Applications Using Operational Amplifiers

Transcription

1 Applcatons Usng peratonal Amplfers Basc op-amp applcatons wth negate feedback: nertng amplfers, nonnertng amplfers, dfferental amplfers, summng amplfers, etc. thers lnear and non-lnear applcatons: Capactely coupled amplfer p-amp amplfers operated from a sngle power supply ntegrators and dfferentators acte flters Current sources Voltage doman conerson crcuts Half-wae and full-wae precson rectfers Precson peak detectors Logarthmc and exponental amplfers Crcuts for multplcaton and dson

2 Capactely coupled amplfer (t) =V + (t) We want to amplfy only the arable sgnal (t) t ole of 3? o t

3 p-amp amplfers operated from a sngle power supply Non-nertng confguraton 5.K 5K How (t) can be amplfed n the case of a unpolar supply?

4 Soluton: VTC translaton +V PS +V PS V BAS V =+V PS / ' V BAS obtanng the basng oltage equalence n steady-state regme

5 V BAS V PS V BAS V PS VBAS VBAS The alue of dc gan s to hgh How can t be lowered to unty? Let s mplement: VBAS V BAS

6 Complete crcut 0K Equalent crcut n steady-state regme V BAS =6Vdc 0K V PS 5.K 5K V BAS V BAS V BAS V BAS =6Vdc Equalent dc crcut? Equalent ac crcut? What s the soluton for an nertng op-amp amplfer wth sngle supply?

7 ntegrator t Tme doman analyss t t t ( t) dt (0) t t t t C C t 0 C t C C tme constant, C 0 t ntegratng constant Cd c dt 0 dt dt C (0) C (0) Problem: Soluton: The op-amp can become saturated due to the dc offset oltage and / or basng currents, because there s no NF n dc (the equalent mpedance of the capactor s nfnte n dc) ntroduce a NF path n dc

8 ntegrator wth NF n dc large enough to be neglected (open-crcut) when compared wth equalent mpedance of the capactor at the workng frequency Ths s the crcut recomanded to be used n applcatons as an ntegrator (lossy ntegrator)

9 ntegrator wth NF n dc Frequency doman analyses A j j Z j ech Z ech jc j C A j j C Acte low-pass flter large enough to be neglected (open-crcut) n comparson wth equalent mpedance of the capactor at the workng frequency Example: =KΩ =00KΩ C=00pF

10 Dfferentator t A d C dt t j A t j j C Z C j C d dt t jc Acte hgh-pass flter f 0 = The crcut acts as a nose amplfer because of the deraton of the nput sgnal. n practcal approaches a small resstor has to be connected n seres wth the capactor. f 0 C

11 p-amp current sources PTNAL the current doesn t depend on L adjustable current f s replaced wth a resstance n seres wth a potentometer the current of the source can be modfed by modfyng oltage controlled current source none of L termnals can be connected to the ground, therefore we hae a floatng load? What f we hae to return the load to ground?

12 NF and PF NF - domnant A o A o r,, r L L L L, 0 A o A, Because L <, > K +, rezults NF, K Howland source L A, 0 Current source wth grounded load PTNAL

13 PTNAL the resstors must be ery well pared n order to hae a perfect (deal) current source (the output resstance tends to nfnte) f 3 4 Practcal soluton: op amp + transstor

14 Current Sources Usng p Amp and T E CEsat CC L V V Better lnearty Current snk, floatng load s t possble to use only one dc supply source?

15 Current Sources Usng p Amp and T cont. Seres-seres NF (oltage-current): the small sgnal output resstance of the current source (seen by L ) s approxmately a tmes larger than the one n the absence of the NF, (wthout the op amp). Adjustable current source: modfyng - oltage-controlled current source a potentometer n seres wth.

16 Current Sources Usng p Amp and T Load returned to ground, current source

17 Voltage doman conerson crcuts Lnear transformatons cd cd ; cd ; mn max mn Solutons max nertng op-amp amplfer cdmn max nonnertng op-amp amplfer cd max max cd max cd mn mn mn

18 Numercal example (;7)V ( ;6)V cd Crcuts Why V EF s necessary? V EF ' + _ nertng cd nonnertng cd ' + _ V EF

19 nertng crcut V EF cd cd mn max cd max mn ' max cd max + _ ' mn cd mn V V EF EF resstor alues? reference oltage? cd V EF mn VTC V cd max EF

20 Numercal example (;7)V ( ;6)V cd Desgn the nertng oltage conerson crcut

21 Nonnertng crcut cd ' + _ V EF resstor alues? reference oltage?

22 Precson (Acte) ectfers Half-Wae ectfer Can not rectfy small sgnals Some oltage (0.7V) s wasted across the conductng dode Precson rectfer: For the rectfed half-wae we need: = Superdode (almost) zero oltage drop across the conductng superdode p amp + NF + D

23 - cannot became negate D 0 half-wae rectfer for poste half-wae rectfcaton of the negate half-wae?

24 PTNAL Shortcomng: <0, D (off), no NF, smple comparator,oa =V L op amp - saturaton slows down the operaton speed lmts the operaton frequency Soluton: aod the saturaton; how?

25 nertng rectfer aodng op-amp saturaton PTNAL VTC 0; D ( on); D ( off ) NF through D and ;, 0.7V opamp - acte regon oa 0; D ( off ); D ( on) NF through D; 0, oa 0.7V opamp - acte regon

26 Full-Wae ectfcaton superdode peratng prncple >0, D -(on), D (off) = <0, D -(on), D (off) =- Precson rectfer How does the crcut look lke? > 0, D -(on), D (off), there s NF only for A, = A A < 0, D -(off), D (on), there s NF only for A, = -(/) = -

27 When the peak detector s requred to hold the alue of the peak for a long tme, the capactor should be buffered. The nd opamp s connected as a oltage follower. D s the essental dode for peak-rectfcaton operaton. Dode D act as a catchng dode to preent negate saturaton, when the nput s less than the output; The st opamp has local NF, through D n conducton, so the output of the st opamp s clamped at a oltage wth one oltage dode drop below the nput oltage. s necessary to assure a small current through D. Precson Poste Peak Detector PTNAL Precson Poste Peak Detector that Holds the Voltage D role? role?

28 Logarthmc amplfer PTNAL BE C S e V BE T C BE V T ln C S V T ln S For <0 - use pnp transstor Lmts of the crcut: - Then range of the output oltage s narrow, hundreds of mv ( s a base to emtter oltage); - Temperature dependence of the output oltage (V T and S ).

29 Exponental amplfer PTNAL C BE C S e V BE T S e V T S e V T

30 Multplcaton crcut PTNAL e ln ln e ln BE4 = BE + BE BE VT ln BE VT ln S BE 4 V 4 T 4 S e Dson crcut? S S

31 Multplcaton and Dson Crcut 3 4 BE BE BE BE S T BE V ln S T BE S T BE V V ln ln T BE V S e 4 4 Equal resstances = No temperature dependence PTNAL