SMDP Workshop on Mixed-Signal VLSI Design, GEC, Goa Comparators, Gm-C Filters, SC Filters in CMOS Technology

Transcription

1 SMDP Workshop on MixedSignal VLSI Design, GEC, Goa Comparators, GmC Filters, SC Filters in CMOS Technology M. Shojaei Baghini, Assistant Prof. Dept. of Electrical Engineering IITBombay

2 Materials presented in this workshop are partly from the following references. CMOS Analog Circuit Design by P. E. Allen et al., Oxford University Press, CMOS Circuit Design, Layout, and Simulation by R. J. Baker et al, IEEE press, reprint 2002, edition CMOS MixedSignal Circuit Design by R. J. Baker, reprint 2008, edition 2002, Wiley. Analog Integrated Circuit Design by D. Johns et al. Wiley, edition 2005.

3 Teaching Methodology The presented lecture is based on a combination of slide presentation, class notes and lab experiments.

4 Comparator Basic Concept V V out (Logic level) V V out V out V OH V OH V OL V OL V IL2 V IH2 V VIH IL V v in =V V IL1 V IH1 v in =V V Without hysteresis With hysteresis

5 Comparator Important specifications Gain Resolution Delay Switching speed Power dissipation Input commonmode range Offset voltage Input noise voltage

6 Twostage opamp as a comparator?

7 Folded cascode opamp as a Comparator? Gain & Delay are important specifications of a comparator.

8 Comparator Block Diagram V i o v o V i o v o V out Preamplifier Decision circuit (Positive feedback) Buffer & latch

9 Comparator Preamplifier example M3 M4 M1 M2 V in V in I Iout 1 2 ( V V ) ss Gm in = in I ss Iout = Gm1 in in 2 ( V V ) M6 I out Gm1 = M5 I out W L 6 gm1 W 2 L 3 All nodes except output Node are lowimpedance. W L W L W L W = L W = L W = L 5 2 4

10 Comparator Decision circuit Example i o i o v o v o M5 M6 M7 M8 M9 Positive feedback

11 Positive Feedback Concept G m i 1 i 2 v 1 v 2 C C G m If i 1 = i 2 v 1 1 i1 ( s) ( s) = Gm sc 1 Gm RHP pole

12 Comparator An example output buffer Selfbiased differential amplifier M6 V in M3 M1 M4 M2 V in V out M5 Highspeed comparators typically use trackandlatch mechanism for the stage after preamplifier.

13 More details will be given in the lecture.

14 GmC Filters Application in continuous time filters (DSL/ADSL, Video applications, very lowfrequency applications ) Need for some sort of tuning circuitry Linearity requirements Noise performance

15 Basic building block of GmC filters Integrator is the main building block of continuous time filters. v in R in = G m R out = v out C v out = v in s C G m G m /C: Unity gain frequency of the integrator

16 Basic building block of GmC filters (cont d) Lossy integrator with singleended input v in v in G m C v out v out Lossy integrator with differential input G m1 C G m2 Similar to a parallel RC = 1 v out v in s C G v v out in m Gm 1 Gm2 = 1 s C G m2

17 Fully differential GmC integrator v in v in G v C m out G v 2C m out 2C Which configuration is preferred?

18 Differential architecture needs commonmode feedback (CMFB) Example M7 M3 M4 I ref2 M1 M2 V in V in V outdiff I ref1 M3 M1 M2 M4 V in V out I ref1 M5 M6 M5 M6 Quiescent value of CM output voltage is very sensitive to I ref1 I ref2. Internal CMFB!

19 CMFB Example M5 M6 V CMFB M1 M2 M3 M4 V out V out V ref gm 1 = gm2 = gm3 = gm4 = gm From the bias voltages of the main circuit V out V VCMFB gm out I = 5 gm V = ref 2 Vout gm5 CM

20 More details will be given in the lecture.

21 SwitchedCapacitor (SC) Filters SC filters are discretetime or sampleddata equivalents of continuoustime filters. SC circuits eliminate resistors in filters by replacing them with capacitors and switches. Filter time constants are a function of capacitor ratios and clock frequency.

22 Emulating a Resistor by a Switched Capacitor Circuit Twophase nonoverlapping clocks are used to control switches S1 & S2. Charge transferred from Vin to Vout during each clock cycle is: Q = C(Vin Vout) Average current flowing from Vin to Vout is: i=q/t = Q.f s i =f s. C(Vin Vout) With the current through the switched capacitor resistor proportional to the voltage across it, the equivalent switched capacitor resistance is: Req = 1/(f s.c)

23 Switchedcapacitor Circuits: Integrator For f signal << f sampling V o = EECS 247 Course slides, UCB ω o = f s x (C s / C I )

24 More details will be given in the lecture.