The three major operations done on biological signals using Op-Amp:

Transcription

1 The three majr peratns dne n blgcal sgnals usng Op-Amp: ) Amplcatns and Attenuatns 2) DC settng: add r subtract a DC 3) Shape ts requency cntent: Flterng

2 Ideal Op-Amp Mst belectrc sgnals are small and requre amplcatns Fgure 3. Op-amp pequ equalent e crcut. cu The tw nputs are and 2. A derental ltage between them causes current lw thrugh the derental resstance d. The derental ltage s multpled by A, the gan the p amp, t generate the utput-ltage surce. Any current lwng t the utput termnal must pass thrugh the utput resstance. Insde the Op-Amp (IC-chp) 20 transstrs resstrs capactr 2

3 Ideal Characterstcs - A = (gan s nnty) t 2- V = 0, when = 2 (n set ltage) 3- d = (nput mpedance s nnty) 4-0 = 0 (utput mpedance s zer) 5- Bandwdth = (n requency respnse lmtatns) and n phase sht Tw Basc ules ule When the p-amp utput s n ts lnear range, the tw nput termnals are at the same ltage. ule 2 N current lws nt r ut ether nput termnal the p amp. 3

4 Inertng Ampler 0 V -0 V 0 V (a) -0 V Slpe = - / (b) G Fgure 3.3 (a) An nertng ampled. Current lwng thrugh the nput resstr als lws thrugh the eedback resstr. (b) The nput-utput plt shws a slpe - / n the central prtn, but the utput saturates at abut ±3 V. Summng Ampler

5 Example 3. The utput a bptental preampler that measures the electrculgram s an undesred dc ltage ±5 V due t electrde halcell ptentals, wth a desred sgnal ±5 V supermpsed. Desgn a crcut that wll balance the dc ltage t zer and prde a gan - 0 r the desred sgnal wthut saturatng the p amp. 0 k 00 k 0 5V 5 k b 20 k b Vltage, V 0 b /2 Tme -5 V -0 (a) (b) Fllwer ( buer) Used as a buer, t preent a hgh surce resstance rm beng laded dwn by a lw-resstance lad. An anther wrd t preent drawng current rm the surce. G 5

6 Nnnertng Ampler 0 V -0 V Slpe = ( )/ 0 V - G -0 V Derental Amplers Derental Gan G d G d ( 3) Cmmn Mde Gan G c Fr deal p amp the nputs are equal then the utput = 0, and the G c =0. N derental ampler perectly rejects the cmmn-mde ltage. Cmmn-mde rejectn rat CMM G CM G d c Typcal alues range rm 00 t 0,000 Dsadantage ne-p-amp derental ampler s ts lw nput resstance 4 6

7 Instrumentatn Amplers Derental Mde Gan 3 G 4 d 2 ( ) Adantages: Hgh nput mpedance, Hgh CM, Varable gan Cmparatr N Hysteress > 2, = -3 V < 2, =3 V re 2-0 V 0 V re -0 V I ( re ) > 0 then = -3 V else = 3 V wll preent erdrng the p-amp 7

8 Cmparatr Wth Hysteress educes multple transtns due t mv nse leels by mng the threshld alue ater each transtn. 0 V Wth hysteress re 2-0 V 0 V - re 3-0 V Wdth the Hysteress = 4 x V 3 ecter (a) x (-x) D D 2 D 4 D 3 = x 0 V -0 V 0 V (b) -0 V = 2 k = k (c) D L = 3 k (a) Full-wae precsn recter. Fr > 0, the nnnertng ampler at the tp s acte (D 2 and D 3 are n), makng > 0. Fr < 0, the nertng ampler at the bttm s acte, makng > 0. Crcut gan may be adjusted wth a sngle pt. (b) Input-utput characterstcs shw saturatn when > 3 V. (c) One-p-amp ull-wae recter. Fr < 0, the crcut behaes lke the nertng ampler recter wth a gan 0.5. Fr > 0, the p amp dscnnects and the passe resstr chan yelds a gan

9 ecter x (-x) D D 2 D 4 D 3 = x (a) x (-x) D a) Fr > 0, D 2 and D 3 cnduct, whereas D and D 4 are reersebased. b) Fr < 0, D and D 4 cnduct, whereas D 2 and D 3 are reersebased. Lgarthmc Amplers I c /9 0 V -0 V 0 V (a) (b) -0 V 0 I C V BE 0.06lg 0.06 lg at 27 C 3 I S 0 Fgure 3.8 (a) A lgarthmc ampler makes use the act that a transstr's V BE s related t the lgarthm ts cllectr current. Wth the swtch thrwn n the alternate pstn, the crcut gan s ncreased by 0. (b) Input-utput characterstcs shw that the lgarthmc relatn s btaned r nly ne plarty; and 0 gans are ndcated. 9

10 Lgarthmc Amplers Antlg (expnental) crcuts are made by nterchangng the resstr and semcnductr. Uses Lg Ampler. Multply arable 2. Dde arable 3. ase arable t a pwer 4. Cmpress large dynamc range nt small nes 5. Lnearze the utput deces Integratrs C t 0 dt c V Z ( j) V ( j ) Z V V Z j j j A large resstr s used t preent saturatn c 2 C r < c 0

11 Integratrs Fgure 3.9 A three-mde ntegratr Wth S pen and S 2 clsed, the dc crcut behaes as an nertng ampler. Thus = c and can be set t any desred ntal cnductn. Wth S clsed and S 2 pen, the crcut ntegrates. Wth bth swtches pen, the crcut hlds cnstant, makng pssble a lesurely readut. Example 3.2 The utput the pezelectrc sensr may be ed drectly nt the negate nput the ntegratr as shwn belw. Analyze the crcut ths charge ampler and dscuss ts adantages. sc = s = 0 = - c dq s / dt = s = K dx/dt s C sc s FET Pez-electrc sensr t Kdx dt Kx C C 0 dt Lng cables may be used wthut changng sensr senstty r tme cnstant.

12 Derentatrs d C dt V j Z ( ) V ( j) Z jc Fgure 3. A derentatr The dashed lnes ndcate that a small capactr must usually be added acrss the eedback resstr t preent scllatn. Gan = G = G Acte Flters- Lw-Pass Flter V V j j j C (a) C / / c = /2 C req Acte lters (a) A lw pass lter attenuates hgh requences 2

13 Acte Flters (Hgh-Pass Flter) V Gan = G = V G j jc j j C (b) C / / c = /2 C req Acte lters (b) A hgh pass lter attenuates lw requences and blcks dc. Acte Flters (Band-Pass Flter) C V V j j C j j C j C G (c) C / / cl = /2 C ch = /2 C req Acte lters (c) A bandpass lter attenuates bth lw and hgh requences. 3

14 Frequency espnse p-amp and Ampler Open-Lp Gan Cmpensatn Clsed-Lp Gan Lp Gan Gan Bandwdth Prduct Slew ate Oset Vltage and Bas Current ead sectn 3.2 Nullng, Drt, Nse ead sectn and 3.3 Derental bas current, Drt, Nse 4

15 Input and Output esstance d d A d L CL a ( A ) Typcal alue d = 2 t 20 M d a A Typcal alue = 40 Phase-Senste Demdulatr Used n many medcal nstruments r sgnal detectn, aeragng, and Nse rejectn 5

16 The ng Demdulatr I c s pste then D and D 2 are rward-based and A = B. S = DB I c s negate then D 3 and D 4 are rward-based and A = c. S = DC c 2 6