Chapter 5 MOSFET 3 - College of · PDF filePerforming the analysis directly on the circuit...

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

C H A P T E R 5

Amplifier Design


The Common-Source

Amplifier

)||)(( 00 rRvgv Dgsm−=

)||( 00 rRgA Dmv −=


Performing the analysis directly on the circuit diagram with the

MOSFET model used implicitly.


The CS amplifier with a source resistance Rs: (a) Circuit without

bias details; (b) Equivalent circuit with the MOSFET

represented by its T model.

sm

Dm

vRg

RgA

+

−=

10

Sm

LDm

s

m

LD

vRg

RRg

Rg

RRA

+

−=

+

−=

1

)||(

1

||0


Figure 5.48 (a) Common-gate (CG) amplifier with bias arrangement omitted. (b)

Equivalent circuit of the CG amplifier with the MOSFET replaced with its T model.

The Common-Gate (CG) Amplifier

Dm

i

v Rgv

vA ==

00


Figure 5.49 Illustrating the need for a unity-gain buffer amplifier.

The Common-Drain Amplifier or Source Follower

m

L

L

i

v

gR

R

v

vA

10

0

+

==

1, 0 =∞= vL AR

mgR

10 =

Summary and Comparison


Biasing in MOS Amplifier



Figure 5.51 The use of fixed bias (constant VGS) can result in a large

variability in the value of ID. Devices 1 and 2 represent extremes among units

of the same type.

Biasing by Fixing VGS


Figure 5.52 Biasing using a fixed voltage at the gate, VG, and a

resistance in the source lead, RS: (a) basic arrangement; (b) reduced

variability in ID

Biasing by Fixing VG and Connecting of Resistance in the Source

DSGSG IRVV −=


Figure 5.52 Biasing using a fixed voltage at the gate, VG, and a resistance in the

source lead, RS: (c) practical implementation using a single supply; (d) coupling of a signal source to the gate using a capacitor CC1; (e) practical implementation

using two supplies.

Biasing by Fixing VG and Connecting of Resistance in the Source


Example 5.12.


Biasing the MOSFET using a large drain-to-gate feedback

resistance, RG.

DDGSDD

DDDDDSGS

IRVV

IRVVV

+=

−==


Figure 5.55 (a) Biasing the MOSFET using a constant-current source I. (b) Implementation of the constant-current source I using a current mirror.

Biasing Using a Constant-Current Source

2

1

'

1 )()(2

1tGSnD VV

L

WkI −=

2

2

'

2 )()(2

1tGSnD

VVL

WkII −==

R

VVVII GSSSDD

REFD

−+

==1

1

2

)/(

)/(

LW

LWII REF=

Current Mirror

Circuit


Figure E5.37


Figure 5.56 Basic structure of the circuit used to realize single-stage,

discrete-circuit MOS amplifier configurations.

Basic Structure


Figure 5.57 (a) Common-source amplifier based on the circuit of Fig. 5.56. (b)

Equivalent circuit of the amplifier for small-signal analysis.

Common Source (CS) Amplifier

Gin RR =

)||||( 0rRRgRR

RG LDm

sigG

G

v+

−=


Figure 5.58 (a) Common-source amplifier with a resistance RS in the

source lead

Common Gate (CG) Amplifier


Figure 5.60 (a) A source-follower amplifier.

(b) Small-signal, equivalent-circuit model.

The Source Follower

m

L

L

sigG

G

v

grR

rR

RR

RG

1)||(

||

0

0

++

=


Figure 5.61 A sketch of the frequency response of a CS amplifier

delineating the three frequency bands of interest.

BW=fH-fL

GB=|AM|BW


Figure 5.62 Small-signal, equivalent-circuit model of a MOSFET in

which the source is not connected to the body.

The role of the Substrate – The Body Effect


Figure 13.17 The CMOS inverter.

The Inverter


Input = Vdd


Input = GND


Figure 13.20 The voltage-transfer characteristic of the CMOS

inverter when QN and QP are matched.


Figure 13.22 Dynamic operation of a capacitively loaded CMOS inverter: (a) circuit;

(b) input and output waveforms; (c) equivalent circuit during the capacitor discharge;

(d) trajectory of the operating point as the input goes high and C discharges through

QN.

Chapter 5 MOSFET 3 - College of · PDF filePerforming the analysis directly on the circuit...

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