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tweetbazaarElectronics - Devices

Nov 2, 2013 (3 years and 9 months ago)

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1

CMOS Circuits

2

Combination and Sequential

3

Static Combinational Network



PMOS

Network

NMOS

Network

Inputs

Output

VDD

CMOS Circuits




Pull
-
up network
-
PMOS



Pull
-
down network
-

NMOS



Networks are complementary
to each other



When the circuit is dormant,
no current flows between
supply lines.



Number of the NMOS
transistors (PMOS transistors)
equals to the number of the
inputs.



Output load is capacitive



4

NAND Gates

Transistors in Parallel


1/Rch
eff
= (1/Rch
1
) + (1/Rch
2
)

Transistors in Series

Rch
eff

= Rch
1

+ Rch
2

5

CMOS NAND Gate

DC Analysis


Two possible scenarios:

1. Both inputs are toggling

2. One input is toggling, the
other one set high


Assumptions:

MP2=MP1=MP

MN1=MN2=MN

W/L for MP = (W/L)p

W/L for MN = (W/L)n


Inverter

VTC

6

Gate Sizing

To obtain equal Rise and
Fall time,

Size the series / parallel
transistors to have an
equivalent of a single PU
or PD inverter transistor
in your design

7

Sizing the CMOS Gate

8

NAND Gates: Analysis

Scenario #1
-

Both inputs are toggling

L
-
H > (W/L)eff = 2(W/L)p

H
-
L > (W/L)eff = 1/2(W/L)n

K
R
|
NAND

= 1/4 K
R
|
INV



Scenario #2
-


One input is toggling

L
-
H > (W/L)eff = (W/L)p

H
-
L > (W/L)eff = 1/2(W/L)n

K
R
|
NAND

= 1/2 K
R
|
INV



Vout

Vin

V

OH

V

OL

Vx2 Vx1



Vin=Vout

Inverter

One input toggling

Two inputs toggling

9

NAND Gates: Analysis

Switching Analysis

Scenario #1
-

Both inputs are toggling

t
PLH
|
NAND

= 1/2t
PLH
|
INVERTER

t
PHL
|
NAND

= 2t
PHL
|
INVERTER




Scenario #2
-


One input is toggling

t
PLH
|
NAND

= t
PLH
|
INVERTER

t
PHL
|
NAND

= 2t
PHL
|
INVERTER



10

NAND Gate: Power Dissipation

P
ac
=
α
.f . C V
DD
2



A B X


0 0 1


1 0 1


0 1 1


1 1 0



α

= P (X=1). P (X=0)


assuming A and B have equal probabilities for 1 and 0

α

= (1/4). (3/4)= 3/16

C = C
L

+ C parasitic


11

Increasing the inputs

12

NOR Gate: Analysis

DC Analysis/ AC Analysis


Two possible scenarios:

1. Both inputs are toggling (one is set low)

2. One input is toggling, the other one set high


Assumptions: AP2=BP1=MP


AN1=BN2=MN

W/L for MP = (W/L)p

W/L for MN = (W/L)n


Compare with a CMOS inverter:


MP/MN

K
R
, and the shift in VTC

Propagation delay
t
PLH
and

t
PHL

13

4 INPUT NOR Gate

VDD

A

B

C

D

A

B

C

D

C

L

X

Very slow rise time and rise delays


Could be compensated by increasing

of PMOS transistor size.


Implications:


Silicon Area


Input capacitance

14

Practical Considerations

1. Minimize the use of NOR gates

2. Minimize the fan
-
in of NOR gates

3. Limit the fan
-
in to 4 for NAND gates

4. Use De morgan’s theorem to reduce the number
of fan
-
in per gate


Example:

15

Complex CMOS Gate

16


Reducing Output Capacitance

17

Pseudo nMOS

18

Pseudo nMOS NAND/NOR Gates

From Lecture #4

For acceptable operation

WN=1.5 WP for our Process
respecting min WP

19

Pseudo nMOS Complex Gates

From Lecture #4

For acceptable operation

WN=1.5 WP for our Process
respecting min WP

20

CASCODE LOGIC

Lad is cross coupled
pMOS transistors


Logic is series and
parallel complementary
transistors

Input and Output are in
Complementary forms

21

CSACODE Inverter/Nand Gate

22

CASCODE Complex Gate

23

DCVS trees for a full adder Sum and Carry Pull
-
Down Networks


S’(A,B,C) = A’BC’ + A’B’C + ABC + AB’C’

S (A,B,C) = A’B’C’ + A’BC + ABC’ + AB’C

C(A,B,C) = AB + BC + AC

24

Transmission Gate

Bi
-
directional switch, passes digital signals

Less complex and more versatile than AND gate

Passes analog signals


Problems:


Large ON resistance during transitions of input signals


Large input and output capacitance


(useful for data storage applications)


Capacitive coupling


Applications:

Multiplexers, encoders, latches, registers

various combinational logic circuits


C

C

A

B

25

NMOS/PMOS as Pass Transistors


Vi

Vo

C

CL

NMOS Transistor



Passes weak “1” signal


Vo = V
DD

-
V
TN


Passes “0” signal undegraded

Vo


Vi

V
DD

-
V
TN

Vi

Vo

C

CL

V
DD

-
V
TN


Vo

Vi

-
V
TP

-
V
TP

PMOS Transistor


Passes “1” signal
undegraded


Passes weak “0” signal


Vo=
-
V
TP

26

TX Gate: Characteristics

Vo

Vin


0V |V
TP
| V
DD
-
V
TN

V
DD

nmos:lin nmos:sat nmos:off

pmos:sat pmos:lin pmos:lin

0

27

AND, NAND

A

B

F

0

0

0

0

1

0

1

0

0

1

1

1

28

OR, NOR

A

B

F

0

0

0

0

1

1

1

0

1

1

1

1

29

A multiplexer

C

A

B

F

C

A

B

F

0

0

0

0

1

0

0

0

0

0

1

1

1

0

1

0

0

1

0

0

1

1

0

1

0

1

1

1

1

1

1

1

30

XOR

A

B

F

0

0

0

0

1

1

1

0

1

1

1

0

31

Four to one multiplexer


32

TX Gate: Layout

VDD

VSS

VO

Vi

C

C

C

C

For data path structure

P+

P+

N+

N+

33

NAND Gates: Layout

Layout

Transistors in Series


Transistors in Parallel

34

NAND Gates: Layout

A

B

X

Metal II

Via

VDD


GND

35

NOR Gate: Layout

A

B

X

V
DD

GND

36

Analysis and Design of Complex
Gate

A B C D E F

A B C D E F

VDD

GND

OUT

active

(diffusion)

n+ layer

metal

polysilicon

contact

p+ layer

N
-
well

Analysis


1. Construct the schematic

2. Determine the logic function.

3. Determine transistor sizes.

4. Determine the input pattern to


cause slowest and fastest


operations.

5. Determine the worst case rise


delay (t
PLH
)and fall delay (t
PHL
)

6. Determine the best case rise


and fall delays.