Lecture 02-Common Source Amp & Small Signal Model

# Lecture 02-Common Source Amp &amp; Small Signal Model -...

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EE 214 Lecture 2 (HO#3) B. Murmann 1 Lecture 2 Common Source Amplifier Small-Signal Model Boris Murmann Stanford University murmann@stanford.edu Copyright © 2004 by Boris Murmann

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EE 214 Lecture 2 (HO#3) B. Murmann 2 Overview Reading – 3.0 (Amplifier basics), 3.1 (Model selection) – 3.2.2 (Common source amplifer) – 1.6.0 . .. 1.6.5 (Small signal MOS model) Introduction – In this lecture, we will use the simple long channel MOS model to construct our first amplifier - a common source stage. Looking at its transfer function, we'll find that treating signals as "small" with respect to the bias conditions allows us to linearize the circuit. Next, we generalize this approach and develop a more universal"small signal model" for MOS devices that are biased in the forward active region.
EE 214 Lecture 2 (HO#3) B. Murmann 3 First-Order MOS Model Summary () 2 2 1 t GS ox D V V L W C I µ S u b- T h r e s o l d ( m a t . ) V t V GS V DS V GS -V t FORWARD ACTIVE TRIODE DS DS t GS ox D V V V V L W C I 2 "VCCS"

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EE 214 Lecture 2 (HO#3) B. Murmann 4 One Way to Amplify Convert input voltage to current using voltage controlled current source (VCCS) Convert back to voltage using a resistor (R) "Voltage gain" = V out / V in – Product of the V-I and I-V conversion factors VCCS R V in V out I
EE 214 Lecture 2 (HO#3) B. Murmann 5 Common Source Amplifier MOS device acts as VCCS I d V i V o V DD () 2 2 1 t i ox D V V L W C I = µ R V V L W C V V t i ox DD o = 2 2 1

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EE 214 Lecture 2 (HO#3) B. Murmann 6 Biasing Need some sort of "battery" that brings input voltage into useful operating region Define V OV =V I -V t , "quiescent point gate overdrive" –V OV =V GS -V t with no input signal applied I D + I d V O + V o V DD V i V I "Bias" "Signal" V I V o V i V O V OV
EE 214 Lecture 2 (HO#3) B. Murmann 7 Relationship Between Incremental Voltages What is V o as a function of V i ? () [] + = + = + = + = + OV i i OV D i i OV ox OV i OV ox o i OV ox DD o O V V V R V I V V V R L W C V V V R L W C V R V V L W C V V V 2 1 2 2 2 1 2 1 2 1 2 2 2 2 µ As expected, this is a nonlinear relationship Nobody likes nonlinear equations, we need a simpler model – Fortunately, a linear approximation to the above expression

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## Lecture 02-Common Source Amp &amp; Small Signal Model -...

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