EE 330 Lect 25 Fall 2011

EE 330 Lect 25 Fall 2011 - EE 330 Lecture 25 Small Signal...

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EE 330 Lecture 25 Small Signal Modeling
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Small signal analysis example V DD R M 1 V IN V OUT V SS V IN =V M sinωt     2 OX OX OUT DD SS T SS T M μC W μC W V V V V R V V R V sin t 2L L Quiescent Output ss Voltage Gain   OX v SS T μC W A V V R L  Assume M 1 operating in saturation region   2 OX OUTQ DD SS T μC W V V V V R 2L    OUT OUTQ V M V V A V sin t Note the ss voltage gain is negative since V SS +V T <0! Review from Last Lecture
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Small signal analysis using nonlinear models R 1 Q 1 V OUT V CC V EE V IN (t) EE t -V V CQ S E I J A e V IN =V M sinωt V M is small   EE EE tt -V -V VV M OUT CC S E 1 S E 1 t V sin t V V J A e R J A e R V      1 OUT CC 1 M I V V R V sin t t R I V       Quiescent Output ss Voltage Gain Review from Last Lecture
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Comparison of Gains for MOSFET and BJT Circuits R Q 1 V IN (t) V OUT V CC V EE BJT MOSFET R 1 V IN (t) V OUT V DD V SS M 1   DQ VM SS T 2I R A VV CQ 1 VB t IR A V  If I DQ R =I CQ R 1 =2V, V SS +V T = -1V, V t =25mV 0 CQ 1 VB t 2V A - =-8 V 25mV   DQ VM SS T 2I R 4V A = - 4 V V -1V  Observe A VB >>A VM Due to exponential-law rather than square-law model Review from Last Lecture
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Operation with Small-Signal Inputs Analysis procedure for these simple circuits was very tedious This approach will be unmanageable for even modestly more complicated circuits Faster analysis method is needed ! Review from Last Lecture
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Small-Signal Analysis I OUTSS V OUTSS V INSS or I INSS Biasing (voltage or current) Nonlinear Circuit I OUTSS V OUTSS V INSS or I INSS Linear Small Signal Circuit Will commit next several lectures to developing this approach Analysis will be MUCH simpler, faster, and provide significantly more insight Applicable to many fields of engineering
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Simple dc Model Small Signal Frequency Dependent Small Signal Better Analytical dc Model Sophisticated Model for Computer Simulations Simpler dc Model Square-Law Model Square-Law Model (with extensions for λ,γ effects) Short-Channel α-law Model BSIM Model Switch-Level Models • Ideal switches • R SW and C GS Small-Signal Analysis
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Operation with Small-Signal Inputs Why was this analysis so tedious? What was the key technique in the analysis that was used to obtain a simple expression for the output (and that related linearly to the input)? Because of the nonlinearity in the device models    M EE tt V sin t -V VV OUT CC S E 1 V V J A R e e   1 CQ OUT CC 1 M I V V R V sin t t R I V         Linearization of the nonlinear output expression at the operating point
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Operation with Small-Signal Inputs   1 CQ OUT CC 1 M I V V R V sin t t R I V         Quiescent Output ss Voltage Gain EE t -V V S E I J A e Small-signal analysis strategy 1. Obtain Quiescent Output (Q-point)
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EE 330 Lect 25 Fall 2011 - EE 330 Lecture 25 Small Signal...

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