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lec22 - 6.012 Microelectronic Devices and Circuits Fall...

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6.012 ­ Microelectronic Devices and Circuits ­ Fall 2005 Lecture 22­1 Lecture 22 ­ Multistage Amplifiers (II) DC Voltage and Current Sources November 29, 2005 Contents : 1. DC voltage sources 2. DC current sources and sinks Reading assignment: Howe and Sodini, Ch. 9, §§ 9.4
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6.012 ­ Microelectronic Devices and Circuits ­ Fall 2005 Lecture 22­2 Key questions How does one synthesize voltage and current sources? How can this be done in an economic way?
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6.012 ­ Microelectronic Devices and Circuits ­ Fall 2005 Lecture 22­3 1. DC voltage sources Features of voltage source: A well controlled voltage voltage does not depend on current drawn from source ( low internal resistance ). I­V characteristics of voltage source: I ideal real 0 0 V S V Equivalent circuit model of voltage source:± V s R S + -
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6.012 ­ Microelectronic Devices and Circuits ­ Fall 2005 Lecture 22­4 Consider MOSFET in ”diode configuration”: I D I D V GS + + V DS V GS V GS =V T 0 0 V T V DS I­V characteristics: W I D = µC ox ( V GS V T ) 2 = W µC ox ( V DS V T ) 2 2 L 2 L Beyond threshold, MOSFET looks like ”diode” with quadratic I­V characteristics.
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6.012 ­ Microelectronic Devices and Circuits ­ Fall 2005 Lecture 22­5 How does one synthesize a voltage source with this? Assume a current source is available. i i D i OUT V DD I REF + v OUT v OUT OUT -I REF V T 0 V GS = V DS takes value needed to sink current: W I D = I REF + i OUT = µC ox ( v OUT V T ) 2 2 L Then: W i OUT = µC ox ( v OUT V T ) 2 I REF 2 L Solving for v OUT : I REF + i OUT v OUT = V T W 2 L µC ox
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6.012
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