Lecture 14CE Stage Pole Frequencies, for VA<∞I/O Impedances of CE StageCB Stage: Pole FrequenciesEmitter FollowerAC Analysis of Emitter FollowerFollower: Zero and Pole FrequenciesEmitter Follower: Input CapacitanceEmitter Follower: Output ImpedanceEmitter Follower as Active InductorCascode StageCascode Stage: Pole FrequenciesCascode Stage: I/O ImpedancesSummary of Cascode Stage BenefitsImpedance of Parallel RC CircuitEE105 Fall 2007 Lecture 14, Slide 1 Prof. Liu, UC BerkeleyLecture 14OUTLINE•Frequency Response (cont’d)–CE stage (final comments)–CB stage–Emitter follower–Cascode stageReading: Chapter 11.4-11.6ANNOUNCEMENTS•Midterm #1 results (undergrad. scores only): •N=74; mean=62.8; median=63; std.dev.=8.42•You may pick up your exam during any TA office hour.•Regrade requests must be made before you leave with your exam.•The list of misunderstood/forgotten points has been updated.253810EE105 Fall 2007 Lecture 14, Slide 2 Prof. Liu, UC BerkeleyCE Stage Pole Frequencies, for VA<∞Note that p,out > p,in CrRgCRoCminTh evinp11, CrRgCrRoCmoutoCoutp111,EE105 Fall 2007 Lecture 14, Slide 3 Prof. Liu, UC BerkeleyI/O Impedances of CE Stage rCrRgCjZoCmin||11 oCCSoutrRCCjZ ||||1EE105 Fall 2007 Lecture 14, Slide 4 Prof. Liu, UC Berkeley•Note that there is no capacitance between input & output nodes No Miller multiplication effect!CB Stage: Pole FrequenciesTXmSXpCgR1||1,CCXYCYpCR1,CSYCCC orCB stage with BJT capacitances shownEE105 Fall 2007 Lecture 14, Slide 5 Prof. Liu, UC BerkeleyEmitter Follower•Recall that the emitter follower provides high input impedance and low output impedance, and is used as a voltage buffer.Follower stage with BJT capacitances shown• CL is the load capacitanceCircuit for small-signal analysis (Av)orEE105 Fall 2007 Lecture 14, Slide 6 Prof. Liu, UC BerkeleyAC Analysis of Emitter Follower •KCL at node X:•KCL at output node: 1)()()(1 2jbjajgCvvminout mLSmSLLmSgCrRgCCRbCCCCCCgRa1vvvoutX011CjvrvCjvvRvvvoutSinoutLoutmCjvvgCjvrv11EE105 Fall 2007 Lecture 14, Slide 7 Prof. Liu, UC BerkeleyFollower: Zero and Pole Frequencies •The follower has one zero:•The follower has two poles at lower frequencies: 1)()()(12jbjajgCvvminout mLSmSLLmSgCrRgCCRbCCCCCCgRa1TmzfCg22121 11)()(ppjjjbjaEE105 Fall 2007 Lecture 14, Slide 8 Prof. Liu, UC BerkeleyEmitter Follower: Input Capacitance•Recall that the voltage gain of an emitter follower isLminRgCCC1orFollower stage with BJT capacitances shownmLLvgRRA1 LmvXRgCCAC11LmvYRgCCAC11•CXY can be decomposed into CX and CY at the input and output nodes, respectively: LinRrR 1EE105 Fall 2007 Lecture 14, Slide 9 Prof. Liu, UC BerkeleyEmitter Follower: Output Impedance CrjCrRRrjRrjCrRrjCrRivZSSSSSXXout/11/111Circuit for small-signal analysis (Rout)or SmxxRvgivv CjrvgivmX1EE105 Fall 2007 Lecture 14, Slide 10 Prof. Liu, UC BerkeleyEmitter Follower as Active Inductor•A follower is typically used to lower the driving impedance RS > 1/gm so that the “active inductor” characteristic on the right is usually observed. CASE 1: RS < 1/gmCASE 2: RS > 1/gm CrjCrRRrjRrjCrRrjCrRivZSSSSSXXout/11/111capacitive behavior inductive behaviorEE105 Fall 2007 Lecture 14, Slide 11 Prof. Liu, UC BerkeleyCascode Stage•Review:–A CE stage has large Rin but suffers from the Miller effect.–A CB stage is free from the Miller effect, but has small Rin.•A cascode stage provides high Rin with minimal Miller effect.1121,mmYXXYvggvvAXYXCC 2 orEE105 Fall 2007 Lecture 14, Slide 12 Prof. Liu, UC BerkeleyCascode Stage: Pole Frequencies 111,2||1CCrRSXp 1212,211CCCgCSmYp 22,1CCRCSLoutpCascode stage with BJT capacitances shown(Miller approximation applied) 222,2TmYpfCgNote thatorEE105 Fall 2007 Lecture 14, Slide 13 Prof. Liu, UC BerkeleyCascode Stage: I/O Impedances 11121||CCjrZin 221||CSLoutCCjRZorEE105 Fall 2007 Lecture 14, Slide 14 Prof. Liu, UC BerkeleySummary of Cascode Stage Benefits•A cascode stage has high output impedance, which is advantageous for–achieving high voltage gain–use as a current source•In a cascode stage, the Miller effect is reduced, for improved performance at high frequencies.EE105 Fall 2007 Lecture 14, Slide 15 Prof. Liu, UC BerkeleyImpedance of Parallel RC
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