EECS 105 Fall 2004 Lecture 41 Lecture 41 Review Frequency Response FET physics Prof J S Smith Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 41 Prof J S Smith Final Exam z z z z Covers the course from the beginning Date Time SATURDAY MAY 15 2004 8 11A Location BECHTEL auditorium One page Two sides of notes Department of EECS University of California Berkeley 1 EECS 105 Spring 2004 Lecture 41 Prof J S Smith Q A about the final Q Are differential amplifiers going to be on the exam A No there will not be a differential amplifier question Q Do we need to know a lot of device physics for BJTs A No there won t be any BJT physics Q Will there be any BJT circuits questions on the exam A No the exam will not have any BJT transistor problems Q How much of the material from before Midterm 1 will be tested in detail A Material before midterm 1 is fair game Q Also are we responsible for Chapter 5 i e digital circuits A No nothing specifically on digital circuits or from chapter 5 Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 41 Prof J S Smith Last Week of Lecture z Monday z Wednesday z Review of Frequency domain analysis of linear circuits Bode plots Frequency Response Semiconductor materials FET physics and models Friday Review of active linear circuits amplifiers wrapup Department of EECS University of California Berkeley 2 EECS 105 Spring 2004 Lecture 41 Prof J S Smith Frequency response CS z When we take into account a finite source impedance in a common source amplifier the capacitances will reduce the voltage swing at the gate at high frequencies Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 41 Prof J S Smith Parasitic Capacitances The transfer function will be a low pass filter with a pole at the frequency determined by the source resistance and the capacitance rs vs Department of EECS University of California Berkeley 3 EECS 105 Spring 2004 Lecture 41 Prof J S Smith High frequency zero At very high frequencies the gain flattens out again because the capacitor couples from the gate to the drain directly as a passive circuit rs vs Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 41 Prof J S Smith Magnitude Bode Plot pole Low frequency o gain Unity current gain 0 dB zero p Department of EECS T z University of California Berkeley 4 EECS 105 Spring 2004 Lecture 41 Prof J S Smith Miller Capacitance CM Effective input capacitance Z in 1 1 j C M 1 AvC gd 1 j C gd 1 j 1 A C vCgd gd Cx AV Cx Vin AV Cx 1 Av Cx Cx Vout Vout 1 1 Av Cx Cx Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 41 Prof J S Smith Frequency response C gd RS v gs C gs g m vin ro vout RS Department of EECS v gs RL C gs CM g m vin ro voutRL University of California Berkeley 5 EECS 105 Spring 2004 Lecture 41 Prof J S Smith Some Examples Common source emitter amplifier AvC gd Negative large number 100 CM 1 AV C gd C gd 100C gd Miller Multiplied Cap has Detrimental Impact on bandwidth Common drain collector amplifier AvC gs Slightly less than 1 CM 1 AV C gs C gs 0C gs Bootstrapped cap has negligible impact on bandwidth Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 41 Prof J S Smith Open Circuit Time Constants z z z For a circuit dominated by a single pole For each capacitor in the circuit you calculate an equivalent resistor seen by capacitor and form a time constant i RiCi The dominant pole then is the sum of these time constants in the circuit p dom Department of EECS 1 1 2 L University of California Berkeley 6 EECS 105 Spring 2004 Lecture 41 z Equivalent Resistance Seen by Capacitor Prof J S Smith For each small capacitor in the circuit Open circuit all other small capacitors Short circuit all big capacitors Turn off all independent sources Replace cap under question with current or voltage source Find equivalent input impedance seen by cap Form RC time constant Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 41 Prof J S Smith Remember For a given capacitor z If the frequency is high compared to the 1 RC for the capacitor in that location in the circuit z That Capacitor can be modeled as a short If the frequency is low compared to the 1 RC for the capacitor in that location in the circuit That Capacitor can be modeled by an open circuit Department of EECS University of California Berkeley 7 EECS 105 Spring 2004 Lecture 41 Prof J S Smith Common Drain Amplifier I DS Cox VGS VT 2 I DS W Cox L W 1 VGS VT 2 L 2 Weak IDS dependence Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 41 Prof J S Smith CD Voltage Gain vout gm 1 vin g mb g m Department of EECS University of California Berkeley 8 EECS 105 Spring 2004 Lecture 41 Prof J S Smith CD Output Resistance Sum currents at output source node Rout ro roc vt it Rout it g m vt g mb vt 1 g m g mb Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 41 Prof J S Smith CD Output Resistance Cont ro roc is much larger than the inverses of the transconductances ignore Rout 1 g m g mb Function a voltage buffer High Input Impedance Low Output Impedance Department of EECS University of California Berkeley 9 EECS 105 Spring 2004 Lecture 41 Prof J S Smith Add capacitors Procedure z Start with small signal two port model z Add device and other capacitors C gs vout vin C gd Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 41 Prof J S Smith Common Gate Amplifier DC bias I SUP I BIAS I DS Department of EECS University of California Berkeley 10 EECS 105 Spring 2004 Lecture 41 Prof J S Smith CG Current buffer iout id it Ai 1 Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 41 Prof J S Smith CG Input Resistance vgs vt We found the approximation Department of EECS Rin 1 g m g mb University of California Berkeley 11 EECS 105 Spring 2004 Lecture 41 Prof J S Smith CG Output Resistance Rout roc ro g m ro RS roc ro 1 g m RS Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 41 Prof J S Smith CG Two Port Model rOC r0 g m r0 RS C gs C gd The function of the CG amp was a current buffer Low input impedance High output impedance The only parasitic capacitances are directly across the Input …
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