MOSFET Common Source Amplifierstorage.googleapis.com/wzukusers/user-19186504/documents...MOSFET...

BITS Pilani, Dubai Campus

Lecture on Microelectronics Circuits Dr. Vilas

Microelectronic Circuits

MOSFET Common Source

Amplifier



Small signal Model

The definition of Transconductance

The definition of output resistance

The definition of voltage gain

OVn

VvGS

Dm V

L

Wk

v

ig

GSGS

'

D

A

IiD

DSo

I

V

i

vr

DD

Dm

i

ov Rg

v

vA



Small signal model

(a) neglecting the channel-length modulation effect

(b) including the effect of channel-length modulation, modeled by output resistance ro = |VA| /ID.



Small signal model

(a) The T model of the MOSFET augmented with the drain-to-source resistance ro.

(b) An alternative representation of the T model.



Single Stage Cs Amplifier

Input resistance with no load

Input resistance

Open-circuit voltage gain

Voltage gain

LRi

ii

i

vR

LRi

ovo

v

vA

i

ov

v

vA

i

ii

i

vR

Open-circuit overall voltage gain

overall voltage gain

LRsig

vov

vG 0

sig

vv

vG 0




Small Signal equivalent circuit




Input resistance

Voltage gain

Overall voltage gain

Output resistance

Gin RR

)////( LDomv RRrgA

)////( oLDmsigG

Gv rRRg

RR

RG

Doout RrR //



Common Source Amplifier

Very high input resistance

Moderately high voltage gain

Relatively high output resistance



CS Amplifier with Source resistance




Input resistance

Voltage gain


Output resistance

Gin RR

Sm

LDmv

Rg

RRgA

1

)//(

Sm

LDm

sigG

Gv

Rg

RRg

RR

RG

1

)//(

Dout RR




Including RS results in a gain reduction by the factor (1+gmRS)

RS takes the effect of negative feedback.



Common Gate Amplifier




Input resistance

Voltage gain


Output resistance

min gR /1

)( LDmv RRgA //

Dout RR

sigm

LDmv

Rg

RRgG

1

)( //




Noninverting amplifier

Low input resistance

Has nearly identical voltage gain of CS amplifier, but the

overall voltage gain is smaller by the factor (1+gmRsig)

Relatively high output resistance

Current follower

Superior high-frequency performance



Common Drain or voltage follower




Input resistance

Voltage gain


Output resistance

Gin RR

mL

Lv

gR

RA

1

mL

L

sigG

Gv

gR

R

RR

RG

1

mout

gR

1




Very high input resistance

Voltage gain is less than but close to unity

Relatively low output resistance

Voltage buffer amplifier

Power amplifier



MOSFET Internal Capacitances

The MOSFET Internal Capacitance and High-Frequency Model

Internal capacitances

The gate capacitive effect

Triode region

Saturation region

Cutoff region

Overlap capacitance

The junction capacitances

Source-body depletion-layer capacitance

Drain-body depletion-layer capacitance





MOSFET operates at triode region

MOSFET operates at saturation region

MOSFET operates at cutoff region

oxgdgs WLCCC 21

0

32

gd

oxgs

C

WLCC

oxgb

gdgs

WLCC

CC 0





Overlap capacitance results from the fact that the source and drain diffusions extend slightly under the gate oxide. The expression for overlap capacitance

Typical value

This additional component should be added to Cgs and Cgd in all preceding formulas.

oxovov CWLC

LLov 1.005.0





Source-body depletion-layer capacitance

Drain-body depletion-layer capacitance

o

SB

sbsb

V

V

CC

＋1

0

o

DB

dbdb

V

V

CC

＋1

0

Csb at zero body-source bias

Magnitude of reverse bias voltage

Built in voltage (0.6V to 0.8V)



High Frequency Response


gmVgsR’L

Cgd

Cgs

G D

S

+

Vo

-

+

Vgs

-

Gsig

'

sigR||RR

sigG

G

sigRR

RV

LDo

'

LR||R||rR





Thevenin’s Equivalent Parallel Combination

This circuit can be further simplified




gmVgsR’L

Cgd

Cgs

G D

S

+

Vo

-

+

Vgs

-

Gsig

'

sigR||RR

sigG

G

sigRR

RV

X

X’

Igd

Cgd is a bridging capacitor , which connects the output node to the input node.




gmVgsR’L

Cgd

Cgs

G D

S

+

Vo

-

+

Vgs

-

Gsig

'

sigR||RR

sigG

G

sigRR

RV

X

X’

Igd

The load current = )( gdgsm IVg

gs

'

Lm

'

LgsmoVRgR)Vg(V




gmVgsR’L

Cgd

Cgs

G D

S

+

Vo

-

+

Vgs

-

Gsig

'

sigR||RR

sigG

G

sigRR

RV

X

X’

Igd

gs

'

Lmgd

gs

'

Lmgsgd

ogsgdgd

VRgsC

VRgVsC

VVsCI

1




gmVgsR’L

Cgd

Cgs

G D

S

+

Vo

-

+

Vgs

-

Gsig

'

sigR||RR

sigG

G

sigRR

RV

X

X’

Igd

At XX’ the existence of Cgd is known through the current Igd, so Cgd can be replaced by Ceq




Look at the input part of the circuit , which is a single time constant circuit of low –pass type.




0

1

1

w

sV

RR

RV

sig

sigG

G

gs

Gsigsig

Lmgdgseqgsin

sigin

RRR

RgCCCCCwhereRC

w

||

)(,

'

'

'

11

circuitSTCoffrequencybreakorcorneriswwhere

0

0




gs

'

Lm

'

LgsmoVRgR)Vg(V

0

1

1

w

sV

RR

RV

sig

sigG

G

gs

0

1

1

w

s)Rg(

RR

R

V

V 'Lm

sigG

G

sig

o

Combining We get,




Which can be expressed in the form

Mid band gain

H

M

sig

o

w

s

A

V

V

1

'

sigin

H

H

'

sigin

H

RC

wfand

RCww

2

1

2

10




Thus we can see that, high frequency response will be that of a low pass filter. With 3dB cut off frequency fH

3dB

20 log|AM|

dBV

V

sig

o

f (Hz)

(log scale)

fH




Find the midband gain AM and the upper 3-dB frequency fH

of a CS amplifier fed with a signal source having an internal

resistance Rsig = 100kΩ. The amplifier has RG = 4.7MΩ, RD , RL

=15kΩ, gm = 1mA/V, r0 = 150kΩ, Cgs = 1pF and Cgd = 0.4pF.

For the CS amplifier for above example, find the value of AM

and the upper 3-dB frequency fH if the source resistance

reduced to Rsig = 10kΩ.

Ans: -7. 12V/V, 3.7MHz.

MOSFET Common Source Amplifierstorage.googleapis.com/wzukusers/user-19186504/documents...MOSFET...

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