Large%20Signal%20Modeling%20of%20Inversion-Mode%20MOS%20Varactors%20in%20VCOs

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Large Signal Modeling of Inversion-Mode MOS
Varactors in VCOs
MOS-AK Meeting 2009
2-3 April 2009 at IHP in Frankfurt (Oder)
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Overview

• Motivation
• Large Signal Modeling of Varactors in VCOs
• Alternative Modeling Concept
• Simulation Results
• Conclusion

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Motivation
Tail-biased differential LCTank VCO
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DSB MOS Varactor
Structure and CV-characteristic
R. L. Bunch and S. Raman, Large-Signal Analysis
of MOS Varaktors in CMOS Gm LC VCOs

• Source-Drain-Bulk are short-circuited and
  connected to Vtune

Disadvantages
Advantages

• Made from standard MOS-cell
• Falling and rising edge of the CV-
  characteristic can be used

• Strongly nonlinear tuning characteristic

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Accumulation Mode MOS Varactor
Structure and CV-characteristic
R. L. Bunch and S. Raman, Large-Signal Analysis
of MOS Varaktors in CMOS Gm LC VCOs

• the p regions of drain and source are
  replaced with n regions

Disadvantages
Advantages

• Wider transition from Cmin to Cmax as
  inversion mode varactors
• Best Cmax / Cmin ratio
• Lowest parasitic resistance

• Not made from standard MOS-cell
• Nonlinear tuning characteristic

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Inversion Mode MOS Varactor
Structure and CV-characteristic
R. L. Bunch and S. Raman, Large-Signal Analysis
of MOS Varaktors in CMOS Gm LC VCOs

• Source-Drain are short-circuited and Bulk is
  connected to supply voltage (PMOS) or ground
  (NMOS)

Disadvantages
Advantages

• Very sharp transition from Cmin to Cmax
• Susceptible to induced substrat noise

• Made from standard MOS-cell
• Best linearity

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Overview

• Motivation
• Large Signal Modeling of Varactors in VCOs
• Alternative Modeling Concept
• Simulation Results
• Conclusion

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Varactors incorporated into VCOs
Vtune1 V
VDD2,5 V
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Large Signal Varactor Modeling after R. L. Bunch
R. L. Bunch and S. Raman, Large-Signal Analysis
of MOS Varaktors in CMOS Gm LC VCOs
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Large Signal Varactor Modeling after A. Abidi I
Oscillating capacitance as Fourier series
Kirchhoff and tank voltage as Fourier series
Complete inductor and capacitor current
Comparing coefficients at every frequency gives
E. Hegazi and A. A. Abidi, Varactor
Characteristics, Oscillator Tuning Curves, and
AM-FM Conversion
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Large Signal Varactor Modeling after A. Abidi II
Graphical ansatz to calculate Ceff
Small signal capacitance approximated with a step
function
E. Hegazi and A. A. Abidi, Varactor
Characteristics, Oscillator Tuning Curves, and
AM-FM Conversion

• Expression for C(v(t)) is needed
• Includes only 1st and 2nd harmonic of the
  nonlinear varactor characteristic
• Includes only the fundamental of the voltage,
  higher harmonics are neglected
• Amplitude of the output signal of the VCO is
  needed

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Overview

• Motivation
• Large Signal Modeling of Varactors in VCOs
• Alternative Modeling Concept
• Simulation Results
• Conclusion

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Differential Equation System for a VCO
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Intrinsic Capacitance Model based on EKV
Cgs / Cgd
Interpolated intrinsic capacitances
Cgb
NMOS transistorWidth 100 µmVtune 0V
Normalized Capacitances
Cbs / Cbd
Gate Voltage
Interpolation function
With
C. Enz, F. Krummenacher and E. Vittoz, An
Analytical MOS Transistor Model Valid in All
Regions of Operation and Dedicated to Low-Voltage
and Low-Current Applications, Analog Integrated
Circuits and Signal Processing, Kluwer, 1995
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Voltage dependant Varactor Capacitance
Capacitance
NMOS
Gate Voltage
Capacitance
PMOS
Gate Voltage
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Simulation Results with IHP SGB25 Technology
Capacitance
NMOS transistorWidth 100 µmVtune 0V
Gate Voltage
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Effective Large Signal Capacitance
Assuming complete symmetry between the two
MOS-varactors
Complete varactor capacitance is a series
connection of two MOSFETs
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Overview

• Motivation
• Large Signal Modeling of Varactors in VCOs
• Alternative Modeling Concept
• Simulation Results
• Conclusion

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Effective Large Signal Capacitance
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Effective Large Signal Capacitance
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Effective Large Signal Capacitance
Vtune 0.2 V
Vtune 0.6 V
Vtune 0.4 V
Vtune 0.8 V
Vtune 2.0 V
Vtune 1.6 V
Vtune 1.0 V
Capacitance
Capacitance
Vtune 1.8 V
Vtune 1.4 V
Tank Amplitude
Tank Amplitude
NMOS transistorWidth 250 µm
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Dimensioning Varactors in the VCO Design Process
Tank amplitude V
Time ns
Design of a 2.4 GHz LC Tank VCO with 20 percent
tuning range
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Overview

• Motivation
• Large Signal Modeling of Varactors in VCOs
• Alternative Modeling Concept
• Simulation Results
• Conclusion

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Conclusion

• An implementation of an analytical small signal
  capacitance model for inversion mode MOS
  varactors based on the EKV model was presented
• Simulation results for the small signal
  capacitance are in good accordance to simulation
  results that were obtained by using Spectre
  simulator
• If the varactors are incorporated into a VCO a
  large signal analysis of the varactor capacitance
  is needed
• Two well-established large signal varactor
  capacitance modeling concepts have been presented
  and analyzed
• An alternative capacitance model in dependency of
  the output signal of the VCO including higher
  harmonics was presented
• Using this nonlinear modeling approach it is
  possible to set up a complete nonlinear VCO model
  that is only dependant of circuit and process
  parameters
• Goal Parameter optimization in advance of the
  actual design flow

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The End

• Thank you for your attention!