oltage gan, G L, of /. The mnus () sgn of the gan ndcates that n and are 8 of phase. -> nerted shape.. The currents eneterng the op amp s zero due to nfnte nput mpedances υ. υ υ due to nfnte open - loop gan A ( OL) on the assumpton that the op amp s deal. A υo ( L) :losed - Loop oltage Gan υ I 6

7 7 Popular Op Amp rcuts The crcut shown below s called a summng amplfer, or summer. N n N n N n n n n N N : KL L L

8 8 Analyss of Ideal OP Amps n the Non- Inertng onfguraton A non-nertng amplfer has a poste closed-loop oltage gan, G L, of ( / ). The nput sgnal s connected to the nonnertng () nput of the op amp, so that the nput and put sgnals wll be n phase. -> non-nerted shape. to) Note :The gan s always poste and greater than (or equal Non - nertng amplfer closed - loop gan KL : n

9 oltage ollower The fgure below shows a ery popular op-amp crcut called a oltage follower wth. (oltage gan equals one.) Ths crcut s also called a unty gan amplfer, buffer amplfer, or solaton amplfer. The name oltage follower deres from the ablty of the put oltage to follow exactly the nput oltage. The extremely hgh nput mpedance of the amplfer permts rtually perfect solaton between source and load and elmnates loadng effects. 9

10 Dfferental Amplfer It s often necessary to amplfy the dfference between two sgnals that are both corrupted by nose or some other form of nterference. In such cases, the dfferental amplfer prodes an naluable tool n amplfyng the desred sgnal whle reectng the nose. KL : n KL ( )

11 Acte lters Amplfers Non - Inertng : : Inertng Amplfers 3dB frequency, : Inertng Amplfers A frst-order, acte, low-pass flter

12 A frst-order, acte, hgh-pass flter crcut. 3dB frequency, : Inertng Amplfers Acte lters (ont.)

13 3 A second-order, acte, band-pass flter crcut. 3dB frequences two, db frequency, : Inertng Amplfers LP HP Acte lters (ont.)

14 Integraton & Dfferentaton Integrator Mathematcal Operatons n Analog omputers d d dt dt ( t ) t t ( t ) dt Dfferentator d dt d dt d dt 4

15 eal OP Amplfers and Ther Lmtatons The amplfer needs dc power supples for ther operaton. The amplfer has two termnals, labeled and - for connecton to the dc supples. (ome amplfers requre only one termnal power supply.) The amplfer characterstcs reman lnear oer only a lmted range of nput and put oltages. or an amplfer operated from two power supples, the put oltage cannot exceed a specfed poste lmt and cannot decrease below a specfed negate lmt < < : oltage supply lmtaton 5

16 eal OP Amplfers and Ther Lmtatons The dfferental open loop oltage gan of real op amps s fnte and decrease wth frequency -> fnte bandwdth. The fgure rght shows the frequency response cure for a typcal 74 op amp. 7.7% (3dB) of ts maxmum alue at Hz - > open loop cutoff frequency 6

17 eal OP Amplfers and Ther Lmtatons The op amp can produce only a fnte rate of change at ts put. -> lew rate The slew rate () s the maxmum possble rate of change of the op amp put oltage. The slew-rate specfcaton of an op amp ndcates how fast the put oltage can change. The slew rate s specfed n /µs. The slew-rate dstorton of a sne wae makes the put waeform appear trangular. d dt max :lew rate lmtaton d dt ( t) Asnt A cost d dt max A put 7