Lecture 16. Complementary metal oxide semiconductor (CMOS) CMOS 1-1

Transcription

1 Lecture 16 Complementary metal oxide semiconductor (CMOS) CMOS 1-1

2 Outline Complementary metal oxide semiconductor (CMOS) Inverting circuit Properties Operating points Propagation delay Power dissipation CMOS 1-2

3 Introduction to CMOS Efficient logic circuit (complementary MOSFET arrangement) with extensive applications technology for constructing integrated circuits, microcontrollers, static RAM, and other digital logic circuits Established by constructing a p-channel and an n- channel MOSFET on the same substrate Because the hole mobility is less than that of the electron mobility, the width of the p-channel MOSFET device is larger than that of the n-channel MOSFET such that Features high input impedance fast switching speeds Note: Typically, W p ~ 2 to 3 times W n lower operating power CMOS 1-3

4 CMOS Architecture CMOS 1-4

5 CMOS Inverter One of CMOS effective use is as an inverter +5 V V GSp I D 5 V 0 V p-channel MOSFET 0 V 5 V V GSn n-channel MOSFET CMOS 1-5

6 CMOS Inverter Basic Operation CMOS 1-6

7 CMOS Inverter Steady State Response CMOS 1-7

8 CMOS Inverter Operating Points When the input is high i.e. v in = V DD, transistor N-MOSFET is on and P-MOSFET is off The resistance from the output to ground is simply the resistance of N-MOSFET (known as R n ) V input V output I D R n CMOS 1-8

9 CMOS Inverter Operating Points When the input is low i.e. v in = 0, transistor P-MOSFET is on and N-MOSFET is off The resistance from V DD to the output is simply the resistance of p-mosfet (known as R p ) I D R p CMOS 1-9

10 CMOS Properties Full rail-to-rail swing (high noise margins) Symmetrical voltage transfer curve always a path to V DD or GND in steady state Extremely high input resistance (gate of MOS transistor is near perfect insulator) nearly zero steady-state input current No static power dissipation no direct path steady-state between power and ground Direct path current during switching Propagation delay is a function of load capacitance and resistance of transistors CMOS 1-10

11 CMOS Inverter Voltage Transfer Curve (VTC) V M :Mid-point voltage at Vin = Vout V M V M CMOS 1-11

12 CMOS Inverter Switch Model of Dynamic Behavior Gate response time is determined by the time to charge C L through R p (discharge C L through R n ) CMOS 1-12

13 Impact of Process Variation on CMOS VTC Curve Process variations (mostly) cause a shift in the switching threshold CMOS 1-13

14 CMOS Inverter Propagation Delay Definition: the inverter propagation delay is the time delay between input and output signals (figure of merit of logic speed) Propagation delays t PHL (propagation delay from High to Low) and t PLH (propagation delay from Low to High) define ultimate speed of logic Typical propagation delays < 100 ps Complex logic systems (e.g., microprocessor) has propagation delays per clock cycle CMOS 1-14

15 Estimation of Propagation Delay Using square-wave at input Average propagation delay CMOS 1-15

16 Estimation of Propagation Delay Oxide capacitance Load capacitance CMOS 1-16

17 CMOS Inverter Power Dissipation Energy from power supply needed to charge up the capacitor through p-channel MOSFET Energy stored in capacitor 2 Energy lost in p-channel MOSFET during charging CMOS 1-17

18 CMOS Inverter Power Dissipation During discharge the n-channel MOSFET dissipates an identical amount of energy If the charge/discharge cycle is repeated f times/second, where f is the clock frequency, the dynamic power dissipation is Note: power dissipation is due to dissipation in both n-channel and p-channel MOSFET In practice many gates do not change state every clock cycle which lowers the power dissipation CMOS 1-18

19 Problem Consider the circuit below, which consists of an NMOS device and PMOS current source load Calculate V M, the voltage midpoint Calculate the total energy dissipated through charging and discharging p-channel MOSFET n-channel MOSFET CMOS 1-19

20 Lecture-related Question Given CMOS Inverter with the following parameters: µ n = 3µ p = 0.06 m 2 V.s, V tn =-V tp = 0.3V, C ox =8 ff/µm 2,V DD = 2V, (W/L) n = 6/1.5, (W/L) p =12/1.5 Find the charging and discharging current knowing that k = ma/v 2 Calculate N-MOSFET and P-MOSFET equivalent resistance Find inverter s average propagation delay CMOS 1-20

21 Lecture Summary Covered material Complementary metal oxide semiconductor (CMOS) Inverting circuit Properties Operating points Propagation delay Power dissipation Material to be covered next lecture Introduction to operational amplifiers and their applications CMOS 1-21