EE 435 Lect 20 Spring 2010

EE 435 Lect 20 Spring 2010 - EE 435 Lecture 20 Linearity in...

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EE 435 Lecture 20 Linearity in Operational Amplifiers -- The differential pairs -- Programmable gain open-loop transconductors
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Alternate Positive Feedback Amplifier C L Q2 Q7 Q1 Q3 Q4 Vi- Vi+ Vout V DD V b1 Q6 Q5 V b2 V b2 .• • • Review from last lecture .•
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Alternate Positive Feedback Amplifier ( ) 4 o6 o4 o2 m1 VO g g g g 1/2 A m g + + = ( ) ] g g [g sC g 1/2 A(s) 4 o6 o4 o2 L m1 m g + + + = •Requires precise matching of g m4 to (g o2 +g o4 +g o6 ) for good gain enhancement •Difficult to match g m terms to g o -type terms .• • • Review from last lecture .•
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Practical Comments about Positive Feedback Gain Enhancement • Significant gain enhancement is possible but most avoid regenerative feedback because of unfounded concerns about closed-loop stability • Accuracy and settling time can be improved with some regenerative feedback • Will become more critical in emerging processes where g m /g o ratios degrade and where supply voltages shrink thus limiting the longstanding cascode process • Regenerative structures can have high sensitivities • Signal swing quite limited in some of the most basic regenerative feedback structures • Most useful in two-stage architecture where regenerative feedback is used in first stage (effects of signal swing are reduced by gain of second stage) .• • • Review from last lecture .•
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Linearity of Amplifiers Single-Stage Two-Stage Linearity of differential pair of major concern Linearity of common-source amplifier is of major concern (since signals so small at output of differential pair) .• • • Review from last lecture .•
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Differential Input Pairs MOS Differential Pair Bipolar Differential Pair .• • • Review from last lecture .•
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MOS Differential Pair 1 2 d V V V = () 2 T S 1 OX D1 V V V 2L W μ C I = 2 T S 2 OX D2 V V V 2L W μ C I = T D2 D1 I I I = + D1 D1 T OX d I I I W μ C 2L V = D2 T D2 OX d I I I W μ C 2L V - = T S 1 OX D1 V V V W μ C 2L I = T S 2 OX D2 V V V W μ C 2L I = .• • • Review from last lecture .•
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MOS Differential Pair () D1 D1 T OX d I I I W μ C 2L V = D2 T D2 OX d I I I W μ C 2L V - = T OX dx I W μ C 2L V ± = What values of V d will cause all of the current to be steered to the left or the right ? .• • • Review from last lecture .•
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2 I T W μ C 2L I OX T W μ C I T () D2 T D2 OX d I I I W μ C 2L V - = .• • • Review from last lecture .•
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() 2 EB OX T V 2L W μ C 2 I = Q-point Calculations W μ C L I V OX T EB = EB dx V 2 V ± = Observe !! .• • • Review from last lecture .•
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() D2 T D2 OX d I I I W μ C 2L V - = 2 I T EB V 2 V 2 V EB affects linearity How linear is the amplifier ? .• • • Review from last lecture .•
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How linear is the amplifier ?
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EE 435 Lect 20 Spring 2010 - EE 435 Lecture 20 Linearity in...

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