ELE6308 Microélectronique analogique et mixte --- Sources de courant Voltage and current references ---

Transcription

1 S G D ELE6308 Microélectronique analogique et mixte --- Sources de courant Voltage and current references --- Mohamad Sawan, Professor Kamal El-Sankary, Ph.D. Laboratoire de neurotechnologies Polystim mohamad.sawan@polymtl.ca M5418 Page 1

2 I. Current Mirrors II. Current Mirrors & V/I references Plan I. Simple II. Cascode/ Improved cascode (high swing) III. Wilson Voltage and current references I. Sensitivity II. Power supply independent references III. Bandgap circuits Page 2

3 CIRCUITS MOS Source de courants Miroir de courant Structure simple iref iout M1 M2 + Vout - Page 3 ELE6308- Microélectronique analogique et mixte

4 Miroir de courant simple (suite) Analyse DC CIRCUITS MOS Source de courants iref iout M1 M2 + Vout V GS I OUT = V = th + 1 mc 2 mc ox 2I ox ref ( W L) 1 ( W L) mc Impédance de sortie r out = r ds2 = 1/(λ I OUT ) 2 2I ox ref ( W L) 1 = ( W L) 2 I ref ( W L) 1 - DV = V GS - V th = mc 2I ox ref ( W L) 1 Page 4 ELE6308- Microélectronique analogique et mixte

5 CIRCUITS MOS Source de courants Miroir de courant «cascode» Analyse DC iref iout + M3 M2 2(Vth +Veff) VA Vout M4 M1 Vth +Veff - V =Veff= V GS - V th à I D =I ref V! V + 2" V OUT th Vout = 2 Vth+ 2 Veff - Veff -Vth + Veff Page 5 ELE6308- Microélectronique analogique et mixte

6 CIRCUITS MOS Source de courants Miroir de courant cascode (suite) Impédance de sortie M2 + Vx ix iref iout M1 - M3 M4 2(Vth+Veff) VA Vth+ Veff + M2 Vout M1 - gm2 Vgs2 gmb2 Vsb2 rds2 Vx + Vsb2 - rds1 L'impédance de sortie est augmentée par (1 + a v2 ): x 1+ ) v i x R g r r out m 2 ds1 ds 2 Page 6 ELE6308- Microélectronique analogique et mixte

7 CIRCUITS MOS Source de courants Miroir de courant Wilson M3 M1 M2 Analyse DC I OUT = ( W L) 2 I REF ( W L) 1 Impédance de sortie r! g r r O m 3 ds3 ds1 Page 7 ELE6308- Microélectronique analogique et mixte

8 CIRCUITS MOS Source de courants Miroir de courant cascode amélioré Avantage: nécessite seulement 2 Veff Inconvénients: énergie et complexité. Page 8 ELE6308- Microélectronique analogique et mixte

9 CIRCUITS MOS Source de courants Miroir de courant cascode amélioré (High swing) Page 9 ELE6308- Microélectronique analogique et mixte

10 I. Current Mirrors II. Current Mirrors & V/I references Plan I. Simple II. Cascode/ Improved cascode (high swing) III. Wilson Voltage and current references I. Sensitivity II. Power supply independent references III. Bandgap circuits Page 10

11 Power supply independent references Power Supply Independence How do you characterize power supply independence? Use the concept of: Application of sensitivity to determining power supply dependence: Thus, the fractional change in the reference current is equal to the sensitivity times the fractional change in the power supply voltage. For example, if the sensitivity is 1, then a 10% change in V DD will cause a 10% change in I REF. Page 11

12 Simple Bias/Reference Circuits Page 12

13 MOSFET-Resistance Voltage References Assume that V DD = 5V, W/L = 2 and R = 100kΩ, Thus, V REF 1.281V and S = Page 13

14 MOSFET-Resistance Voltage References This circuit allows V REF to be larger. If the current in R 1 (and R 2 ) is small compared to the current flowing through the transistor, then Page 14

15 Simple Current Reference Page 15

16 MOS Widlar Current Reference => => Where Page 16

17 Bootstrapped Current Source/Sink Start up Principle: If T4 = T5, then I 1 I 2. However, the T1-R loop gives: Solving these two equations gives The output current, I out = I 1 = I 2 can be solved as: Power supply independent!! Page 17

18 Temperature Dependence The objective is to minimize the fractional temperature coefficient defined as, parts per million per C or ppm/ C Note that V REF, V T, β, and R are all functions of temperature. Page 18

19 Temperature Dependence Bootstrapped Current Source/Sink The output current was given as: Assume that: If the resistor is polysilicon, then The temperature performance of this circuit is not that good. Page 19

20 Temperature-Independent References Reference voltages and/or currents with little dependence to temperature prove useful in many analog circuits. If two quantities with opposite temperature coefficient are added with proper weighting, the resultant quantity theoretically exhibits zero temperature coefficient. For V1 and V2 with opposite temperature dependence, the coefficients c1 and c2 can be chosen in such a way that Thus, the reference voltage coefficient. exhibits zero temperature Page 20

21 Bandgap Voltage Reference The most popular technique for both Bipolar and CMOS technologies. Generates a fixed dc reference voltage that does not change with temperature. Cancels the negative temperature dependence of a PN junction with a positive temperature dependence from a PTAT (proportional-to-absolutetemperature) circuit. The term with negative temperature dependence is the forward-biased voltage of a diode (usually base-emitter junction): VBE The PTAT term is realized by amplifying the voltage difference of two forward-biased diodes ( i.e., base-emitter junctions): VBE1 - VBE2 Page 21

22 Bandgap Voltage Reference Page 22

23 Bipolar Circuit Page 23

24 CMOS circuits In CMOS technologies, where the independent bipolar transistors are not available, parasitic lateral bipolar transistors are used. Page 24

25 CMOS circuits Page 25

26 CMOS circuits Page 26

27 CMOS circuits Usually make n=integer 2-1, e.g. n=8 Layout: Page 27

28 CMOS circuits If V BE of a unit device is 700mV at room temperature and I=50µA Decided to use n=8 Page 28

29 CMOS circuits Page 29

30 CMOS circuits For low voltage reference: Add currents proportional to V BE and kt/q, instead of stacking voltages Page 30