CSE477 L04 CMOS Inverter.1Irwin&Vijay, PSU, 2002 ECE484 VLSI Digital Circuits Fall 2010 Lecture 04: CMOS Inverter (static view) Mary Jane Irwin ( mji.

CSE477 L04 CMOS Inverter.1 Irwin&Vijay, PSU, 2002

ECE484VLSI Digital Circuits

Fall 2010

Lecture 04: CMOS Inverter (static view)

Mary Jane Irwin ( www.cse.psu.edu/~mji ) www.cse.psu.edu/~cg477

Modified by Dr. George Engel (SIUE)

[Adapted from Rabaey’s Digital Integrated Circuits, ©2002, J. Rabaey et al.]


CMOS Inverter: A First Look

VDD

Vout

CL

Vin


CMOS Inverter: Steady State Response

VDD

Rn

Vout = 0

Vin = V DD

VDD

Rp

Vout = 1

Vin = 0

VOL = 0VOH = VDD

VM = f(Rn, Rp)


CMOS Properties

Full rail-to-rail swing high noise margins Logic levels not dependent upon the relative device sizes

transistors can be minimum size ratioless

Always a path to Vdd or GND in steady state low output impedance (output resistance in k range) large fan-out (albeit with degraded performance)

Extremely high input resistance (gate of MOS transistor is near perfect insulator) nearly zero steady-state input current

No direct path steady-state between power and ground no static power dissipation

Propagation delay function of load capacitance and resistance of transistors


Review: Short Channel I-V Plot (NMOS)

0

0.5

1

1.5

2

2.5

0 0.5 1 1.5 2 2.5

I D (

A)

VDS (V)

X 10-4

VGS = 1.0V

VGS = 1.5V

VGS = 2.0V

VGS = 2.5V

Lin

ear

de

pe

nde

nce

NMOS transistor, 0.25um, Ld = 0.25um, W/L = 1.5, VDD = 2.5V, VT = 0.4V


Review: Short Channel I-V Plot (PMOS)

-1

-0.8

-0.6

-0.4

-0.2

00-1-2

I D (

A)

VDS (V)

X 10-4

VGS = -1.0V

VGS = -1.5V

VGS = -2.0V

VGS = -2.5V

PMOS transistor, 0.25um, Ld = 0.25um, W/L = 1.5, VDD = 2.5V, VT = -0.4V

All polarities of all voltages and currents are reversed


Transforming PMOS I-V Lines

IDSp = -IDSn

VGSn = Vin ; VGSp = Vin - VDD

VDSn = Vout ; VDSp = Vout - VDDVout

IDn

VGSp = -2.5

VGSp = -1

Mirror around x-axisVin = VDD + VGSp

IDn = -IDp

Vin = 1.5

Vin = 0

Vin = 1.5

Vin = 0

Horiz. shift over VDD

Vout = VDD + VDSp

Want common coordinate set Vin, Vout, and IDn


CMOS Inverter Load Lines

0

0.5

1

1.5

2

2.5

0 0.5 1 1.5 2 2.5

I Dn (

A)

Vout (V)

X 10-4

Vin = 1.0V

Vin = 1.5V

Vin = 2.0V

Vin = 2.5V

0.25um, W/Ln = 1.5, W/Lp = 4.5, VDD = 2.5V, VTn = 0.4V, VTp = -0.4V

Vin = 0V

Vin = 0.5V

Vin = 1.0V

Vin = 1.5V

Vin = 0.5VVin = 2.0V

Vin = 2.5V

Vin = 2VVin = 1.5V

Vin = 1V

Vin = 0.5V

Vin = 0V

PMOS NMOS


CMOS Inverter VTC

0

0.5

1

1.5

2

2.5

0 0.5 1 1.5 2 2.5

Vin (V)

Vo

ut (

V)

NMOS offPMOS res

NMOS satPMOS res

NMOS satPMOS sat

NMOS resPMOS sat NMOS res

PMOS off


CMOS Inverter: Switch Model of Dynamic Behavior

VDD

Rn

Vout

CL

Vin = V DD

VDD

Rp

Vout

CL

Vin = 0

Gate response time is determined by the time to charge CL through Rp (discharge CL through Rn)


