h25_lecture17_2

h25_lecture17_2 - Lecture 17 Compensation Two-Stage OTA...

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EE 214 Lecture 17 (HO#25) B. Murmann 1 Lecture 17 Compensation Two-Stage OTA Design Considerations Boris Murmann Stanford University murmann@stanford.edu Copyright © 2004 by Boris Murmann EE 214 Lecture 17 (HO#25) B. Murmann 2 Overview Reading – 9.4.1, 9.4.2, 9.4.3 (Compensation) Introduction – In this lecture, we will continue to look at the pole splitting technique as a method for achieving sufficient phase margin in a two-stage amplifier. While using a Miller capacitance for compensation helps move the non-dominant pole to higher frequency, it also introduces an undesired zero in the transfer function. Today, we'll discuss several options on how to cope with this artifact. – In addition, we will review a possible design strategy for two- stage OTAs. Since there exist many degrees of freedom in this topology, a thorough hand analysis is essential, and can help minimize, if not completely eliminate, the time needed for a large number of Spice iterations.
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EE 214 Lecture 17 (HO#25) B. Murmann 3 Two-Stage OTA with C c Detailed analysis shows: R 1 C 1 R 2 v o v i C 2 C c M 2 M 1 () [] 1 c 2 c 2 1 2 1 2 c 1 2 2 m 1 c 1 2 c 2 2 m c 2 2 m 1 1 m i o C C C C C C R R s C R R g R C C R C C s 1 g C s 1 R g R g v v s a + + + + + + + + = = Very messy… EE 214 Lecture 17 (HO#25) B. Murmann 4 Dominant Pole Approximation We can write the denominator as 2 1 2 2 1 2 1 1 1 1 1 1 p p s p p s p s p s s D + + = = Since in a practical design outcome we'll have |p 1 |<<|p 2 |, we can approximate 2 1 2 1 1 1 p p s p s s D + With this simplification, we can now easily identify p 1 and p 2 by comparing the coefficients with the expression from the previous slide
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EE 214 Lecture 17 (HO#25) B. Murmann 5 Final Result •Q u e s t i o n s – How can we design an amplifier with these complex expressions?
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h25_lecture17_2 - Lecture 17 Compensation Two-Stage OTA...

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