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1、Chapter 10 Operational AmplifiersChapter 10 Operational AmplifiersChapter 10 Operational AmplifiersIntroductionDifferential Amplifier CircuitDifferential and Common-mode OperationBIFET,BIMOS,and CMOS Differential Amplifier CircuitsOP-Amp BasicsOP-Amp Unit Specifications OutlineChapter 10 Operational
2、 AmplifiersAnalog ICs include operational amplifiers,analog multipliers,A/D converters,D/A converters,PLL,etc.A complete op amp is realized by combining analog circuit building blocks.The bipolar op-amp has the general purpose variety and is designed to fit a wide range of specifications.The termina
3、l characteristics is nearly ideal.10.1 Introduction Chapter 10 Operational Amplifiers 10.2 Differential Amplifier Circuit The differential amplifier(pair)configuration is the most widely used building block in analog IC design.BJT differential amplifier is the basis of a very-high-speed logic circui
4、t family,called emitter-coupled logic(ECL).There are 2 reasons why differential amplifiers are so well suited for IC fabrication.(1)The performance of the differential pair depends on the matching between the two sides of the circuits.IC fabrication is capable of providing matched devices.(2)Differe
5、ntial pair uses more components(nearly twice)than singleended circuits.IC fabrication could make numbers of transistors with low cost.Chapter 10 Operational AmplifiersThere are 2 reasons for using differential in preference to singleended amplifiers.(1)Differential circuits are much less sensitive t
6、o noise and interference than single-ended circuits.(2)It enables us to bias the amplifier and to couple amplifier stage without the need of bypass and coupling capacitors which are impossible to fabricate economically by IC technology.Differential Amplifier Circuit Chapter 10 Operational Amplifiers
7、Reasons:Direct coupling between signal source and amplifier will easily cause temperature Drift(zero drift).Chapter 10 Operational AmplifiersChapter 10 Operational AmplifiersThe differential pair with a“large”differential input signal.Q1 is on and Q2 is off.Current I entirely flows in Q1.10.3 Differ
8、ential and Common-mode Operation Differential mode OperationChapter 10 Operational AmplifiersThe differential pair with a large differential input signal of polarity opposite to that in(b).Q2 is on and Q1 is off.Current I entirely flows in Q2.Differential mode Operation Chapter 10 Operational Amplif
9、iersDifferential mode Operation The differential pair with a small differential input signal vi.Small signal operation or linear amplifier.Assuming the bias current source I to be ideal and thus I remains constant with the change in vCM.Increment in Q1 and decrement in Q2.Chapter 10 Operational Ampl
10、ifiersLarge-Signal OperationChapter 10 Operational AmplifiersLarge-Signal OperationNonlinear curves.Linear segments.Maximum value of input differential voltages as a small-signal amplifier can be used as a fast current switchEnlarge the linear segment by including equal resistance Re in series with
11、the emitters.Chapter 10 Operational AmplifiersLarge-Signal OperationThe transfer characteristics of the BJT differential pair(a)can be linearized by including resistances in the emitters.Chapter 10 Operational AmplifiersSmall-signal operation of the BJT differential amplifier The currents and voltag
12、es in the differential amplifier when a small differential input signal vid is applied.Chapter 10 Operational AmplifiersA simple technique for determining the signal currents in a differential amplifier excited by a differential voltage signal vid;dc quantities are not shown.Small-Signal OperationCh
13、apter 10 Operational AmplifiersSmall-Signal OperationA differential amplifier with emitter resistances.Only signal quantities are shown(in color).Chapter 10 Operational AmplifiersInput Differential ResistanceInput differential resistance is finite.The resistance seen between the two bases is equal t
14、o the total resistance in the emitter circuit multiplied by(1+).Input differential resistance of differential pair with emitter resistors.Chapter 10 Operational AmplifiersDifferential Voltage GainDifferential voltage gainOutput voltage taken single-endedOutput voltage taken differentiallyChapter 10
15、Operational AmplifiersDifferential Voltage GainDifferential voltage gain of the differential pair with resistances in the emitter loadsOutput voltage taken single-endedOutput voltage taken differentially The voltage gain is equal to the ratio of the total resistance in the collector circuit to the t
16、otal resistance in the emitter circuit.Chapter 10 Operational AmplifiersDifferential Half-Circuit AnalysisDifferential input signals.Single voltage at joint emitters is zero.The circuit is symmetric.Equivalent common-emitter amplifiers in(b).Chapter 10 Operational AmplifiersDifferential Half-Circuit
17、 AnalysisThis equivalence applies only for differential input signals.Either of the two common-emitter amplifiers can be used to find the differential gain,differential input resistance,frequency response,and so on,of the differential amplifier.Half circuit is biased at I/2.The voltage gain(with the
18、 output taken differentially)is equal to the voltage of half circuit.Chapter 10 Operational AmplifiersDifferential Half-Circuit AnalysisThe differential amplifier fed in a single-ended manner.Signal voltage at the emitter is not zero.Almost identical to the symmetric one.Chapter 10 Operational Ampli
19、fiersCommon-mode OperationThe differential pair with a common-mode input signal vCM.Two transistors are matched.Current source with infinite output resistance.Current I divide equally between two transistors.The difference in voltage between the two collector is zero.The differential pair rejects th
20、e common-mode input signal as long as two transistors remain in active region.Chapter 10 Operational AmplifiersCommon-Mode Gain The differential amplifier fed by a common-mode voltage signal vicm.Chapter 10 Operational AmplifiersCommon-Mode Gain Equivalent“half-circuits”for common-mode calculations.
