13_Lecutre

13_Lecutre - EE114 Lecture 13 Lecture 13 Current Mirrors R...

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EE114 Lecture 13 B. Murmann 1 EE114 (HO #17) Lecture 13 Current Mirrors R. Dutton, B. Murmann Stanford University B. Murmann 2 EE114 (HO #17) Basic Analysis ( λ =0) M1 “computes” gate-source voltage M2 sees same V GS , and therefore I O =I I (to first order) – Note that this result holds even with large variations in V t or μ C ox that are common to M1 and M2 L W C I V V V ox i t I gs μ + ! = 2 ( ) ( ) 1 2 1 2 1 = ! μ ! μ = t GS ox t GS ox I O V V L W C V V L W C I I I I W/L I O V I V O M 1 W/L M 2
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EE114 Lecture 13 B. Murmann 3 EE114 (HO #17) Typical Design Objectives Minimize error in I O Minimize compliance voltage V omin Minimize capacitance C o Maximize R o Want to be able to scale currents, i.e. I O =K I I I I I O V I V O R o , C o + V omin - B. Murmann 4 EE114 (HO #17) Current Scaling (1) The above equation suggests that we can set K by scaling either or both the widths and lengths of the two MOSFETS – This deserves a closer look ( ) ( ) K V V L W C V V L W C I I ! t GS ox t GS ox I O = ! μ ! μ = 1 1 2 2 2 1 2 1 I I W 1 /L 1 I O V I V O M 1 W 2 /L 2 M 2
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EE114 Lecture 13 B. Murmann 5 Current Scaling (2) The solid curve to the right shows how the current of a modern MOSFET changes with 1/L Very different from our long channel model, which predicts I D 1/L Bottom line – Impossible to make current mirrors with reasonable accuracy unless L 1 =L 2 EE114 (HO #17) 6 8 10 12 14 200 250 300 350 400 450 NMOS I D vs. 1/L in 90-nm technology. V GS =V DS =500mV (V t ~300mV) 1/L [ μ m -1 ] I D [ μ A ] long channel model real device B. Murmann 6 Current Scaling (3) The remaining (and only option) for scaling the current is to use devices with different widths Basic possibility from a layout perspective (e.g. K=2) EE114 (HO #17) M1 W1 M2 W2=2W1
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EE114 Lecture 13 B. Murmann 7 Current Scaling (4) Unfortunately, transistors in modern technology do
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This note was uploaded on 11/02/2011 for the course EE 114 taught by Professor Murmann during the Fall '08 term at Stanford.

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13_Lecutre - EE114 Lecture 13 Lecture 13 Current Mirrors R...

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