CMOS Topic 5 -_cmos_inverter

cmos_inverter

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EE603 – CMOS IC DESIGN
Topic 5 – CMOS Inverter
Faizah Amir
POLISAS
TEKNOLOGI
TE RAS
P E MBANGUNAN
Lesson Learning Outcome
1) To explain the Switch Models of CMOS inverter
2) To explain the properties of static CMOS Inverter:
a. CMOS Voltage Transfer Characteristic (VTC)
b. Switching Threshold
c. Noise Margin
3) To explain the performance of static CMOS Inverter in
terms of:
a. Process Variation
b. Supply Voltage Scaling
4) To explain the dynamic behaviour of CMOS inverter

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Inverter Switch Models
⇒When V
inis high and equal to V
DD, the NMOS transistor is ON, while the
PMOS is OFF. A direct path exists between V
outand the ground node,
resulting in a steady-state value of 0V.
⇒When the input voltage is low (0 V), NMOS transistor is OFF, while PMOS
transistors in ON. A direct path exists between V
DDand V
out, resulting in a
steady-state value of V
DD.
CMOS Properties
1. High noise margins : output high level = V
DD
output low level = 0V
2. Ratioless gate – gates will work correctly for any ratio of
PMOS size to NMOS size , so the transistors can be minimum
size.
3. Low output impedance (output resistance in kWrange) which
makes it less sensitive to noise and disturbances.
4. Extremely high input resistance (MOS transistor gate is a
virtually perfect insulator and draws no dc input current.) ⇒
large fan-out.
5. Low static power consumption because no direct path exists
between the supply and ground rails under steady-state
operating conditions .

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NMOS & PMOS Operation in CMOS
NMOS & PMOS I-V Curve

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NMOS & PMOS I-V Curve
Combining the VI characteristics
of the n-device and p-device
I
DSp= -I
DSn
V
GSn = Vin ; V
GSp = Vin - V
DD
V
DSn= Vout ; V
DSp= Vout - V
DD
For a DC operating points to be valid, the
currents through NMOS and PMOS
devices must be equal.
The DC points are located at the
intersection of corresponding load lines,
as marked with dots on the graph.
CMOS Inverter VTC
CMOS Inverter VTC is produced from both
NMOS and PMOS IV curve.

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CMOS Inverter VTC
CMOS Inverter Ideal VTC
dIdeally, VTC
appears as an
inverted step-
function.
dWe can see
the precise
switching
between ON
and OFF.
Logic ‘1’ output
Logic ‘0’ output
CMOS Inverter VTC
VTC for real CMOS Inverter
dIn real
devices, a
gradual
transition
region exists.
dWe cannot
see the precise
switching
between ON
and OFF.

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CMOS INVERTER
●V
M, The switching
threshold is defined as the
point where the intersection
of the VTC curve and the
line given by V
out= V
in.
●The switching threshold
voltage presents the
midpoint of the switching
characteristics.
●A good inverter must
have the value V
M= V
DD/2
●At switching threshold,
V
in= V
out= V
M
V
M
V
out= V
in
Switching Threshold
CMOS INVERTER
Noise Margin
Typical inverter transfer characteristics
Input Low Voltage, V
IL
– V
ILis at point ‘a’ on the plot where
the slope dV
in/dV
out= -1
– V
in such that V
in< V
IL= logic 0
Input High Voltage, V
IH
– V
IHis at point ‘b’ on the plot where
the slope dV
in/dV
out= -1
– V
insuch that V
in> V
IH= logic 1
Noise Margin
– measure of how stable inputs are
with respect to signal interference
– NM
H= V
OH -V
IH
– NM
L= V
IL-V
OL
– large N
MH and N
ML is desired for
noise immunity:
NM
H = V
DD–V
IH
NM
L = V
IL - 0
●a
●b

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NOISE MARGIN
NOISE MARGIN

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CMOS INVERTER
Effect of Transistor Size on VTC
When designing static CMOS circuits, it is advisable to
balance the driving strengths of the transistors by
making the width of the PMOS two or three times
than the width of NMOS .
Ideally :
CMOS INVERTER
Effect of Transistor Size on VTC
IEffect on switching threshold:
V
M= V
DD/2, exactly in the middle.
IEffect on noise margin:
V
IH and V
ILboth are close to V
Mand
noise margin is good.
If :

