Lecture 12Review: Cascode Stage RoutAnother View of a Cascode StageTemperature and Supply-Voltage Dependence of Bias CurrentConcept of a Current MirrorCurrent Mirror CircuitryBad Current Mirror Example 1Bad Current Mirror Example 2Multiple Copies of IREFCurrent ScalingExample: Scaled CurrentsFractional ScalingExample: Different Mirroring RatiosEffect of Base CurrentsImproved Mirroring AccuracyDifferent Mirroring Ratio AccuracyPNP Current MirrorGeneration of IREF for a PNP-BJT Current MirrorCurrent Mirror with Discrete BJTsEE105 Fall 2007 Lecture 12, Slide 1 Prof. Liu, UC BerkeleyLecture 12OUTLINE•Cascode Stage (cont’d)–supplementary remarks•Current MirrorsReading: Chapter 9.2ANNOUNCEMENTS•Late Pre-Lab assignments will no longer be accepted!•Review session: 3-5PM Friday (10/5) in 306 Soda (HP Auditorium)•Midterm #1 (Thursday 10/11, 3:30PM-5:00PM) location:•106 Stanley Hall: Students with last names starting with A-L•306 Soda Hall: Students with last names starting with M-ZEE105 Fall 2007 Lecture 12, Slide 2 Prof. Liu, UC BerkeleyReview: Cascode Stage Rout•The impedance seen looking into the collector can be boosted significantly by using a BJT for emitter degeneration, with a relatively small reduction in headroom. 121112112||||)]||(1[rrrgRrrrrrgROOmoutOOOmoutEE105 Fall 2007 Lecture 12, Slide 3 Prof. Liu, UC BerkeleyAnother View of a Cascode Stage•Instead of considering a cascode as Q2 degenerating Q1, we can also think of it as Q1 stacked on top of Q2 (current source) to boost Q2’s output impedance.EE105 Fall 2007 Lecture 12, Slide 4 Prof. Liu, UC BerkeleyTemperature and Supply-Voltage Dependence of Bias Current•Circuits should be designed to operate properly over a range of supply voltages and temperatures.•For the biasing scheme shown below, I1 depends on the temperature as well as the supply voltage, since VT and IS depend on temperature.CCBEVVSVRRRVeIITBE212/1EE105 Fall 2007 Lecture 12, Slide 5 Prof. Liu, UC BerkeleyConcept of a Current Mirror•Circuit designs to provide a supply- and temperature-independent current exist, but require many transistors to implement. “golden current source”•A current mirror is used to replicate the current from a “golden current source” to other locations.EE105 Fall 2007 Lecture 12, Slide 6 Prof. Liu, UC BerkeleyCurrent Mirror Circuitry•Diode-connected QREF produces an output voltage VX that forces Icopy1 to be equal to IREF, if Q1 is identical to QREF.REFREFSScopyIIII,1,1REFSREFTScopyTXIIVIIVV,1,1lnlnCurrent mirror concept Generation of required VBECurrent Mirror CircuitryEE105 Fall 2007 Lecture 12, Slide 7 Prof. Liu, UC BerkeleyBad Current Mirror Example 1•If the collector and base of QREF are not shorted together, there will not be a path for the base currents to flow, so that Icopy is zero.EE105 Fall 2007 Lecture 12, Slide 8 Prof. Liu, UC BerkeleyBad Current Mirror Example 2•Although it provides a path for base currents to flow, this biasing approach is no better than a resistive voltage divider.EE105 Fall 2007 Lecture 12, Slide 9 Prof. Liu, UC BerkeleyMultiple Copies of IREF•Multiple copies of IREF can be generated at different locations by applying the current mirror concept to multiple transistors.REFREFSjSjcopyIIII,,,EE105 Fall 2007 Lecture 12, Slide 10 Prof. Liu, UC BerkeleyCurrent Scaling•By scaling the emitter area of Qj by a factor of n with respect to the emitter area of QREF, Icopy,j is scaled by a factor of n with respect to IREF.–This is equivalent to placing n unit-sized transistors in parallel. REFjcopynII ,EE105 Fall 2007 Lecture 12, Slide 11 Prof. Liu, UC BerkeleyExample: Scaled CurrentsEE105 Fall 2007 Lecture 12, Slide 12 Prof. Liu, UC BerkeleyFractional Scaling•A fraction of IREF can be created in Q1 by scaling up the emitter area of QREF.REFcopyII31TXSREFVVII exp3TXScopyVVII expEE105 Fall 2007 Lecture 12, Slide 13 Prof. Liu, UC BerkeleyExample: Different Mirroring Ratios•Using the concept of current scaling and fractional scaling, Icopy1 = 0.05mA and Icopy2 = 0.5mA, derived from a single 0.2mA reference current source (IREF).EE105 Fall 2007 Lecture 12, Slide 14 Prof. Liu, UC BerkeleyEffect of Base Currents 111 nnIIREFcopycopycopyREFCREFInIII ,nIIcopyREFC,EE105 Fall 2007 Lecture 12, Slide 15 Prof. Liu, UC BerkeleyImproved Mirroring Accuracy•Use QF (rather than IREF) to supply the base currents of QREF and Q1, reduce the mirroring error by a factor of . 1112nnIIREFcopycopycopyFEFCInIII ,, 112,nIIcopyFBREFCFBREFIII,,nIIcopyREFC,EE105 Fall 2007 Lecture 12, Slide 16 Prof. Liu, UC BerkeleyDifferent Mirroring Ratio Accuracy 222115410154REFcopyREFcopyIIII1, copyREFCII REFCFBREFIII,,4REFCcopycopyFCIIII,21,421,15copyFBII EE105 Fall 2007 Lecture 12, Slide 17 Prof. Liu, UC BerkeleyPNP Current Mirror•A PNP BJT current mirror can be used as a current-source load for an NPN BJT amplifier stage.EE105 Fall 2007 Lecture 12, Slide 18 Prof. Liu, UC BerkeleyGeneration of IREF for a PNP-BJT Current Mirror•Neglecting base currents, the currents flowing through QM and QREF2 are the same.EE105 Fall 2007 Lecture 12, Slide 19 Prof. Liu, UC BerkeleyCurrent Mirror with Discrete BJTs•If QREF and Q1 are discrete NPN BJTs, IREF and Icopy1 can differ dramatically, due to IS
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