Lect2UP140_(100325)

Lect2UP140_(100325) - Lecture 140 The MOS Switch and Diode...

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Lecture 140 – The MOS Switch and Diode (3/25/10) Page 140-1 CMOS Analog Circuit Design © P.E. Allen - 201 LECTURE 140 – THE MOS SWITCH AND MOS DIODE LECTURE ORGANIZATION Outline • MOSFET as a switch • Influence of the switch resistance • Influence of the switch capacitors - Channel injection - Clock feedthrough • Using switches at reduced values of V DD • MOS Diode • Summary CMOS Analog Circuit Design, 2 nd Edition Reference Pages 113-124 Lecture 140 – The MOS Switch and Diode (3/25/10) Page 140-2 CMOS Analog Circuit Design © P.E. Allen - 201 Switch Model An ideal switch is a short-circuit when ON and an open-circuit when OFF. V C = controlling terminal for the switch ( V C high ± switch ON, V C low ± switch OFF) Actual switch: r on = resistance of the switch when ON r off = resistance of the switch when OFF V OS = offset voltage when the switch is ON I off = offset current when the switch is OFF I A and I B are leakage currents to ground C A and C B are capacitances to ground C AC and C BC = parasitic capacitors between the control terminal and switch terminals + V C A B I AB V AB R AB = 0 Ω ( V C = high) R AB = ∞Ω ( V C = low) 060526−03 060526-04 C AC C BC C A C B V C I A r OFF I OFF r ON C AB V OS A B C I B
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Lecture 140 – The MOS Switch and Diode (3/25/10) Page 140-3 CMOS Analog Circuit Design © P.E. Allen - 201 MOS Transistor as a Switch 060526-05 Bulk AB (S/D) (D/S) C (G) A B On Characteristics of a MOS Switch Assume operation in active region ( v DS < v GS - V T ) and v DS small. i D = μ C ox W L ± ² ² ³ ´ µ µ ( v GS - V T ) - v DS 2 v DS · μ C ox W L ( v GS - V T ) v DS Thus, R ON ± v DS i D = 1 μ C ox W L ( v GS - V T ) OFF Characteristics of a MOS Switch If v GS < V T , then i D = I OFF = 0 when v DS · 0V. v DS > 0, then R OFF ± 1 i D ¸ = 1 I OFF ± ² Lecture 140 – The MOS Switch and Diode (3/25/10) Page 140-4 CMOS Analog Circuit Design © P.E. Allen - 201 MOS Switch Voltage Ranges If a MOS switch is used to connect two circuits that can have analog signal that vary from 0 to 1V, what must be the value of the bulk and gate voltages for the switch to work properly? Circuit 1 Circuit 2 (0 to 1V) (S/D) (0 to 1V) (D/S) Bulk Gate Fig.4.1-3 • To insure that the bulk-source and bulk-drain pn junctions are reverse biased, the bulk voltage must be less than the minimum analog signal for a NMOS switch. • To insure that the switch is on, the gate voltage must be greater than the maximum analog signal plus the threshold for a NMOS switch. Therefore: V Bulk ± 0V V Gate (on) > 1V + V T V Gate (off) ± Unfortunately, the large value of reverse bias bulk voltage causes the threshold voltage to increase.
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Lecture 140 – The MOS Switch and Diode (3/25/10) Page 140-5 CMOS Analog Circuit Design © P.E. Allen - 201 Current-Voltage Characteristics of a NMOS Switch The following simulated output characteristics correspond to triode operation of the MOSFET.
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This note was uploaded on 03/19/2012 for the course EE 3050 at Georgia Tech.

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Lect2UP140_(100325) - Lecture 140 The MOS Switch and Diode...

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