Analog Integrated Circuits Fundamental Building Blocks Fundamental - PowerPoint PPT Presentation

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

Outline  references as independent sources, parameters  simple dividers as voltage reference  MOS or bipolar diode voltage references  MOS or bipolar diode voltage references  a Zener diode voltage reference  self biased current references (self biased current mirrors)  bipolar and MOS Widlar current references (PTAT)  V Th and V BE references (CTAT)  bootstrapping and supply voltage independence  bandgap references Analog Integrated Circuits – Fundamental building blocks – Current and voltage references 2

References - definitions  current and voltage references → active implementations of independent sources  the output current or voltage is independent on load, temperature and supply voltage  reference → better precision, sensitivity and temperature coefficient than average circuitry → closer to ideal sources circuitry → closer to ideal sources   X X 1 y  ref   ref  y TC S  Xref  T X y X ref ref Sensitivity = the relative variation Temperature coefficient = the of the output voltage or current X ref temperature sensitivity of the output with respect to the parameter y voltage or current X ref , normalized to 1°  references often exploit  physical dependences in integrated components (e.g. temperature dependence of a junction voltage or the thermal voltage)  advantages rising from circuit topologies (e.g. resistor ratio independent on temperature) Analog Integrated Circuits – Fundamental building blocks – Current and voltage references 3

A voltage divider as reference Passive: R   V 2 V ref  DD R R 1 2  V   V V R V R V R R R R   ref ref         V V S S 1 1 DD DD 2 2 DD DD 1 1 2 2 DD DD V   V V R R R V ref DD ref 1 2 2 DD → a 1% variation of V DD produces 1% variation passive active of the reference voltage Active:   V V | V |   DD Thn Thp V V  ref Thn      I I  2       I I V V V V D 1 D 2   1 1 n n   D D 1 1 n n GS GS 1 1 Thn Thn     2 p     1 I V | V |  D 2 p SG 2 Thp  V V V    C W ref V    C W S DD DD DD     p ox 2  n ox 1 ; V   V V ref n p 2 L 2 L    DD ref V V | V | n 1 2 DD Thn  Thp p Analog Integrated Circuits – Fundamental building blocks – Current and voltage references 4

A bipolar or MOS diode voltage reference MOS:  V V   DD ref V V ref Th  R  V V V ref V    S DD DD DD   V  V V ref   2 V R V V DD ref ref DD ref  V 1    ref   Bipolar: TC f TC , TC , TC , TC   Vref  VDD VCC VTh R T V ref    V V   CC ref V V V V ln ln      non-linear variation of V ref with I → the  non-linear variation of V ref with I → the ref ref T T RI   sensitivity decreases with V ref S  non-linearity more effective for bipolar  V  V V V  ref   V S CC CC T transistors (exponential vs. quadratic) DD   V   V V V V V ref CC ref ref CC ref  V ref range relatively low ( V BE or V GS ) Analog Integrated Circuits – Fundamental building blocks – Current and voltage references 5

A bipolar or MOS diode voltage reference  extending the output voltage range with an additional voltage divider MOS:      V V R    DD ref   V 1 1 V       ref ref Th Th   R R R R       2  V V V  ref   V S DD DD DD   V  V V ref   2 V R V V DD ref ref DD ref  V 1      ref   Bipolar: TC f TC , TC , TC , TC     Vref Vref VDD VCC VDD VCC VTh VTh R R   T T V V  ref     V V R   CC ref V V 1 1 ln     ref T R RI  the reference voltage still depends on     2 S all the temperature sensitive components  V  V V V ref V    S CC CC T DD   V   V V V V V ref CC ref ref CC ref Analog Integrated Circuits – Fundamental building blocks – Current and voltage references 6

A Zener diode voltage reference  the Zener effect → avalanche breakdown of a pn junction under the effect of a large enough reverse biasing  the large intrinsic electric field created by the wide depletion region breaks minority carrier bonds → voltage drop relatively constant and well defined with changing current  abrupt current to voltage dependence of the diode decreases supply sensitivity reverse junction current abrupt  supply voltages often 1.2V-1.5V breakdown  typical V BV in CMOS larger than V BV =ct. 4-5V  drawback → relatively large V BV → inappropriate for low voltage applications reverse junction voltage Analog Integrated Circuits – Fundamental building blocks – Current and voltage references 7

Self biased current mirror reference  uses V ref of the diode voltage reference to control a current source   V V V V S S DD DD I I out ref  if the transistors are matched and balanced in voltage → supply voltage sensitivity inherited from I ref ref  possible improvement → scale the I ref sensitivity with a coeffcient smaller than unity V   V  S k S ; k 1 DD DD I I out ref  partly linear transformation of V ref into I out → Widlar current reference (mirror) Analog Integrated Circuits – Fundamental building blocks – Current and voltage references 8

Widlar current references  MOS and bipolar implementations are possible      V V I R  V V I R GS 1 GS 2 out 2 BE 1 BE 2 out 2     I I     ref     ref V V V V ln ln V V V V         BE BE 1 1 T T GS GS 1 1 Th Th   I I     S 1 1     I I      V V ln out V V out     BE 2 T GS 2 Th  I     S 2 2     I I V I 1 1 1       ref   T S 2         ref I I ln ln     I I 4 4 R R out out R R I I I I out out     2 2         2 R 2 out S 1   2 2 1 2 V V   V V V   V S T S S DSat 1 S DD DD DD DD I I I I   V R I 2 V V out ref out ref         T 2 out DSat 1 DSat 2   1 1 Analog Integrated Circuits – Fundamental building blocks – Current and voltage references 9

V Th and V BE current references  similar to Widlar references but voltage to current conversion entirely linear I  ref V Th  V   I GS 1   out MOS out MOS R R R R 2 2   I ref V ln   T I V     S 1 I BE 1  out BJT R R 2 2  increased output resistance due to the cascode effect of M 2 and Q 2  lowest possible supply voltage sensitivity among self biased references V V   V V   MOS: S DSat 1 S V V Bipolar: S T S DD DD DD DD   I I  I I 2 V V V out ref out ref DSat 1 Th BE 1 Analog Integrated Circuits – Fundamental building blocks – Current and voltage references 10

Supply independent references - principle  self biasing ties the output voltage or current to V DD - V CC → inherent supply sensitivity  idea: define I ref as function of I out → supply in dependence in any Widlar, V Th or V BE reference  implies a positive feedback loop and the double definition of I out → bootstrapping implies a positive feedback loop and the double definition of I out → bootstrapping What is this ?? I ref I out Analog Integrated Circuits – Fundamental building blocks – Current and voltage references 11

Bootstrapped current references – startup  stable operating point → I ref = I out  positive feedback loop → two stable operating points, one at the origin ( I ref = I out =0)  the startup circuit prevents the loop to settle in the origin and is deactivated once it starts to converge to the desired operating point starts to converge to the desired operating point I I ref out  I ref = I out =0 → V GS 1 =0 → V GS 7 >> → M 7 pumps current into M 1 → I out and I ref rise  when I ref and I out are large enough, V GS 1 increases pushing V GS 7 to 0 and the startup circuit is disabled ( I D 7 =0) Analog Integrated Circuits – Fundamental building blocks – Current and voltage references 12