Fundamentals of Microelectronicsocw.snu.ac.kr/sites/default/files/NOTE/ch07.pdf · 1 Fundamentals...

1

Fundamentals of Microelectronics

CH1 Why Microelectronics?

CH2 Basic Physics of Semiconductors

CH3 Diode Circuits

CH4 Physics of Bipolar Transistors

CH5 Bipolar Amplifiers

CH6 Physics of MOS Transistors

CH7 CMOS Amplifiers

CH8 Operational Amplifier As A Black Box

CH16 Digital CMOS Circuits

2

Chapter 7 CMOS Amplifiers

7.1 General Considerations

7.2 Common-Source Stage

7.3 Common-Gate Stage

7.4 Source Follower

7.5 Summary and Additional Examples

CH7 CMOS Amplifiers 3

Chapter Outline

CH7 CMOS Amplifiers 4

MOS Biasing

Voltage at X is determined by VDD, R1, and R2.

VGS can be found using the equation above, and ID can be found by using the NMOS current equation.

Soxn

THDD

THGS

THGSoxnDSDGSDD

RL

WC

V

VRR

VRVVVVV

VVL

WCIRIV

RR

VR

1

2

2

1 &&

1

21

21

2

11

2

21

2

CH7 CMOS Amplifiers 5

Self-Biased MOS Stage

The circuit above is analyzed by noting M1 is in saturation and no potential drop appears across RG.

DDDSGSDDVIRVRI

CH7 CMOS Amplifiers 6

Current Sources

When in saturation region, a MOSFET behaves as a current source.

NMOS draws current from a point to ground (sinks current), whereas PMOS draws current from VDD to a point (sources current).

CH7 CMOS Amplifiers 7

Common-Source Stage

DDoxnv

Dmv

RIL

WCA

RgA

2

0

CH7 CMOS Amplifiers 8

Operation in Saturation

In order to maintain operation in saturation, Vout cannot fall below Vin by more than one threshold voltage.

The condition above ensures operation in saturation.

THGSDDDD

THDDDDGS

THoutin

VVVIR

VIRVV

VVV

)(

Example 7.4

9

10

CS Stage with =0

Lout

in

Lmv

RR

R

RgA

CH7 CMOS Amplifiers 11

CS Stage with 0

However, Early effect and channel length modulation affect

CE and CS stages in a similar manner.

OLout

in

OLmv

rRR

R

rRgA

||

||

CH7 CMOS Amplifiers 12

CS Gain Variation with Channel Length

Since is inversely proportional to L, the voltage gain actually becomes proportional to the square root of L.

D

oxn

D

oxn

vI

WLC

I

L

WC

A

2

2

CH7 CMOS Amplifiers 13

CS Stage with Current-Source Load

To alleviate the headroom problem, an active current-source load is used.

This is advantageous because a current-source has a high output resistance and can tolerate a small voltage drop across it.

21

211

||

||

OOout

OOmv

rrR

rrgA

CH7 CMOS Amplifiers 14

PMOS CS Stage with NMOS as Load

Similarly, with PMOS as input stage and NMOS as the load,

the voltage gain is the same as before.

)||(212 OOmv

rrgA

CH7 CMOS Amplifiers 15

CS Stage with Diode-Connected Load

Lower gain, but less dependent on process parameters.

12

2

1

2

1

2

1

||||1

/

/1

OO

m

mv

m

mv

rrg

gA

LW

LW

ggA

16

CS Stage with Diode-Connected PMOS Device

Note that PMOS circuit symbol is usually drawn with the

source on top of the drain.

21

1

2||||

1oo

m

mvrr

ggA

CH7 CMOS Amplifiers 17

CS Stage with Degeneration

Similar to bipolar counterpart, when a CS stage is

degenerated, its gain, I/O impedances, and linearity change.

0

11

0

Dv

S DS

m

RA

R RR

g r

0

1

S

m

D

v

Rg

RA

CH7 CMOS Amplifiers 18

Example of CS Stage with Degeneration

A diode-connected device degenerates a CS stage.

21

11

mm

Dv

gg

RA

CH7 CMOS Amplifiers 19

CS Stage with Gate Resistance

Since at low frequencies, the gate conducts no current, gate resistance does not affect the gain or I/O impedances.

0G

RV

CH7 CMOS Amplifiers 20

Output Impedance of CS Stage with Degeneration

Similar to the bipolar counterpart, degeneration boosts

output impedance.

OSOmoutrRrgr

CH7 CMOS Amplifiers 21

Output Impedance Example (I)

When 1/gm is parallel with rO2, we often just consider 1/gm.

22

11

111

mm

mOoutgg

grR

CH7 CMOS Amplifiers 22

Output Impedance Example (II)

In this example, the impedance that degenerates the CS

stage is rO, instead of 1/gm in the previous example.

