MIT6_012S09_lec24

MIT6_012S09_lec24 - Lecture 24 Multistage Amplifiers (I)...

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6.012 Spring 2009 Lecture 24 1 Lecture 24 Multistage Amplifiers (I) MULTISTAGE AMPLIFIER Outline 1. Introduction 2. CMOS multi-stage voltage amplifier 3. BiCMOS multistage voltage amplifier 4. BiCMOS current buffer 5. Coupling amplifier stages Reading Assignment: Howe and Sodini, Chapter 9, Sections 9-1-9.3
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6.012 Spring 2009 Lecture 24 2 1. Introduction What amplifying stages should be used and in what order? What devices should be used, BJT or MOSFET? How is biasing to be done? Issues: Most often, single stage amplifier does not accomplish design goals: Need more gain than could be provided by a single stage Need to adapt to specified R S and R L to maximize efficiency Multistage amplifier V BIAS
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6.012 Spring 2009 Lecture 24 3 Summary of single stage amplifier characteristics Stage A vo , A io R in R out Key Function Common Source Transcon- ductance amplifier Common Drain Voltage Buffer Common Gate Current buffer Common Emitter Transcon- ductance amplifier Common Collector buffer Common Base Current buffer ) // ( oc o m vo r r g A = r o // r oc A vo g m g m + g mb 1 g m + g mb A io ≈ − 1 A vo =− g m ( r o // r oc ) mb m g g + 1 r oc //[ r o (1 + g m R S )] π r r o // r oc A vo 1 A io ≈ − 1 ( ) L oc o o R r r r // // β π + 1 g m + R S β o m g 1 r oc //[ r o + g m r π // R S () )] Differences between BJT’s and MOSFETs r = o g m g mb g m g m = I C V th > g m = 2 W L μ C ox I D r o = V A I C > r o = 1 λ I D BJT MOSFET
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6.012 Spring 2009 Lecture 24 4 2. CMOS Multistage Voltage Amplifier Goals: High voltage gain, A vo High input resistance, R in Low output resistance, R out Good starting point: Common-Source stage: R in = A vo =-g m (r o //r oc ), probably insufficient R out = (r o //r oc ), too high
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6.012 Spring 2009 Lecture 24 5 Add second CS stage to get more gain: CMOS Multistage Voltage Amplifier (contd.) R in = A vo =g m1 (r o1 //r oc1 ) g m2 (r o2 //r oc2 ) R out = (r o2 //r oc2 ), still too high Add CD stage at output (to reduce R out ): R in = A vo = g m 1 r o 1 || r oc 1 () g m 2 r o 2 || r oc 2 g m 3 g m 3 + g mb 3 R out = 1 g m 3 + g mb 3
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6.012 Spring 2009
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This note was uploaded on 11/07/2011 for the course COMPUTERSC 6.012 taught by Professor Charlesg.sodini during the Spring '09 term at MIT.

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MIT6_012S09_lec24 - Lecture 24 Multistage Amplifiers (I)...

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