EECS 105 Spring 2002 Lecture 31 R T Howe Lecture 31 Last time Short circuit current gain of CE and CS amps Unity gain frequency T Today Frequency response of the CE as voltage amp The Miller approximation Dept of EECS University of California at Berkeley EECS 105 Spring 2002 Lecture 31 R T Howe Common Emitter Voltage Amplifier Small signal model omit Ccs due to avoid complicated analysis Dept of EECS University of California at Berkeley EECS 105 Spring 2002 Lecture 31 R T Howe CE Voltage Amp Small Signal Model Dept of EECS University of California at Berkeley EECS 105 Spring 2002 Lecture 31 R T Howe Frequency Response KCL at input and output nodes analysis is made complicated due to Z branch see H S pp 639 640 Vout Vin r ro roc RL 1 j z g m r RS 1 j p1 1 j p 2 Low frequency gain gm Zero z T C C Dept of EECS University of California at Berkeley EECS 105 Spring 2002 Lecture 31 R T Howe Poles 1 p1 C Rout C RS r C 1 g m Rout p2 Dept of EECS RS r Rout C Rout C RS r C 1 g m Rout University of California at Berkeley EECS 105 Spring 2002 Lecture 31 R T Howe Decoupling Input and Output the Miller Approximation Results of complete analysis not exact and little insight Look at how Z affects the transfer function find Zin Dept of EECS University of California at Berkeley EECS 105 Spring 2002 Lecture 31 R T Howe Input Impedance Zin j I t Vt Vout Z At output node g mVt Rout Vout g mVt I t Rout Why I t Vt AvC Vt Z Z in Vt I t Dept of EECS Z 1 AvC University of California at Berkeley EECS 105 Spring 2002 Lecture 31 R T Howe Miller Capacitance CM Effective input capacitance 1 1 Z in j CM 1 AvC 1 j C 1 j 1 A C vC Cx Vin AvCx Vout What about the role of Cx when viewed from the output port Dept of EECS University of California at Berkeley EECS 105 Spring 2002 Lecture 31 R T Howe Some Examples Common emitter source amplifier AvC Negative large number 100 Common collector drain amplifier AvC Dept of EECS Slightly less than 1 University of California at Berkeley EECS 105 Spring 2002 Lecture 31 R T Howe CE Amplifier using Miller Approx Use Miller to transform C Analysis is straightforward now single pole Dept of EECS University of California at Berkeley EECS 105 Spring 2002 Lecture 31 R T Howe Comparison with Exact Analysis Miller result 1 p1 Exact result C Rout C p1 RS r C 1 g m Rout 1 Dept of EECS University of California at Berkeley
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