EE 435 Lect 12 Spring 2010

EE 435 Lect 12 Spring 2010 - EE 435 Lecture 12 Cascaded...

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EE 435 Lecture 12 Cascaded Amplifiers -- Two-Stage Op Amp Design
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Routh-Hurwitz Stability Criteria: A third-order polynomial s 3 +a 2 s 2 +a 1 s+a 0 has all poles in the LHP iff all coefficients are positive and a 1 a 2 >a 0 • Very useful in amplifier and filter design • Can easily determine if poles in LHP without finding poles • But tells little about how far in LHP poles may be • RH exists for higher-order polynomials as well Review from Last Time
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Similar implications on amplifier even if not a basic voltage feedback amplifier 1 2 OUT 1 VF IN 2 V1 R 1+ V R A = = V R 1+ 1+ A R ⎛⎞ ⎜⎟ ⎝⎠ 1 2 OUT 1 VF IN 2 R - V R A = = V R 1+ 1+ A R 1 21 R β = R+R OUT V VF IN 1 V V A A = = V R 1+A RR + 2 V 1 OUT VF IN 1 V -R A R V A = = V R 1+A + These circuits have • same β • same dead network • same characteristic polynomial • same poles • different zeros () D s =1+A β (expressed as polynomial) Review from Last Time
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Cascaded Amplifier Issues Four or more amplifier cascades - problems even larger than for three stages -- seldom used in industry ! -- seldom used in industry ! Two amplifier cascades 0TOT 0TOT A 2 β k A 4 β > > -- widely used in industry but compensation is essential ! Three amplifier cascades - for ideally identical stages 3 0 β A 8 > Single-stage amplifiers -- widely used in industry, little or no concern about compensation Note: Some amplifiers that are termed single-stage amplifiers in many books and papers are actually two-stage amplifiers and some require modest compensation. Some that are termed two- stage amplifiers are actually three-stage amplifiers. These invariable have a very small gain on the first stage and a very large bandwidth. The nomenclature on this summary refers to the number of stages that have reasonably large gain. Review from Last Time
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Fundamental Amplifier Design Issues Single-Stage Low Gain Op Amps Single-Stage High Gain Op Amps Two-Stage Op Amp – Compensation – Breaking the Loop Other Basic Gain Enhancement Approaches Other Issues in Amplifier Design Summary Remarks
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Two-stage op amp design It is essential to know the poles of the op amp since there are some rather strict requirements about the relative location of the poles
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Poles and Zeros of Amplifiers V DD M 1 M 2 V B2 M 3 M 4 V IN V IN M 5 V DD M 6 M 7 V B2 M 8 M 9 C L M 10 V OUT C 1 C 2 C 3 C 4 C 5 C 6 C 7 C 8 • There are a large number of parasitic capacitors in an amplifier (appprox 5 for each transistor) • Many will appear in parallel but the number of equivalent capacitors can still be large • Order of transfer function is equal to the number of non-degenerate energy storage elements • Obtaining the transfer function of a high-order network is a lot of work !
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This note was uploaded on 01/31/2012 for the course EE 345 taught by Professor Geiger during the Fall '11 term at Iowa State.

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EE 435 Lect 12 Spring 2010 - EE 435 Lecture 12 Cascaded...

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