3 13 2008 Lecture 14 OUTLINE Frequency Response cont d CB stage Emitter follower Cascode stage Reading Chapter 11 4 11 6 EE105 Spring 2008 Lecture 14 Slide 1 Prof Wu 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 RCCY CY C CCS p X 1 T 1 RS CX gm CX C EE105 Spring 2008 EE105 Fall 2007 Lecture 14 Slide 2 Prof Wu UC Berkeley 1 3 13 2008 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 Spring 2008 Circuit for small signal analysis Av ro Lecture 14 Slide 3 Prof Wu 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 Spring 2008 EE105 Fall 2007 Lecture 14 Slide 4 gm r g m Prof Wu UC Berkeley 2 3 13 2008 Follower Zero and Pole Frequencies vout vin C j gm a j 2 b j 1 1 a RS C C C C L C C L gm b RS C C RS 1 g m r CL g m The follower has one zero z gm 2 fT C The follower has two poles at lower frequencies j j 1 a j 2 b j 1 1 p1 p2 EE105 Spring 2008 Lecture 14 Slide 5 Prof Wu 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 X 1 Av C C 1 g m RL C 1 CY 1 C g m RL Av Rin r 1 RL EE105 Spring 2008 EE105 Fall 2007 Cin C Lecture 14 Slide 6 C 1 g m RL Prof Wu UC Berkeley 3 3 13 2008 Emitter Follower Output Impedance ro Circuit for small signal analysis Rout 1 v i X g m v r j C Z out vx v ix g m v RS v X RS r C j r RS r RS 1 iX r C j 1 EE105 Spring 2008 j r R r RS RS r C 1 S for low j R for high 1 S 1 r C 1 Lecture 14 Slide 7 Prof Wu UC Berkeley Emitter Follower as Active Inductor Z out v X RS r C j r RS r RS iX r C j 1 1 CASE 1 RS 1 gm 1 R S gm 1 capacitive behavior j r RS RS r C j 1 1 r C 1 CASE 2 RS 1 gm 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 Spring 2008 EE105 Fall 2007 Lecture 14 Slide 8 Prof Wu UC Berkeley 4 3 13 2008 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 g m2 C X 2C XY EE105 Spring 2008 Lecture 14 Slide 9 Prof Wu UC Berkeley Cascode Stage Pole Frequencies Cascode stage with BJT capacitances shown Miller approximation applied p X ro p Y 1 RS r 1 C 1 2C 1 1 1 CCS1 C 2 2C 1 g m2 Note that p out EE105 Spring 2008 EE105 Fall 2007 Lecture 14 Slide 10 p Y gm2 2 fT 2 C 2 1 RL CCS 2 C 2 Prof Wu UC Berkeley 5 3 13 2008 Cascode Stage I O Impedances ro Z in r 1 EE105 Spring 2008 1 j C 1 2C 1 Z out RL Lecture 14 Slide 11 1 j C 2 CCS 2 Prof Wu UC Berkeley Summary of Cascode Stage Benefits A cascode stage has high output impedance which is advantageous for achieving hi i hi high h voltage l gain i use as a current source In a cascode stage the Miller effect is reduced for improved performance at high frequencies EE105 Spring 2008 EE105 Fall 2007 Lecture 14 Slide 12 Prof Wu UC Berkeley 6
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