21、Chapter 10 Operational AmplifiersCommon-Mode Gain Common-mode voltage gainOutput voltage taken single-endedOutput voltage taken differentiallyChapter 10 Operational AmplifiersCommon-Mode Rejection Ratio Common-mode rejection ratioOutput voltage taken single-endedOutput voltage taken differentiallyTh
22、is is true only when the circuit is symmetric.Mismatch on CMRRChapter 10 Operational AmplifiersInput Common-Mode Resistance Definition of the input common-mode resistance Ricm.The equivalent common-mode half-circuit.Chapter 10 Operational AmplifiersInput Common-Mode Resistance Input common-mode resi
23、stanceInput common-mode resistance is very large.Chapter 10 Operational AmplifiersExample Chapter 10 Operational AmplifiersExample(contd)Evaluate the following:The input differential resistance.The overall differential voltage gain(neglect the effect of ro).The worst-case common-mode gain if the two
24、 collector resistance are accurate within 1%.The CMRR,in dB.The input common-mode resistance(suppose the Early voltage is 100V).Chapter 10 Operational Amplifiers Other Nonideal characteristic of the Differential AmplifierInput offset voltage VosIdeal differential pair is perfectly matched,but practi
25、cal circuits exhibitsMismatches that result in a dc Vo is not zero.Vo is the output dc offset Voltage.Input offset voltage Vos:Input bias and offset currents Iosperfectly matchedInput Common-Mode RangeChapter 10 Operational Amplifiers10.4 BIFET,BIMOS,and CMOS Differential Amplifier Circuits Chapter
26、10 Operational AmplifiersOperation with a Common Mode Input VoltageChapter 10 Operational AmplifiersOperation with a Common Mode Input VoltageSymmetry circuit.Common-mode voltage.Current I divides equally between two transistors.The difference between two drains is zero.The differential pair rejects
27、 the common-mode input signals.Chapter 10 Operational AmplifiersOperation with a Differential Input VoltageThe MOS differential pair with a differential input signal vid applied.With vid positive:vGS1 vGS2,iD1 iD2,and vD1 vD2;thus(vD2-vD1)will be positive.With vid negative:vGS1 vGS2,iD1 vD2;thus(vD2
28、-vD1)will be negative.Chapter 10 Operational AmplifiersOperation with a Differential Input VoltageDifferential input voltage.Response to the differential input signal.The current I can be steered from one transistor to the other by varying the differential input voltage in the range:When differentia
29、l input voltage is very small,the differential output voltage is proportional to it,and the gain is high.Chapter 10 Operational AmplifiersLarge-Signal OperationTransfer characteristic curvesNormalized plots of the currents in a MOSFET differential pair.Note that VOV is the overdrive voltage at which
30、 Q1 and Q2 operate when conducting drain currents equal to I/2.Chapter 10 Operational AmplifiersLarge-Signal OperationNonlinear curves.Maximum value of input differential voltage.When vid=0,two drain currents are equal to I/2.Linear segment.Linearity can be increased by increasing overdrive voltage(
31、see next slide).Price paid is a reduction in gain(current I is kept constant).Chapter 10 Operational AmplifiersLarge-Signal OperationThe linear range of operation of the MOS differential pair can be extended by operating the transistor at a higher value of VOV.Chapter 10 Operational AmplifiersSmall-
32、Signal Operation of MOS Differential PairLinear amplifierDifferential gainCommon-mode gainCommon-mode rejection ratio(CMRR)Mismatch on CMRRChapter 10 Operational AmplifiersDifferential Gaina common-mode voltage applied to set the dc bias voltage at the gates.vid applied in a complementary(or balance
33、d)manner.Chapter 10 Operational AmplifiersDifferential GainSignal voltage at the joint source connection must be zero.Chapter 10 Operational AmplifiersDifferential GainAn alternative way of looking at the small-signal operation of the circuit.Chapter 10 Operational AmplifiersDifferential GainDiffere
34、ntial gainOutput taken single-endedOutput taken differentiallyAdvantages of output signal taken differentiallyReject common-mode signalIncrease in gain by a factor of 2(6dB)Chapter 10 Operational AmplifiersDifferential GainMOS differential amplifier with ro and RSS taken into account.Chapter 10 Oper
35、ational AmplifiersDifferential GainEquivalent circuit for determining the differential gain.Each of the two halves of the differential amplifier circuit is a common-source amplifier,known as its differential“half-circuit.”Chapter 10 Operational AmplifiersDifferential GainDifferential gainOutput take
