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lec5 - 6.976 High Speed Communication Circuits and Systems...

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6.976 High Speed Communication Circuits and Systems Lecture 5 High Speed, Broadband Amplifiers Michael Perrott Massachusetts Institute of Technology Copyright © 2003 by Michael H. Perrott
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M.H. Perrott MIT OCW Broadband Communication System Example: high speed data link on a PC board - We’ve now studied how to analyze the transmission line effects and package parasitics - What’s next? V L C 1 R L L 1 Delay = x Characteristic Impedance = Z o Transmission Line Z 1 V in C 2 die Adjoining pins Connector Controlled Impedance PCB trace package On-Chip Driving Source
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M.H. Perrott MIT OCW High Speed, Broadband Amplifiers The first thing that you typically do to the input signal is amplify it Function - Boosts signal levels to acceptable values - Provides reverse isolation Key performance parameters - Gain, bandwidth, noise, linearity V L C 1 R L L 1 Delay = x Characteristic Impedance = Z o Transmission Line Z 1 V in C 2 die Adjoining pins Connector Controlled Impedance PCB trace package On-Chip Driving Source Amp V out
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M.H. Perrott MIT OCW Basics of MOS Large Signal Behavior (Qualitative) S D G C channel = C ox (V GS -V T ) V GS V DS =0 S D G V GS V D = V S D G V GS V D > V Triode Pinch-off Saturation V DS I D I D I D I D Triode Pinch-off Saturation V Overall I-V Characteristic
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M.H. Perrott MIT OCW Basics of MOS Large Signal Behavior (Quantitative) S D G C channel = C ox (V GS -V T ) V GS V DS =0 S D G V GS V D = V S D G V GS V D > V Triode Pinch-off Saturation I D I D I D I D = µ n C ox W L (V GS - V T - V DS /2)V DS I D µ n C ox W L (V GS - V T )V DS for V DS << V GS - V T I D = µ n C ox W L 1 2 (V GS -V T ) 2 (1+ λ V DS ) (where λ corresponds to channel length modulation) V = V GS -V T V = µ n C ox W 2I D L
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M.H. Perrott MIT OCW Analysis of Amplifier Behavior Typically focus on small signal behavior - Work with a linearized model such as hybrid- π - Thevenin modeling techniques allow fast and efficient analysis To do small signal analysis: R S R G R D v in v out V bias I D 1) Solve for bias current I d 2) Calculate small signal parameters (such as g m, r o ) 3) Solve for small signal response using transistor hybrid- π small signal model Small Signal Analysis Steps
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M.H. Perrott MIT OCW MOS DC Small Signal Model Assume transistor in saturation: Thevenin modeling based on the above R S R G R D R D R S R G -g mb v s v gs v s r o g m v gs g m = µ n C ox (W/L)(V GS - V T )(1 + λ V DS ) = 2 µ n C ox (W/L)I D (assuming λ V DS << 1) C ox 2q ε s N A 2 2| Φ p | + V SB γ g m where γ = g mb = In practice: g mb = g m /5 to g m /3 λ I D 1 r o = I D
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M.H. Perrott MIT OCW Capacitors For MOS Device In Saturation S D G V GS V D > V I D L D L D overlap cap: C ov = WL D C ox + WC fringe B C gc C cb C ov C jdb C jsb C ov Side View gate to channel cap: C gc = C ox W(L-2L D ) channel to bulk cap: C cb - ignore in this class S D Top View W E L E E source to bulk cap: C jsb = 1 + V SB Φ B C j (0) 1 + V SB Φ B C jsw (0) WE + (W + 2E) junction bottom wall cap (per area) junction sidewall cap (per length) drain to bulk cap: C jsd = 1 + V DB Φ B C j (0) 1 + V DB Φ B C jsw (0) WE + (W + 2E) 2 3 (make 2W for "4 sided" perimeter in some cases) L
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M.H. Perrott
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