Lecture 14 10 ANNOUNCEMENTS 38 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 OUTLINE Frequency Response cont d CE stage final comments CB stage Emitter follower Cascode stage Reading Chapter 11 4 11 6 EE105 Fall 2007 Lecture 14 Slide 1 Prof Liu UC Berkeley 25 CE Stage Pole Frequencies for VA p in 1 RThev Cin 1 g m RC ro C p out 1 1 C RC ro Cout 1 g R r m C o Note that p out p in EE105 Fall 2007 Lecture 14 Slide 2 Prof Liu UC Berkeley I O Impedances of CE Stage Z in 1 r j C 1 g m RC ro C EE105 Fall 2007 Lecture 14 Slide 3 Z out 1 RC ro j C CCS Prof Liu UC Berkeley CB Stage Pole Frequencies Note that there is no capacitance between input output nodes No Miller multiplication effect CB stage with BJT capacitances shown p Y ro 1 RC CY CY C CCS p X 1 T 1 RS C X gm C X C EE105 Fall 2007 Lecture 14 Slide 4 Prof Liu UC Berkeley Emitter 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 capacitance EE105 Fall 2007 Lecture 14 Slide 5 Circuit for small signal analysis Av ro Prof Liu UC Berkeley AC Analysis of Emitter Follower v X vout v KCL at node X vout v vin vout v v v 0 1 1 RS r j C j C v v vout KCL at output node g m v 1 1 r j C j C L C R 1 j a S C C C C L C C L gm vout gm C R C vin a j 2 b j 1 b R C 1 S L S EE105 Fall 2007 Lecture 14 Slide 6 gm r g m Prof Liu UC Berkeley Follower Zero and Pole Frequencies vout vin C 1 j gm a j 2 b j 1 RS a C C C C L C C L gm b RS C C R S 1 gm r CL gm The follower has one zero gm z 2 fT C The follower has two poles at lower frequencies j j 1 a j b j 1 1 p1 p2 2 EE105 Fall 2007 Lecture 14 Slide 7 Prof Liu UC Berkeley Emitter Follower Input Capacitance Recall that the voltage gain of an emitter follower is Av Follower stage with BJT capacitances shown ro RL RL 1 gm CXY can be decomposed into CX and CY at the input and output nodes respectively C C X 1 Av C 1 g m RL C 1 C CY 1 g m RL Av C Cin C 1 g m RL Rin r 1 RL EE105 Fall 2007 Lecture 14 Slide 8 Prof Liu UC Berkeley Emitter Follower Output Impedance ro Circuit for small signal analysis Rout 1 v i X g m v r j C v x v ix g m v RS Z out j 1 v X RS r C j r RS r RS r RS RS r C j iX r C j 1 1 1 1 r C EE105 Fall 2007 Lecture 14 Slide 9 Prof Liu UC Berkeley Emitter Follower as Active Inductor Z out j 1 v X RS r C j r RS r RS r RS RS r C j iX r C j 1 1 1 1 r C CASE 1 RS 1 gm CASE 2 RS 1 gm capacitive behavior inductive behavior 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 EE105 Fall 2007 Lecture 14 Slide 10 Prof Liu UC Berkeley Cascode 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 ro Av XY 1 vX 1 g m1 vY gm2 C X 2C XY EE105 Fall 2007 Lecture 14 Slide 11 Prof Liu UC Berkeley Cascode Stage Pole Frequencies Cascode stage with BJT capacitances shown Miller approximation applied p X ro 1 RS r 1 C 1 2C 1 p Y 1 1 CCS1 C 2 2C 1 g m2 Note that p Y p out EE105 Fall 2007 Lecture 14 Slide 12 gm2 2 fT 2 C 2 1 RL CCS 2 C 2 Prof Liu UC Berkeley Cascode Stage I O Impedances ro 1 Z in r 1 j C 1 2C 1 EE105 Fall 2007 Lecture 14 Slide 13 Z out RL 1 j C 2 CCS 2 Prof Liu UC Berkeley Summary 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 14 Prof Liu UC Berkeley
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