Lecture 11-Offset Voltage & Current Mirror

Lecture 11-Offset Voltage & Current Mirror -...

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EE 214 Lecture 11 (HO#16) B. Murmann 1 Lecture 11 Offset Voltage Current Mirrors Boris Murmann Stanford University murmann@stanford.edu Copyright © 2004 by Boris Murmann
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EE 214 Lecture 11 (HO#16) B. Murmann 2 Overview Reading – 3.5.6.6, 3.5.6.7 (Input Offset Voltage) – 4.1, 4.2 (Current Mirrors) References – M. Pelgrom et al., "Matching properties of MOS transistors," IEEE J. Solid- State Circuits, Oct. 1989. – P.G. Drennan et al., "Understanding MOSFET mismatch for analog design," IEEE J. Solid-State Circuits, March 2003. Introduction – So far, we have worked with perfectly balanced differential pairs. In practice, there is always some imbalance due to matching errors between circuit elements. In this lecture, we will consider the input referred offset of a differential pair as an example. Following this discussion, we will take a closer look at practical implementations of current mirrors with emphasis on high swing biasing techniques.
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EE 214 Lecture 11 (HO#16) B. Murmann 3 Offset Voltage Defined as the input voltage one needs to apply to make the differential output of an amplifier equal to zero There is no offset in a perfectly balanced differential amplifier Two fundamentally different sources of offset – Systematic • Due to an "imbalance by design" • See examples later in this class – Random • Due to mismatch between nominally identical devices
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EE 214 Lecture 11 (HO#16) B. Murmann 4 Resistor Mismatch Want large g m /I D to mitigate this error (low V OV ) I TAIL R R+ R 0V V DD V os + = R R R I R I d d 1 2 1 D d D d D d d d I I I I I I I I R R + + = = + 1 2 1 1 2 1 1 1 2 1 R R V I g I I os D m D d R R I g V D m os 1
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EE 214 Lecture 11 (HO#16) B. Murmann 5 Current Factor Mismatch L W C ox µ β = ( ) 2 2 2 1 ov ov V V + = + = 1 2 2 2 1 ov ov V V + + 2 1 1 1 2 1 OV os ov ov V V V V 1 D m os I g V I TAIL R R+ R 0V V DD V os ∆β
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EE 214 Lecture 11 (HO#16) B. Murmann 6 Threshold Mismatch I TAIL V t1 R R 0V V DD V os V t2 t t t os V V V V = = 2 1
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Lecture 11-Offset Voltage & Current Mirror -...

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