Lect2UP300-(100328)

# Lect2UP300-(100328) - Lecture 300 Low Voltage Op Amps Page...

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Lecture 300 – Low Voltage Op Amps (3/28/10) Page 300-1 CMOS Analog Circuit Design © P.E. Allen - 2010 LECTURE 300 – LOW VOLTAGE OP AMPS LECTURE ORGANIZATION Outline • Introduction • Low voltage input stages • Low voltage gain stages • Low voltage bias circuits • Low voltage op amps • Summary CMOS Analog Circuit Design, 2 nd Edition Reference Pages 415-432 Lecture 300 – Low Voltage Op Amps (3/28/10) Page 300-2 CMOS Analog Circuit Design © P.E. Allen - 2010 INTRODUCTION Implications of Low-Voltage, Strong-Inversion Operation • Reduced power supply means decreased dynamic range • Nonlinearity will increase because the transistor is working close to V DS (sat) • Large values of ± because the transistor is working close to V DS (sat) • Increased drain-bulk and source-bulk capacitances because they are less reverse biased. • Large values of currents and W/L ratios to get high transconductance Small values of currents and large values of W/L will give small V DS (sat) Severely reduced input common mode range • Switches will require charge pumps

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Lecture 300 – Low Voltage Op Amps (3/28/10) Page 300-3 CMOS Analog Circuit Design © P.E. Allen - 2010 What are the Limits of Power Supply? The limit comes when there is no signal range left when the dc drops are subtracted from V DD . Minimum power supply (no signal swing range): V DD (min.) = V T + 2 V ON For differential amplifiers, the minimum power supply is: V DD (min.) = 3 V ON However, to have any input common mode range, the effective minimum power supply is, V DD (min.) = V T + 2 V ON 060802-01 V DD V PB 1 V NB 1 M1 M2 M3 M4 + - V ON V T + V ON + - + - V T + V ON V ON + - 060802-02 V PB 1 V DD V NB 1 + - V ON + - V ON + - V ON + - V T + V ON + - V T + V ON M1 M2 M3 M4 M5 Lecture 300 – Low Voltage Op Amps (3/28/10) Page 300-4 CMOS Analog Circuit Design © P.E. Allen - 2010 Minimum Power Supply Limit – Continued The previous consideration of the differential amplifier did not consider getting the signal out of the amplifier. This will add another V ON . 060802-03 V PB 1 V DD V NB 1 + - V ON + - V ON + - V ON + - V T + V ON + - V T + V ON M1 M2 M3 M4 M5 V PB 1 V PB 2 M6 M7 M8 M9 + - V ON V T + V ON + - V T Therefore, V DD (min.) = V T + 3 V ON This could be reduced to 3 V ON with the floating battery but its implementation probably requires more than 3 V ON of power supply. Note the output signal swing is V T + V ON while the input common range is V ON .