Relative Transistor Sizing

When designing static CMOS circuits, balance the driving strengths of the transistors by making the PMOS section wider than the NMOS section to

maximize the noise margins and obtain symmetrical characteristics


Switching Threshold

VM where Vin = Vout (both PMOS and NMOS in saturation since VDS = VGS)

VM rVDD/(1 + r) where r = kpVDSATp/knVDSATn

Switching threshold set by the ratio r, which compares the relative driving strengths of the PMOS and NMOS transistors

Want VM = VDD/2 (to have comparable high and low noise margins), so want r 1

(W/L)p kn’VDSATn(VM-VTn-VDSATn/2)

(W/L)n kp’VDSATp(VDD-VM+VTp+VDSATp/2)

=


Switch Threshold Example

In our generic 0.25 micron CMOS process, using the process parameters from slide L03.25, a VDD = 2.5V, and a minimum size NMOS device ((W/L)n of 1.5)

VT0(V) (V0.5) VDSAT(V) k’(A/V2) (V-1)

NMOS 0.43 0.4 0.63 115 x 10-6 0.06

PMOS -0.4 -0.4 -1 -30 x 10-6 -0.1

(W/L)p 115 x 10-6 0.63 (1.25 – 0.43 – 0.63/2)

(W/L)n -30 x 10-6 -1.0 (1.25 – 0.4 – 1.0/2)= x x = 3.5

(W/L)p = 3.5 x 1.5 = 5.25 for a VM of 1.25V


Simulated Inverter VM

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

0 1 10(W/L)p/(W/L)n

VM (

V)

VM is relatively insensitive to variations in device ratio

setting the ratio to 3, 2.5 and 2 gives VM’s of 1.22V, 1.18V, and 1.13V

Increasing the width of the PMOS moves VM towards VDD

Increasing the width of the NMOS moves VM toward GND

.1

Note: x-axis is semilog

~3.4


Noise Margins Determining VIH and VIL

0

1

2

3

VIL VIHVin

Vo

ut

VOH = VDD

VM

By definition, VIH and VIL are where dVout/dVin = -1 (= gain)

VOL = GND

A piece-wise linear approximation of VTC

NMH = VDD - VIH

NML = VIL - GND

Approximating: VIH = VM - VM /g VIL = VM + (VDD - VM )/g

So high gain in the transition region is very desirable


CMOS Inverter VTC from Simulation

0

0.5

1

1.5

2

2.5

0 0.5 1 1.5 2 2.5

Vin (V)

Vo

ut (

V)

0.25um, (W/L)p/(W/L)n = 3.4(W/L)n = 1.5 (min size)VDD = 2.5V

VM 1.25V, g = -27.5

VIL = 1.2V, VIH = 1.3VNML = NMH = 1.2(actual values are VIL = 1.03V, VIH = 1.45VNML = 1.03V & NMH = 1.05V)

Output resistance low-output = 2.4khigh-output = 3.3k


Gain Determinates

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

0 0.5 1 1.5 2

Vin

gai

n

Gain is a strong function of the slopes of the currents in the saturation region, for Vin = VM

(1+r)g ---------------------------------- (VM-VTn-VDSATn/2)(n - p )

Determined by technology parameters, especially channel length modulation (). Only designer influence through supply voltage and VM (transistor sizing).


Impact of Process Variation on VTC Curve

0

0.5

1

1.5

2

2.5

0 0.5 1 1.5 2 2.5

Vin (V)

Vo

ut (

V)

Nominal

Good PMOSBad NMOS

Bad PMOSGood NMOS

Process variations (mostly) cause a shift in the switching threshold


Scaling the Supply Voltage

0

0.5

1

1.5

2

2.5

0 0.5 1 1.5 2 2.5Vin (V)

Vo

ut (

V)

Device threshold voltages are kept (virtually) constant

0

0.05

0.1

0.15

0.2

0 0.05 0.1 0.15 0.2

Vin (V)

Vo

ut (

V)

Gain=-1

Device threshold voltages are kept (virtually) constant

CSE477 L04 CMOS Inverter.1Irwin&Vijay, PSU, 2002 ECE484 VLSI Digital Circuits Fall 2010 Lecture 04: CMOS Inverter (static view) Mary Jane Irwin ( mji.

Documents

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v dd v gsp

v slide

v dd v dsp

v dd v dsn

look v dd v

v nmos

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