Analog Integrated Circuits Fundamental Building Blocks Fundamental - PowerPoint PPT Presentation

Analog Integrated Circuits Fundamental Building Blocks Fundamental Building Blocks Current mirrors Faculty of Electronics Telecommunications and Information Technology Information Technology Gabor Csipkes Bases of Electronics Department

Outline  current source biasing – voltage sources  MOS transistor current mirrors  fundamental current mirror  fundamental current mirror  cascode current mirror  low swing cascode current mirror  unbalanced and symmetrical Wilson current mirrors  bipolar transistor current mirrors  fundamental current mirror  fundamental current mirror with β compensation  fundamental current mirror with resistive emitter degeneration  fundamental current mirror with resistive emitter degeneration  cascode current mirror  unbalanced and symmetrical Wilson current mirrors Analog Integrated Circuits – Fundamental building blocks – Current mirrors 2

Current mirrors – principles of operation  integrated current sources with transistors need bias voltages → voltage sources 1 g R R m in out 1: n Is this a voltage Current mirror source?  key parameters to consider:  input resistance → must be as small as possible → current input  input resistance → must be as small as possible → current input  output resistance → must be as large as possible → current output  minimum required output voltage  required input voltage  current gain → precision imposed by the application Analog Integrated Circuits – Fundamental building blocks – Current mirrors 3

The fundamental MOS current mirror NMOS PMOS Small signal model V 1 V     R in R out r    V V V V V in out DS 2 I g I in DSat 1 Th o min DSat 2 in m 1 out         1 V     V V 1 V I I   n n out out     2 2 GS GS Th Th 2 2 DS DS 2 2 n n out out  current gain :  current gain :      1 V     I V V 1 V in in 1 GS Th 1 DS 1    2 V  Δ β – geometry mismatch    n 1 Th  2 V  Δ V Th – threshold voltage mismatch DSat Analog Integrated Circuits – Fundamental building blocks – Current mirrors 4

The MOS cascode current mirror NMOS PMOS     V V V V V in DSat 3 Th 3 DSat 1 Th 1   f V BS 3    V V V V (!) o min DSat 4 DSat 2 Th 2 1 1 1 1   R in g g m 1 m 3       R r r g g r r out DS 2 DS 4 m 4 mb 4 DS 2 DS 4 Small signal model Analog Integrated Circuits – Fundamental building blocks – Current mirrors 5

The MOS cascode current mirror  the V GS voltages of M 3 and M 4 balance the fundamental mirror M 1 -M 2 → n is accurately defined    V V V V in in out out DS DS 1 1 DS DS 2 2 V V DS 2 DS 1 The current gain is very close to unity even when the input- output voltage imbalance Δ V is significant Analog Integrated Circuits – Fundamental building blocks – Current mirrors 6

The MOS low swing cascode current mirror  the cascode current mirror is not optimal in terms of V omin → the gate voltage of M 4 must be decreased by V Th 2 1  V 2 V o min DSat V G 4 V G 4  V V DS 1 DS 2 4 3   V V V DS 1 DS 2 R R triode region Analog Integrated Circuits – Fundamental building blocks – Current mirrors 7

The MOS low swing cascode current mirror NMOS PMOS   V V V in DSat 1 Th 1   V V V o min DSat 4 DSat 2 1 R   R in g m 1       R r r g g r r out DS 2 DS 4 m 4 mb 4 DS 2 DS 4 Small signal model Analog Integrated Circuits – Fundamental building blocks – Current mirrors 8

The Wilson current mirror NMOS PMOS     V V V V V in DSat 3 Th 3 DSat 2 Th 2    V V V V o min DSat 3 DSat 2 Th 2   g g g g   R R m m 2 2 m m 3 3 in g g m 1 m 3 g g r r  R m 1 m 3 DS 1 DS 3 out g m 2 Small signal V in and V out create voltage imbalance model model between V between V DS 1 and V DS 2 and V Accuracy issues for the current gain n Analog Integrated Circuits – Fundamental building blocks – Current mirrors 9

