Copyright 2001, Regents of University of CaliforniaEECS 42 Intro. electronics for CS Fall 2001 Lecture 6: 9/17/01 A.R. NeureutherVersion Date 9/17/01Lecture 6: September 17th, 2001Charging and Discharging of RC Circuits(Transients)A) Digital pulse environmentB) RC responseC) Review of simple exponentialsReading: Schwarz and Oldham 8.1+ HandoutThe following slides were derived from those prepared by Professor Oldham For EE 40 in Fall 01Copyright 2001, Regents of University of CaliforniaEECS 42 Intro. electronics for CS Fall 2001 Lecture 6: 9/17/01 A.R. NeureutherVersion Date 9/17/01Review of charging and discharging in RC Circuits(an enlightened approach)• Before we continue with formal circuit analysis - lets review RC circuits• Rationale: Every node in a circuit has capacitance to ground, like it or not, and it’s the charging of these capacitances that limits real circuit performance (speed)Relevance to digital circuits: We communicate with pulses We send beautiful pulses outBut we receive lousy-looking pulses and must restore themtimevoltageRC charging effects are responsible …. So lets review them.timevoltageCopyright 2001, Regents of University of CaliforniaEECS 42 Intro. electronics for CS Fall 2001 Lecture 6: 9/17/01 A.R. NeureutherVersion Date 9/17/01Simplification for time behavior of RC Circuits• Before any input change occurs we have a dc circuit problem (that is we can use dc circuit analysis to relate the output to the input).• Long after the input change occurs things “settle down” …. Nothing is changing …. So again we have a dc circuit problem.We call the time period during which the output changes the transienttimevoltagetimevoltageWe can predict a lot about the transient behavior from the pre- and post-transient dc solutionsCopyright 2001, Regents of University of CaliforniaEECS 42 Intro. electronics for CS Fall 2001 Lecture 6: 9/17/01 A.R. NeureutherVersion Date 9/17/01What environment do pulses face?• Every wire in a circuit has resistance.• Every junction (called nodes) has capacitance to ground and other nodes.• The active circuit elements (transistors) add additional resistance in series with the wires, and additional capacitance in parallel with the node capacitance.A pulse originating at node I will arrive delayed and distorted at node O because it takes time to charge C through RIOInput node Output nodegroundRCVin+-If we focus on the circuit which distorts the pulses produced by Vin, it consists simply of R and C. (Vin is just the time-varying source which produces the input pulse.)Copyright 2001, Regents of University of CaliforniaEECS 42 Intro. electronics for CS Fall 2001 Lecture 6: 9/17/01 A.R. NeureutherVersion Date 9/17/01The RC Circuit to Study(All single-capacitor circuits reduce to this one)• R represents total resistance (wire plus whatever drives the input node)Input node Output nodegroundRC• C represents the total capacitance from node to the outside world (from devices, nearby wires, ground etc)Copyright 2001, Regents of University of CaliforniaEECS 42 Intro. electronics for CS Fall 2001 Lecture 6: 9/17/01 A.R. NeureutherVersion Date 9/17/01SWITCHING PROPERTIES OF L, CCapacitor Inductorv is continous i is continousI can jump V can jumpDo not short circuit acharged capacitor(produces ∞ current)Do not
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