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EE 435 Lect 19 Spring 2010

# EE 435 Lect 19 Spring 2010 - EE 435 Lecture 19 Gain...

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EE 435 Lecture 19 Gain Enhancement with Positive Feedback Linearity in Operational Amplifiers -- The differential pairs

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g meq Enhancement with Driven Counterpart Circuit 3 1 3 1 0 o o m m V g g g g A + + = L m m C g g GB 3 1 + = Both gain and GB enhancement .• Review from last lecture .• • •
Other Methods of Gain Enhancement OCC OQC QC M V0 g g g A + = Two Strategies: 1. Decrease denominator of A V0 2. Increase numerator of A V0 Consider again decreasing the denominator Recall: OX OCC OQC QC M V0 g g g g A + = Is it possible to come up with circuits that will provide a subtraction of conductance in the denominator ? .• Review from last lecture .• • •

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But - can we easily build circuits with this property? But – the inverting amplifier may be more difficult to build than the op amp itself! Gain Enhancement with Regenerative Feedback YES – simply by cross-coupling the outputs in a fully differential structure .• Review from last lecture .• • •
Gain Enhancement with Regenerative Feedback ( ) s mF1 V0 L oF1 oP1 mP1 g A sC g g g = + + If g mP1 = g oF1 +g oP1 , the dc gain will become infinite !! V0 A mP1 g oP1 oF1 g g + 0 mF1 oF1 oP1 g g g + .• Review from last lecture .• • •

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Gain Enhancement with Regenerative Feedback L mP1 oP1 oF1 C g g g - p + = ( ) s mF1 V0 L oF1 oP1 mP1 g A sC g g g = + + If g mP1 > g oF1 +g oP1 , the pole will be in the RHP !! This will make the op amp unstable This is the major reason most have avoided using the structure ! Positive Feedback is BAD !! .• Review from last lecture .• • •