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Handout_8_-_MOSFET_Amplifier_-_AC_Analysis

# Handout_8_-_MOSFET_Amplifier_-_AC_Analysis - EE 101A Winter...

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1 EE 101A / Winter 10 Handout #8 EE 101A MOSFET Amplifier – Small Signal Analysis Reading – Sedra & Smith, Chapter 4.6 (skip 4.6.8) 1. Motivation To simplify the analysis of transistor circuits. In previous set of lecture notes, it has been shown that in a common source amplifier, an appropriate DC bias is needed to ensure that the MOSFET operates in the saturation region. 0 2 4 6 8 10 12 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 VIN VO 0 2 4 6 8 10 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 VIN TIME 0 2 4 6 8 10 12 0 2 4 6 8 10 12 14 16 TIME VO

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2 v I = v GS = V GS + v gs i DS = I DS + i ds v O = v DS = V DS + v ds Assume NMOS is in saturation region   2 gs ' n gs t GS ' n 2 t GS ' n 2 t gs GS ' n 2 t GS ' n DS 2 t GS ' n DS v L 2 W K v V V L W K V V L 2 W K V v V L 2 W K V v L 2 W K v 1 V v L 2 W K i If the AC signal v gs is sine wave, the 1 st term is a DC signal, the 2 nd term is a sine wave proportional to the input signal, the 3 rd term is a distortion due to the non-linear behavior of the MOSFET. In practice, it is desirable to reduce the 3 rd term. If v gs << 2(V GS – V t ) , the 3 rd term is much smaller than the 2 nd term. ds DS gs t GS ' n 2 t GS ' n DS i I v V V L W K V V L 2 W K i (1) 2 t GS ' n DS V V L 2 W K I DC component (2) gs t GS ' n ds v V V L W K i AC component (3) (Note that I DS is the drain current when the input voltage is V GS , DC only. See Equation 2 of previous handout.) ds DS gs t GS ' n D 2 t GS ' n D DD gs t GS ' n 2 t GS ' n D DD DS D DD DS v V v V V L W K R V V L 2 W K R V v V V L W K V V L 2 W K R V i R V v (4) 2
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Handout_8_-_MOSFET_Amplifier_-_AC_Analysis - EE 101A Winter...

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