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CMOS INVERTER
Impact of process variation on VTC Curve
The “good” transistor has:
smaller oxide thickness
smaller length
higher width
smaller threshold voltage
The “bad” transistor has:
larger oxide thickness
larger length
lower width
larger threshold voltage
Conclusion:
The variations cause a small shift
in the switching threshold, but that
the operation of the gate is not
affected.
Process variations (mostly) cause a
shift in the switching threshold.
●
●
●
CMOS INVERTER
Impact of process variation on VTC Curve
Result from changing (W/L)
p / (W/L)
nratio:
•Inverter threshold V
M¹V
DD/2
•Rise and fall delays are unequal
•Noise margins are not equal

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CMOS INVERTER
Impact of supply voltage scaling
• The inverter characteristic can still
be obtained although the supply
voltage is small (not even large
enough to turn the transistors on.)
• How does this happen?
Because of sub-threshold
operation of the transistors.
The sub-threshold currents are
sufficient to switch the gate
between low and high levels,
and provide enough gain to
produce acceptable VTCs.
• However, the very low value of the
switching currents will slow down
the operation of the transistor.
CMOS Inverter : Dynamic Behaviour
The Switch Model of Dynamic CMOS Inverter

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CMOS Inverter : Dynamic Behaviour
Parasitic Capacitances of CMOS inverter
Parasitic capacitances, influencing the transient behaviour of
the cascaded inverter pair.
CMOS Inverter : Dynamic Behaviour
Parasitic Capacitances – Miller Effect
The Miller effect— A capacitor that has identical but opposite
voltage swings at both its terminals can be replaced by a capacitor
to ground, whose value is two times the original value.

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CMOS Inverter : Dynamic Behaviour
Sources of parasitic capacitance:
i. Intrinsic MOS transistor capacitances
ii. Extrinsic MOS transistor (fan-out)
capacitances
iii. Wiring (interconnect) capacitance
Sources of Parasitic Capacitances
•MOS Intrinsic Capacitances
Structure capacitances
Channel capacitances
Diffusion capacitances

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Intrinsic Capacitances
MOS Structure Capacitance
Overlap Capacitance (linear):
C
GSO= C
GDO= C
oxx
dWL = Co WL
D
Intrinsic Capacitances
Overlap Capacitance
In reality the gate overlaps
source and drain.
So, the parasitic overlap
capacitances:
C
GS(overlap) = C
oxW L
D
C
GD(overlap) = C
oxW L
D

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Intrinsic Capacitances
MOS Channel Capacitances
•The gate-to-channel capacitance depends upon the
• operating region and the terminal voltages
Intrinsic Capacitances
MOS Channel Capacitances
Average Channel Capacitance

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Intrinsic Capacitances
MOS Diffusion Capacitances
•The junction (or diffusion) capacitance is from the
reverse biased source-body and drain-body p-n
junctions.
Intrinsic Capacitances
Source Junction View

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Extrinsic Capacitance
Extrinsic (Fan-out) Capacitance
The extrinsic, or fanout, capacitance is the total gate
capacitance of the loading PMOS and NMOS transistors.
C
fanout= C
gate(NMOS) + C
gate(PMOS)
= (C
GSOn+ C
GDOn+ W
nL
nC
ox) +
(C
GSOp+ C
GDOp+ W
pL
pC
ox)
Simplification of the actual situation
• Assumes all the components of C
gateare between V
outand
GND (or V
DD)
• Assumes the channel capacitances of the loading gates are
constant
MOS Capacitance Model

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Summary
1. When designing static CMOS circuits, the width of PMOS
must be made two or three times than the width of NMOS
in order to obtain symmetrical switching threshold, V
M and
noise margin.
2.The variations of NMOS and PMOS in CMOS cause a small
shift in the switching threshold, but that the operation of the
gate is not affected.
3. The inverter characteristic can still be obtained although the
supply voltage is scaled down, as long as the minimum
supply voltage is higher than thermal voltage
4. Parasitic capacitances slower the switching speeds
=>Bigger capacitance means more charges are needed to change
voltage.
Past Years Questions
1. a) Sketch a complete CMOS inverter circuit diagram.
(2 marks)
2. Explain the CMOS inverteroperation.
(2 marks)
3. Based of the Voltage Transfer Characteristics (VTC) curve
below, explain the transition region when both NMOS and
PMOS are in saturation. (5 marks)

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