MIT6_012S09_lec12

MIT6_012S09_lec12 - Lecture 12 Digital Circuits (II) MOS...

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Lecture 12 Digital Circuits (II) MOS INVERTER CIRCUITS Outline NMOS inverter with resistor pull-up –The inverter • NMOS inverter with current-source pull-up • Complementary MOS (CMOS) inverter • Static analysis of CMOS inverter Reading Assignment: Howe and Sodini; Chapter 5, Section 5.4 6.012 Spring 2009 Lecture 12 1
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1. NMOS inverter with resistor pull-up: Dynamics C L pull-down limited by current through transistor – [shall study this issue in detail with CMOS] C L pull-up limited by resistor (t PLH RC L ) Pull-up slowest V IN : LO HI V IN : HI LO V OUT : HI LO V OUT : LO HI V DD C L R V DD C L R pull-down pull-up 6.012 Spring 2009 Lecture 12 2
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1. NMOS inverter with resistor pull-up: Inverter design issues Noise margins ↑ ⇒ |A v | ↑⇒ R ↑ ⇒ |RC L | ↑⇒ slow switching g m ↑ ⇒ |W| ↑⇒ big transistor (slow switching at input) During pull-up we need: High current for fast switching But also high incremental resistance for high noise margin. 6.012 Spring 2009 Lecture 12 3 Trade-off between speed and noise margin.
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2. NMOS inverter with current-source pull-up High current throughout voltage range v SUP > 0 i SUP = 0 for v SUP ≤ 0 i SUP = I SUP + v SUP / r oc for v SUP > 0 High small-signal resistance r oc. Equivalent circuit models : I—V characteristics of current source: i SUP I SUP v SUP 1 r oc v SUP i SUP + _ I SUP r oc i SUP v SUP + _ r oc large-signal model small-signal model 6.012 Spring 2009 Lecture 12 4
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NMOS inverter with current-source pull-up Static Characteristics Inverter characteristics : High r oc high noise margins V IN V OUT V DD C L i SUP i D v OUT = v DS V IN = V GS V DD I SUP + V DD r oc V OUT V IN 1 2 3 4 1 2 3 4 (a) (b) 6.012 Spring 2009 Lecture 12 5
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MIT6_012S09_lec12 - Lecture 12 Digital Circuits (II) MOS...

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