EE101: Op Amp circuits (Part 4) - Department of Electrical Engineering

66 Pages·2015·0.94 MB·English

EE101: Op Amp circuits (Part 4)
M. B. Patil
[email protected]
www.ee.iitb.ac.in/~sequel
DepartmentofElectricalEngineering
IndianInstituteofTechnologyBombay
M.B.Patil,IITBombay
IfVi (cid:29)VD,thediodedropcanbeignored.
However,ifVi issmall,e.g.,Vi =0.2sinωt V,thenthecircuitdoesnotrectify,and
Vo(t)=0V.
Precisionrectifiercircuitsovercomethisdrawback.
Half-wave rectifier
Consideradioderectifier:
V
o
V
V o
i D
R
slope=1
V
V i
D
M.B.Patil,IITBombay
Precisionrectifiercircuitsovercomethisdrawback.
Half-wave rectifier
Consideradioderectifier:
V
o
V
V o
i D
R
slope=1
V
V i
D
IfVi (cid:29)VD,thediodedropcanbeignored.
However,ifVi issmall,e.g.,Vi =0.2sinωt V,thenthecircuitdoesnotrectify,and
Vo(t)=0V.
M.B.Patil,IITBombay
Half-wave rectifier
Consideradioderectifier:
V
o
V
V o
i D
R
slope=1
V
V i
D
IfVi (cid:29)VD,thediodedropcanbeignored.
However,ifVi issmall,e.g.,Vi =0.2sinωt V,thenthecircuitdoesnotrectify,and
Vo(t)=0V.
Precisionrectifiercircuitsovercomethisdrawback.
M.B.Patil,IITBombay
Considertwocases:
(i) Disconducting: Thefeedbackloopisclosed,andthecircuitlookslike(except
forthediodedrop)thebufferwehaveseenearlier.
Sincetheinputcurrenti−≈0,iR =iD.
Further,V+−V−= Vo1 = Vo+0.7V ≈0V →Vo =Vi.
A A
V V
Thissituationarisesonlyifi >0(sincethediodecanonlyconductinthe
D
forwarddirection),i.e.,Vo >0→Vi =Vo >0V.
V
o
i
D
i− V Vo
Vi o1 VD slope=1
i
R
R
V
i
Half-wave precision rectifier
V
o
Vi D
R
M.B.Patil,IITBombay
Sincetheinputcurrenti−≈0,iR =iD.
Further,V+−V−= Vo1 = Vo+0.7V ≈0V →Vo =Vi.
A A
V V
Thissituationarisesonlyifi >0(sincethediodecanonlyconductinthe
D
forwarddirection),i.e.,Vo >0→Vi =Vo >0V.
V
o
slope=1
V
i
Half-wave precision rectifier
i
D
Vo i− V Vo
Vi D R Vi o1 VD i
R
R
Considertwocases:
(i) Disconducting: Thefeedbackloopisclosed,andthecircuitlookslike(except
forthediodedrop)thebufferwehaveseenearlier.
M.B.Patil,IITBombay
Thissituationarisesonlyifi >0(sincethediodecanonlyconductinthe
D
forwarddirection),i.e.,Vo >0→Vi =Vo >0V.
V
o
slope=1
V
i
Half-wave precision rectifier
i
D
Vo i− V Vo
Vi D R Vi o1 VD i
R
R
Considertwocases:
(i) Disconducting: Thefeedbackloopisclosed,andthecircuitlookslike(except
forthediodedrop)thebufferwehaveseenearlier.
Sincetheinputcurrenti−≈0,iR =iD.
Further,V+−V−= Vo1 = Vo+0.7V ≈0V →Vo =Vi.
A A
V V
M.B.Patil,IITBombay
V
o
slope=1
V
i
Half-wave precision rectifier
i
D
Vo i− V Vo
Vi D R Vi o1 VD i
R
R
Considertwocases:
(i) Disconducting: Thefeedbackloopisclosed,andthecircuitlookslike(except
forthediodedrop)thebufferwehaveseenearlier.
Sincetheinputcurrenti−≈0,iR =iD.
Further,V+−V−= Vo1 = Vo+0.7V ≈0V →Vo =Vi.
A A
V V
Thissituationarisesonlyifi >0(sincethediodecanonlyconductinthe
D
forwarddirection),i.e.,Vo >0→Vi =Vo >0V.
M.B.Patil,IITBombay
Half-wave precision rectifier
V
o
i
D
Vo i− V Vo
Vi D R Vi o1 VD i slope=1
R
R
V
i
Considertwocases:
(i) Disconducting: Thefeedbackloopisclosed,andthecircuitlookslike(except
forthediodedrop)thebufferwehaveseenearlier.
Sincetheinputcurrenti−≈0,iR =iD.
Further,V+−V−= Vo1 = Vo+0.7V ≈0V →Vo =Vi.
A A
V V
Thissituationarisesonlyifi >0(sincethediodecanonlyconductinthe
D
forwarddirection),i.e.,Vo >0→Vi =Vo >0V.
M.B.Patil,IITBombay
(ii) Disnotconducting→Vo =0V.
WhataboutVo1?
SincetheOpAmpisnowintheopen-loopconfiguration,averysmallVi is
enoughtodriveittosaturation.
NotethatCase(ii)occurswhenVi <0V. SinceV+−V−=Vi−0=Vi is
negative,Vo1 isdrivento−Vsat.
V
o
V
V o
V o1
i
V =0
o
R
V
i
Half-wave precision rectifier
V
o
Vi D
R
M.B.Patil,IITBombay