1211 OOOmoutrrrgR

CH7 CMOS Amplifiers 23

CS Core with Biasing

Degeneration is used to stabilize bias point, and a bypass

capacitor can be used to obtain a larger small-signal

voltage gain at the frequency of interest.

Dm

G

v

S

m

D

G

vRg

RRR

RRA

Rg

R

RRR

RRA

21

21

21

21

||

||,

1||

||

CH7 CMOS Amplifiers 24

Common-Gate Stage

Common-gate stage is similar to common-base stage: a

rise in input causes a rise in output. So the gain is positive.

DmvRgA

CH7 CMOS Amplifiers 25

Signal Levels in CG Stage

In order to maintain M1 in saturation, the signal swing at Vout

cannot fall below Vb-VTH.

Example 7.12

26

CH7 CMOS Amplifiers 27

I/O Impedances of CG Stage

The input and output impedances of CG stage are similar to those of CB stage.

DoutRR

m

ing

R1

0

CH7 CMOS Amplifiers 28

CG Stage with Source Resistance

When a source resistance is present, the voltage gain is equal to that of a CS stage with degeneration, only positive.

S

m

D

v

Rg

RA

1

CH7 CMOS Amplifiers 29

Generalized CG Behavior

When a gate resistance is present it does not affect the gain and I/O impedances since there is no potential drop across it ( at low frequencies).

The output impedance of a CG stage with source resistance is identical to that of CS stage with degeneration.

OSOmout

rRrgR 1

CH7 CMOS Amplifiers 30

Example of CG Stage

Diode-connected M2 acts as a resistor to provide the bias current.

DOS

m

OmoutRrR

grgR ||||

11

2

11

Smm

Dm

in

out

Rgg

Rg

v

v

21

1

1

CH7 CMOS Amplifiers 31

CG Stage with Biasing

R1 and R2 provide gate bias voltage, and R3 provides a path

for DC bias current of M1 to flow to ground.

Dm

Sm

m

in

out RgRgR

gR

v

v

/1||

/1||

3

3

CH7 CMOS Amplifiers 32

Source Follower Stage

1v

A

CH7 CMOS Amplifiers 33

Source Follower Core

Similar to the emitter follower, the source follower can be analyzed as a resistor divider.

LO

m

LO

in

out

Rrg

Rr

v

v

||1

||

Example 7.17

34

CH7 CMOS Amplifiers 35

Source Follower Example

In this example, M2 acts as a current source.

21

1

21

||1

||

OO

m

OOv

rrg

rrA

CH7 CMOS Amplifiers 36

Output Resistance of Source Follower

The output impedance of a source follower is relatively low, whereas the input impedance is infinite ( at low frequencies); thus, a good candidate as a buffer.

L

m

LO

m

outR

gRr

gR ||

1||||

1

1|| if 0

1

Do O D

m m O

RR r R

g g r

CH7 CMOS Amplifiers 37

Source Follower with Biasing

RG sets the gate voltage to VDD, whereas RS sets the drain

current.

The quadratic equation above can be solved for ID.

2

2

1THSDDDoxnD

VRIVL

WCI

CH7 CMOS Amplifiers 38

Supply-Independent Biasing

If Rs is replaced by a current source, drain current IDbecomes independent of supply voltage.

CH7 CMOS Amplifiers 39

Example of a CS Stage (I)

M1 acts as the input device and M2, M3 as the load.

321

3

321

3

1

||||||1

||||||1

OOO

m

out

OOO

m

mv

rrrg

R

rrrg

gA

CH7 CMOS Amplifiers 40

Example of a CS Stage (II)

M1 acts as the input device, M3 as the source resistance,

and M2 as the load.

3

31

2

||11

O

mm

Ov

rgg

rA

CH7 CMOS Amplifiers 41

Examples of CS and CG Stages

With the input connected to different locations, the two circuits, although identical in other aspects, behave differently.

1_

2

1O

v CG

S

m

rA

Rg

_ 1 2 2 2 1(1 ) ||v CS m m O S O OA g g r R r r

CH7 CMOS Amplifiers 42

By replacing the left side with a Thevenin equivalent, and

recognizing the right side is actually a CG stage, the

voltage gain can be easily obtained.

Example of a Composite Stage (I)

21

11

mm

Dv

gg

RA

CH7 CMOS Amplifiers 43

Example of a Composite Stage (II)

This example shows that by probing different places in a circuit, different types of output can be obtained.

Vout1 is a result of M1 acting as a source follower whereas Vout2 is a result of M1 acting as a CS stage with degeneration.

1

2

2

43

32

1||

1

||||1

m

O

m

OO

m

in

out

gr

g

rrg

v

v