Lect15_VCOs IV

Lect15_VCOs IV - ECE 894 RF and mmWave Front-end Circuit...

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ECE 894 RF and mmWave Front-end Circuit Design Prof. Waleed Khalil Lecture VCOs IV 16 1
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NMOS Differential LC Oscillator (1/2) Large voltage swing to improve VCO phase noise Differential tapped inductor and differential capacitor used to save area Differential inductor has larger Q than two single-ended inductors Condition for oscillation: g m /2 >= 1/R p g m /2 is typically set 2-3x higher than 1/Rp to guarantee oscillation PMOS LC oscillator can also be used but output swing below 0V can impact varactor type 2 L/2 I bias v m v p L/2 C v s I m I p L Center-Tapped Diff Ind. C v DD v DD v max v max -v max -v max L C R p R in =-2/g m I bias v p v s
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NMOS Differential LC Oscillator (2/2) Square wave current pulses are assumed At high frequency, differential pair current is rather sinusoidal 3 I p I bias t t I bias I m T osc v DD 2v max -2v max v p -v m t T osc ( ) ( ) 2 diff fund p n fund p n fund p bias p v vv I I R IR π = = −⋅ = 1 2 diff fund bias p v =
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CMOS Differential LC Oscillator (2/2) Condition for oscillation: (g mn + g mp ) /2 >= 1/R p g m >= 1/R p (Assuming g mn = g mp = g m ) 4 I bias v n v p C v s L L C R p R in =-2/g mn I bias v p v s R in =-2/g mp 4 diff fund bias p v IR π =
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CMOS vs. NMOS LC VCO Power efficient Double the output swing for same bias current ½ gm required for startup Achieves better phase noise for a given bias current Double the output swing per I bias Symmetric rise and fall time reduces transistor contribution to phase noise 5 Maximum output swing Positive swing is not restricted by PMOS headroom
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Lect15_VCOs IV - ECE 894 RF and mmWave Front-end Circuit...

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