模拟集成电路的分析与设计：Chapter 14-Oscillators.ppt

单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,Oscillation condition,Barkhausen criteria(,巴克豪森准则）,.,In a negative-feedback circuit has a loop gain that satisfies two conditions:,The circuit may oscillate at,0,:,In fact,in order to ensure oscillation in the presence of temperature and process variations,we typically choose the loop gain to be at least twice or three times the required value.,Ring Oscillators(1),Maximum frequency-dependent phase shift is 90,o,so the total shift is 270,o,smaller than 360,o,no oscillation happens.,One-stage system.,Ring Oscillators(2),Maximum frequency-dependent phase shift is 180,o,the total shift is 360,o,it seems that oscillation may happen.,However,it only“latches up”rather than oscillates:if V,E,V,F,M,1,offV,E,until V,E,=V,DD.,Two-stage system.,Ring Oscillators(3),Frequency-dependent phase shift is 270,o,so it can satisfy the requirement of oscillation easily.,.,Three-stage system.,Ring Oscillators(4),For oscillating,each stage contributes 60,o,:,.,If the transfer function of each stage is.,Total transfer function is.,Calculating the minimum voltage gain per stage:,.,When A,0,=2,circuit oscillate at frequency of,Ring Oscillators(5),What if A,0,2?,Use model Fig.14.10 and feedback concept:,Ring Oscillators(6),The denominator of transfunction can be expanded as,The closed-loop system exhibits three poles:,Ring Oscillators(7),For different values of A,0,the locations of the poles:,Ring Oscillators(8),For A,0,2,the two complex poles exhibit a positive real part and hence give rise to a growing sinusoid.Neglecting the effect of s,1,the output waveform can be expressed as,The small-signal oscillation frequency become as,Ring Oscillators(9),Use three-stage CMOS inverters to implement,no resistors.,Initialized at V,DD,oscillate at frequency of 1/(6T,D,),Initialized at the trip point(V,trip,),if no noise,the circuit would remain in this stage indefinitely;However,noise will disturb this balance,yielding oscillating at frequency of ,when amplitude grows and the circuit becomes nonlinear,the oscillation frequency shifts to 1/(6T,D,).,Ring Oscillators(10),Can also use differential implementation.,Ring Oscillators(11),Five-stage single-ended and Four-stage differential ring osillators:,Does not invert,equal to 3-stage,In most applications,three to five stages provide optimum performance,Voltage-Controlled Oscillators(VCO)(1),Use voltage to tune the oscillators,The ideal VCO is a linear control,as following:,where K,VCO,represents the“gain”or“sensitivity”of VCO.,Voltage-Controlled Oscillators(2),Parameters for VCO:,Center Frequency,Tuning Range,Tuning Linearity,Output Amplitude,Power Dissipation,Supply and Common-Mode Rejection,Output Signal Purity,Voltage-Controlled Oscillators(3),M,3,and M,4,work in the,triode region,each acting as a variable resistor,The time constant,can be obtained as:,The delay of the circuit is roughly proportional to,yielding,Linearly proportional to V,cont,Oscillator tuning:,Voltage-Controlled Oscillators(4),Other tuning techniques are available(p514525).,Voltage-Controlled Oscillators(5),Mathematical Model of VCOs,Voltage-Controlled Oscillators(6),Mathematical Model of VCOs,V,2,is faster than V,1,Voltage-Controlled Oscillators(7),Mathematical Model of VCOs,2,1,2,1,If know phase,then frequency,Voltage-Controlled Oscillators(8),Mathematical Model of VCOs,If know frequency,then phase:,In particular,since for a VCO:,Therefore:,Essential in the analysis of VCOs and PLLs,the initial phase,0,is usually uniportant and is assumed zero:,Voltage-Controlled Oscillators(9),When used in PLL,the output of VCOs is the“excess phase”,ex,and the input is V,cont,the relation between them can be expressed as,Act as an ideal integrator,providing a transfer function:,Questions for Chap.14(1),(1)Explain,Barkhausen criteria,?,(2)Explain why one-stage and two-stage system can not oscillate,while three-stage system can oscillate.,(3)Explain briefly the oscillating principle of the three-stage CMOS converters.,(4)Explain briefly the oscillator tuning.,Questions for Chap.14(2),Q14.3;,Q14.15.,