Lecture 12ANNOUNCEMENTS•Late PreLab assignments will no longer be accepted!•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‐ZOUTLINE• Cascode Stage (cont’d)–supplementary remarks• Current MirrorsEE105 Fall 2007 Lecture 12, Slide 1Prof. Liu, UC BerkeleyReading: Chapter 9.2Review: Cascode Stage Rout• The impedance seen looking into the collector can be boosted significantly by using a BJT for emitter degeneration, with asignificantly by using a BJT for emitter degeneration, with a relatively small reduction in headroom.||)]||(1[rrrrrgR++=()121112112||||)]||(1[πππrrrgRrrrrrgROOmoutOOOmout≈++=EE105 Fall 2007 Lecture 12, Slide 2Prof. Liu, UC BerkeleyAnother View of a Cascode Stage• Instead of considering a cascode as Q2degenerating Qwe can also think of it asQstack ed on top ofQQ1, we can also think of it as Q1 stack ed on top of Q2 (current source) to boost Q2’s output impedance. EE105 Fall 2007 Lecture 12, Slide 3Prof. Liu, UC BerkeleyTemperature and Supply‐Voltage Dependence of Bias CurrentDependence of Bias Current• Circuits should be designed to operate properly over a range of supply voltages and temperaturesa range of supply voltages and temperatures.• For the biasing scheme shown below, I1 depends on the temperature as well as the supply voltage, sincethe temperature as well as the supply voltage, since VTand ISdepend on temperature.VVSReIITBE/1=CCBEVRRRV212+≅EE105 Fall 2007 Lecture 12, Slide 4Prof. Liu, UC BerkeleyConcept of a Current Mirror• Circuit designs to provide a supply‐ and temperature‐independent current exist, but require many p , q ytransistors to implement.Æ “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 5Prof. Liu, UC BerkeleyCurrent Mirror Circuitry• Diode‐connected QREFproduces an output voltage VX that forcesIto be equal toIifQis identical toQforces Icopy1 to be equal to IREF, if Q1 is identical to QREF.Current mirror concept Generation of required VBECurrent Mirror Circuitry⎞⎜⎛⎞⎜⎛REFcopyII1REFScopyIIII1,1=⎟⎠⎞⎜⎜⎝⎛=⎟⎠⎞⎜⎜⎝⎛=REFSREFTScopyTXIIVIVV,1,1lnlnEE105 Fall 2007 Lecture 12, Slide 6Prof. Liu, UC BerkeleyREFREFScopyI,1Bad Current Mirror Example 1• If the collector and base of QREFare 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 7Prof. Liu, UC BerkeleyBad Current Mirror Example 2Alth h it id th f b t t fl•Although it provides a path for base currents to flow, this biasing approach is no better than a resistive voltage dividerEE105 Fall 2007 Lecture 12, Slide 8Prof. Liu, UC Berkeleyvoltage divider. Multiple Copies of IREF• Multiple copies of IREFcan be generated at different locations by applying the current mirror concept tolocations by applying the current mirror concept to multiple transistors.REFjSjIII,=EE105 Fall 2007 Lecture 12, Slide 9Prof. Liu, UC BerkeleyREFREFSjcopyIII,,Current Scaling• By scaling the emitter area of Qj by a factor of n with respect to the emitter area ofQIis scaled by arespect 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.q pgpnII=EE105 Fall 2007 Lecture 12, Slide 10 Prof. Liu, UC BerkeleyREFjcopynII=,Example: Scaled Currents EE105 Fall 2007 Lecture 12, Slide 11 Prof. Liu, UC BerkeleyFractional Scaling• A fraction of IREF can be created in Q1by scaling up the emitter area ofQthe emitter area of QREF.⎞⎜⎜⎛=XVIIexp3⎞⎜⎜⎛=XVIIexp⎠⎜⎜⎝=TSREFVIIexp3⎠⎜⎜⎝=TScopyVIIexpII1=EE105 Fall 2007 Lecture 12, Slide 12 Prof. Liu, UC BerkeleyREFcopyII3=Example: Different Mirroring Ratios• Using the concept of current scaling and fractional scalingI= 0 05mA andI= 05mA derivedscaling, Icopy1 = 0.05mA and Icopy2= 0.5mA, derived from a single 0.2mA ref erence current source (IREF).EE105 Fall 2007 Lecture 12, Slide 13 Prof. Liu, UC BerkeleyEffect of Base CurrentsββcopycopyREFCREFInIII ++=,ββnInIIcopyREFC=,()111++=nnIIREFcopyEE105 Fall 2007 Lecture 12, Slide 14 Prof. Liu, UC Berkeley()11++nβImproved Mirroring Accuracy• Use QF (rather than IREF) to supply the base currents of QREFand Q1, reduce the mirroring error by a factor of β.QREF Q1, g yβcopycopyIIII+=≅REFCFBREFIII,,+=ββFEFCnII+=≅,,⎞⎜⎛+≅11IIcopy⎠⎜⎝+≅12,nIpyFBβnIIcopyREFC=,()111++=nnIIREFcopyEE105 Fall 2007 Lecture 12, Slide 15 Prof. Liu, UC Berkeley()112++nβDifferent Mirroring Ratio Accuracy REFCFBREFIII,,4+=βββREFCcopycopyFCIIII,21,4++≅βββ21,15βcopyFBII ≅1=REFIIβ21154β+copyI215410+=REFcopyII1, copyREFCII =EE105 Fall 2007 Lecture 12, Slide 16 Prof. Liu, UC Berkeley24β+PNP Current Mirror• A PNP BJT current mirror can be used as a current‐source load for an NPN BJT amplifier stagesource load for an NPN BJT amplifier stage.EE105 Fall 2007 Lecture 12, Slide 17 Prof. Liu, UC BerkeleyGeneration of IREF for a PNPBJT Current MirrorPNP‐BJT Current Mirror• Neglecting base currents, the currents flowing throughQandQare the sameEE105 Fall 2007 Lecture 12, Slide 18 Prof. Liu, UC Berkeleythrough QMand QREF2are the same.Current Mirror with Discrete BJTs• If QREF and Q1 are discrete NPN BJTs, IREF and Icopy1 can differ dramatically due toImismatchdiffer dramatically, due to IS mismatch.EE105 Fall 2007 Lecture 12, Slide 19 Prof. Liu, UC
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