36、n single-endedOutput taken differentiallyChapter 10 Operational AmplifiersCommon-Mode GainThe MOS differential amplifier with a common-mode input signal vicm.Chapter 10 Operational AmplifiersCommon-Mode GainEquivalent circuit for determining the common-mode gain(with ro ignored).Each half of the cir
37、cuit is known as the“common-mode half-circuit.”Chapter 10 Operational AmplifiersCommon-Mode GainCommon-mode gainOutput taken single-endedOutput taken differentiallyChapter 10 Operational AmplifiersCommon-Mode Rejection RatioCommon-mode rejection ratio(CMRR)Output taken single-endedOutput taken diffe
38、rentiallyThis is true only when the circuit is perfectly matched.Chapter 10 Operational AmplifiersMismatch on CMRREffect of RD mismatch on CMRREffect of gm mismatch on CMRRChapter 10 Operational AmplifiersMismatch on CMRRDetermine the common-mode gain resulting from a mismatch in the gm values of Q1
39、 and Q2.Common-mode half circuit is not available due to mismatch in circuit.The nominal value gm.Chapter 10 Operational AmplifiersMismatch on CMRREffect of gm mismatch on CMRRChapter 10 Operational AmplifiersOP-Amp Specifications-DC Offset Parameters Chapter 10 Operational Amplifiers BJT Current Mi
40、rrorChapter 10 Operational AmplifiersA Simple BJT Current SourceChapter 10 Operational Amplifiers Current SteeringChapter 10 Operational AmplifiersCurrent-Mirror Circuits with Improved PerformanceTwo performance parameters need to be improved:a.The accuracy of the current transfer ratio of the mirro
41、r.b.The output resistance of the current source.Chapter 10 Operational AmplifiersCascode MOS Current MirrorChapter 10 Operational AmplifiersCurrent Mirror with Base-Current CompensationChapter 10 Operational AmplifiersThe Wilson Bipolar Current MirrorChapter 10 Operational AmplifiersThe Wilson MOS C
42、urrent MirrorChapter 10 Operational AmplifiersThe Widlar Current SourceChapter 10 Operational AmplifiersMOSFET Current SourceChapter 10 Operational AmplifiersThe Basic MOSFET Current MirrorChapter 10 Operational AmplifiersOutput CharacteristicChapter 10 Operational AmplifiersMOS Current-Steering Cir
43、cuitsChapter 10 Operational AmplifiersThe Wilson MOS Current MirrorChapter 10 Operational AmplifiersCascode MOS Current MirrorChapter 10 Operational AmplifiersGeneral descriptionThe input stageThe intermediate stage The output stageThe biasing circuitsDevice parameters10.5 OP-Amp Basics The 741 Op-A
44、mp CircuitChapter 10 Operational AmplifiersChapter 10 Operational Amplifiers General Description24 transistors,few resistors and only one capacitorTwo power suppliesShort-circuit protection Chapter 10 Operational Amplifiers The Input StageThe input stage consists of transistors Q1 through Q7.Q1-Q4 i
45、s the differential version of CC and CB configuration.High input resistance.Current source(Q5-Q7)is the active load of input stage.It not only provides a high-resistance load but also converts the signal from differential to single-ended form with no loss in gain or common-mode rejection.Chapter 10
46、Operational Amplifiers The Intermediate StageThe intermediate stage is composed of Q16,Q17 and Q13B.Common-collector configuration for Q16 gives this stage a high input resistance as well as reduces the load effect on the input stage.Common-emitter configuration for Q17 provides high voltage gain be
47、cause of the active load Q13B.Capacitor Cc introduces the miller compensation to insure that the op amp has a very high unit-gain frequency.Chapter 10 Operational Amplifiers The Output StageThe output stage is the efficient circuit called class AB output stage.Voltage source composed of Q18 and Q19
48、supplies the DC voltage for Q14 and Q20 in order to reduce the cross-over distortion.Q23 is the CC configuration to reduce the load effect on intermediate stage.Short-circuit protection circuitryForward protection is implemented by R6 and Q15.Reverse protection is implemented by R7,Q21,current sourc
49、e(Q24,Q22)and intermediate stage.Chapter 10 Operational Amplifiers The Output Stage(a)The emitter follower is a class A output stage.(b)Class B output stage.Chapter 10 Operational AmplifiersThe Output StageWave of a class B output stage fed with an input sinusoid.Positive and negative cycles are una
50、ble to connect perfectly due to the turn-on voltage of the transistors.This wave form has the nonlinear distortion called crossover distortion.To reduce the crossover distortion can be implemented by supplying the constant DC voltage at the base terminals.Chapter 10 Operational Amplifiers The Output