Analog Circuit Design: Current References & Mirror Design Problems
Explore current reference design challenges, mirror design problems, and precision current sources in analog circuit design. Learn about cascode current mirror specifications, component variables, and temperature coefficients for stability. Bipolar transistors in CMOS processes and supply-independent current references are also discussed.
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Presentation Transcript
INEL 5265 Current References 3/18/2025 Last Lecture Current Mirrors Chp. #4 Current Mirror Design Problems Homework #1 & #2 Vdd Vdd M10 M11 Vdd=3.3V M12 Vdd Vb Vdd M13 Iref Vdd Vout Iout Iref Vx Vy Vin M8 M4 M3 Ibias M9 Rbias M1 M7 Ibias M2 M6 M5 Analog Circuit Design
INEL 5265 Current References 3/18/2025 Homework #2 Revisited! Design the cascode current mirror for the following specifications: Iout = 2 Iin=[0-10] A Vout(min) < 0.5V Vin < 0.9V VDD Iin Iout Ibias 1. 2. M2=2 M1 Vout(min)=Vds1(sat)+Vds2(sat) = 0.5V Choose Vds1(sat)=Vds2(sat)=0.25V Find W/L for M4,M2, and M1 Vin(max)=Vgs1=Vton+Vds1(sat)=0.95V does not satisfy Vds1(sat)=0.2V in order to satisfy Find new W/L for M1 and M2 Vgs3=Vgs4+Vds2(sat) Vds3(sat)=Vds4(sat)+Vds2(sat)=0.45V Find W/L for M3 + M3 M4 3. Vout 4. M1 M2 _ 2 Analog Circuit Design
INEL 5265 Current References 3/18/2025 Current References Revisited! An independent current source that has a high degree of precision and stability VDD ? Iout Iref Important Concepts: 1. Sensitivity: All Circuits Have Performance That Varies As The Values Of The Components Change Temperature Time (Aging) Phase Of The Moon Component Tolerance In Manufacturing Etc M1 M2 Ideally a current reference should be independent of: power supply temperature noise process variations 2. Temperature Coefficient: 3 Analog Circuit Design
INEL 5265 Current References 3/18/2025 Simple Current Reference VDD Process and Temperature Dependant TCVton - negative 10% Vto variation ? Iout= Iref R Linear dependence on VDD Q2 Q1 V V DD ton I out R AE1=AE2 Process and Temperature Dependant TCR (depends on material) 30% R variation 4 Analog Circuit Design
INEL 5265 Current References 3/18/2025 An Improved Current Reference VDD Iout= TCIout= Iout Iref Output current is no longer sensitive to Vdd to some degree! R1 M2 V out BE I M1 R 2 R2 TC TC TC I V R 2 out BE Temperature Coefficient of VBE Temperature Coefficient of R2 BJTs in a MOS process? 5 Analog Circuit Design
INEL 5265 Current References 3/18/2025 Bipolar Transistors in CMOS n-well Process vertical parasitic BJT lateral parasitic BJT 6 Analog Circuit Design
INEL 5265 Current References 3/18/2025 An Supply Independent Current Reference Assume: VDD VDD AE2= N AE1 M2= M1 M3= M4 I1=Iref/N TCIout=TCBJT-TCR-TCCMOS + VSG - M3 M4 Vp Iref M2 M1 R1 Q1 Threshold Voltage has a positive dependence on temperature Q2 R(T) = R(T0) [1 + TCRl (T- T0) + TCR2 (T-R2 (T- T)2] vertical bjts 7 Analog Circuit Design
INEL 5265 Current References 3/18/2025 An Supply Independent Current Reference CMOS! Assume: VDD VDD M2= N M1 M3= M4 Iout=Iref=N IM1 TCIout= TCR + TCCMOS M3 M4 Vp Iref M2 M1 Where IDO= current at VGS=VTH R1 1. Saturation 2. Sub-Vth 8 Analog Circuit Design
INEL 5265 Current References 3/18/2025 Current Reference Higher Supply Regulation! Assume: VDD VDD M2= N M1 M3= M4 Iout= TCIout= M3 M4 Vp Iref M2 M1 R1 Sub-Vth 9 Analog Circuit Design
INEL 5265 Current References 3/18/2025 Linear TC Cancellation VDD VDD VDD VDD M3 M6 M5 VDD VDD VDD M4 M10 M8 M9 M2 M1 M7 M11 R1 R2 Assume: M2= N M1 M3= M4 10 Analog Circuit Design
