MIT6_012S09_lec22

MIT6_012S09_lec22 - Lecture 22 Frequency Response of...

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Unformatted text preview: Lecture 22 Frequency Response of Amplifiers (II) VOLTAGE AMPLIFIERS Outline 1. Full Analysis 2. Miller Approximation 3. Open Circuit Time Constant 6.012 Spring 2009 Lecture 22 1 Reading Assignment: Howe and Sodini, Chapter 10, Sections 10.1-10.4 Common Emitter Amplifier + V BIAS + V + R S R L i SUP i OUT V s v OUT + Operating Point Analysis v s =0, R S = 0, r o , r oc , R L Find V BIAS such that I C =I SUP with the BJT in the forward active region V BIAS 6.012 Spring 2009 Lecture 22 2 Frequency Response Analysis of the Common Emitter Amplifier V s r R S C C V + V out R L + g m V r o r oc + Frequency Response Set V BIAS = 0. Substitute BJT small signal model (with capacitors) including R S , R L , r o , r oc Perform impedance analysis 6.012 Spring 2009 Lecture 22 3 1. Full Analysis of CE Voltage Amplifier Replace voltage source and resistance with current source and resistance using Norton Equivalent V s r R S C C V + V out R L + g m V r o r oc + C + 6.012 Spring 2009 Lecture 22 4 Node 1: Node 2: ( ) out in s V V C j V C j R V I + + = ( ) out out out m V V C j R V V g = + I s R' in R' in = R S r R' out = r o r oc R L R' out C V + V out g m V Full Frequency Response Analysis (contd.) Re-arrange 2 and obtain an expression for V Substituting it into 1 and with some manipulation, we can obtain an expression for V out / I s : V R g j C ) R in out ( m out = I 1 + j R R ) 2 R s ( out C + R in C + R in C + g m out R in C out R in C C Changing input current source back to a voltage source: g R r 1 C V out V s = g m R out R S + r 1 j g m 1 + j R out C + R in C 1 + g m R out ( ) + R in C ( ) 2 R out R in C C V out V s = A vo 1 + j 1 ( ) 1 + j 2 ( ) = A vo 1 j 1 + 2 ( ) 2 1 2 The gain can be expressed as: where A vo is the gain at low frequency and 1 and...
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MIT6_012S09_lec22 - Lecture 22 Frequency Response of...

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