Lect2UP160_(100325)

Lect2UP160_(100325) - Lecture 160 Current Mirrors and...

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Lecture 160 – Current Mirrors and Simple References (3/25/10) Page 160-1 CMOS Analog Circuit Design © P.E. Allen - 2010 LECTURE 160 – CURRENT MIRRORS AND SIMPLE REFERENCES LECTURE ORGANIZATION Outline • MOSFET current mirrors • Improved current mirrors • Voltage references with power supply independence • Current references with power supply independence • Temperature behavior of voltage and current references CMOS Analog Circuit Design, 2 nd Edition Reference Pages 134-153 Lecture 160 – Current Mirrors and Simple References (3/25/10) Page 160-2 CMOS Analog Circuit Design © P.E. Allen - 2010 MOSFET CURRENT MIRRORS What is a Current Mirror? A current mirror replicates the input current of a current sink or current source as an output current. The output current may be identical to the input current or can be a scaled version of it. 060528-01 V DD Current Mirror i IN V DD i OUT = Ki IN V DD Current Mirror I IN V DD I OUT = KI IN i in Ki out i OUT i IN The above current mirrors are referenced with respect to ground. Current mirrors can also be referenced with respect to V DD and current sink inputs and outputs.
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Lecture 160 – Current Mirrors and Simple References (3/25/10) Page 160-3 CMOS Analog Circuit Design © P.E. Allen - 2010 Characterization of Current Mirrors A current mirror is basically nothing more than a current amplifier. The ideal characteristics of a current amplifier are: • Output current linearly related to the input current, i out = A i i in • Input resistance is zero • Output resistance is infinity Also, the characteristic V MIN applies not only to the output but also the input. V MIN (in) is the range of v in over which the input resistance is not small V MIN (out) is the range of v out over which the output resistance is not large Graphically: Therefore, R out , R in , V MIN (out), V MIN (in), and A i will characterize the current mirror. Current Mirror + - v in i in + - v out i out v in i in V MIN± (in) Slope = 1/ R in i in v out i out V MIN± (out) Slope = 1/ R out i out 1 A i Fig. 300-01 Input Characteristics Transfer Characteristics Output Characteristics Lecture 160 – Current Mirrors and Simple References (3/25/10) Page 160-4 CMOS Analog Circuit Design © P.E. Allen - 2010 Simple MOS Current Mirror Circuit: Assume that v DS 2 > v GS - V T 2 , then i O i I = ± ² ² ³ ´ µ µ L 1 W 2 W 1 L 2 ± ² ² ³ ´ µ µ V GS - V T 2 V GS - V T 1 2 · ¸ ¸ ¹ º » » ¼ 1 + ½ v DS 2 1 + v DS 1 ± ² ² ³ ´ µ µ K 2 K 1 If the transistors are matched, then K 1 ’ = K 2 ’ and V T 1 = V T 2 to give, i O i I = ± ² ² ³ ´ µ µ L 1 W 2 W 1 L 2 ± ² ² ³ ´ µ µ 1 + v DS 2 1 + v DS 1 If v DS 1 = v DS 2 , then i O i I = ± ² ² ³ ´ µ µ L 1 W 2 W 1 L 2 Therefore the sources of error are: 1.) v DS 1 ¾ v DS 2 2.) M1 and M2 are not matched.
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Lect2UP160_(100325) - Lecture 160 Current Mirrors and...

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