The balanced Wilson current mirror NMOS PMOS     V V V V V in DSat 3 Th 3 DSat 2 Th 2    V V V V o min DSat 3 DSat 2 Th 2   g g g g   R R m m 2 2 m m 3 3 in g g m 1 m 3 g g r r  R m 1 m 3 DS 1 DS 3 out g m 2 Small signal model Analog Integrated Circuits – Fundamental building blocks – Current mirrors 10

The fundamental bipolar current mirror NPN PNP   V V V V in BE 1 o min CE 2 1   R R r in out CE 2 g m 1             I I V V V V I I V V 2 1       S S 2 2 1 1 CE CE 2 2 CE CE 1 1     n n S S 2 1    I V V  I V V      S 1 CE 1 EA  current gain : n S 1 in EA    1 I V V    1 S 2 1 CE 2 CE 1       I I V V     n S 2 S 1 CE 1 EA   I 2  Δ V – input-output voltage imbalance S 1 Analog Integrated Circuits – Fundamental building blocks – Current mirrors 11

The fundamental current mirror with β compensation NPN PNP 1    V V V V V  R  R r o min CE 2 in BE 1 BE 3 in g out CE 2 m 1  current gain → β replaced by β ( β +1) → n much closer to the ideal value when the  current gain → β replaced by β ( β +1) → n much closer to the ideal value when the input and the output are balanced in voltage        1  I I V     n S 2 S 2 n 1          I 1 2 I V V   S 1 S 1 in EA Analog Integrated Circuits – Fundamental building blocks – Current mirrors 12

The degenerated fundamental current mirror NPN PNP 1   R R    R r R g r R     in 1 V V I R V V I R out CE 2 2 m 2 CE 2 2 g in BE 1 in 1 o min CE 2 out 2 m 1 R  n 1       V V I R I R V V I I R R  current gain :  current gain : R R BE BE 1 1 in in 1 1 BE BE 2 2 out out 2 2 2  remember to adjust the emitter areas of Q 1 and Q 2 A I R    2 S 2 1 n proportionally with the current in each branch !!! A I R 1 S 1 2  n is still affected by the finite β and by Δ V Analog Integrated Circuits – Fundamental building blocks – Current mirrors 13

The bipolar cascode current mirror NPN PNP g r r r  1 1 R m 4 CE 4 CE 2 BE 4   R     out   V V V V V V g in in BE 1 BE 3 o min CE 4 BE 2 g g   r r 1 m 2   m 1 m 3 BE 4 CE 2 g   m 1  current gain → β influences the accuracy while the  2 I   n S 2 fundamental mirror Q 1 -Q 2 is balanced by the cascode     2 I 4 2 transistors Q 3 -Q 4 S 1 Analog Integrated Circuits – Fundamental building blocks – Current mirrors 14

The bipolar Wilson current mirror asymmetrical balanced     V V V V V V  current gain → β influences the accuracy o min CE 4 BE 2 in BE 1 BE 3 while the fundamental mirror Q 1 -Q 2 is    r g g balanced by the cascode transistors Q 3 -Q 4   BE 3 m 2 m 3 R R in in   g g g g g g r r m 2 m 1 m 3 BE 3    2 I 2   n S 2 r  g g r r CE 1     2 I 4 2 m 1 m 3 CE 1 CE 3 S 1 r  R BE 3 out   r r   g 1 CE 1 g CE 1   m 2 m 1 r r   BE 3 BE 3 Analog Integrated Circuits – Fundamental building blocks – Current mirrors 15

Bibliography  P.E. Allen, D.R. Holberg, CMOS Analog Circuit Design , Oxford University Press, 2002  B. Razavi, Design of Analog CMOS Integrated Circuits , McGraw-Hill, 2002  D. Johns, K. Martin, Analog Integrated Circuit Design , Wiley, 1996  P.R.Gray, P.J.Hurst, S.H.Lewis, R.G, Meyer, Analysis and Design of Analog Integrated Circuits , Wiley,2009  R.J. Baker, CMOS Circuit Design, Layout and Simulation , 3 rd edition, IEEE Press, 2010 Analog Integrated Circuits – Fundamental building blocks – Current mirrors 16