12.5 Common Source Amplifiers

vin?

vo?

Voltage gainAv=vo/ vin?

Common-Source Amplifiers

•C1 and C2 are coupling capacitors and Cs is the bypass capacitor. The capacitors are intended to have large impedances for the dc signal and very small impedances for the ac signal.

large is or ,0,0

0,

2

1

CfZc

fZc

fCjZc

Common-Source Amplifiers

•For DC analysis, the capacitors are replaced by open circuits to determine the quiescent operation point (Q point). The transconductance gm for the small-signal equivalent circuit is also determined.

•For AC analysis, the capacitor are replaced by short circuits to determine the ac voltage gain Av=vo/vin.

Coupling capacitors

Bypass capacitors

DC voltage sourcesDC Analysis

The Small-Signal Equivalent Circuit

•In small-signal midband analysis of FET amplifiers, the coupling capacitors, bypass capacitors, and dc voltage sources are replaced by short circuits. •The FET is replaced with its small-signal equivalent circuit. Then, we write circuit equations and derive useful expressions for gains, input impedance, and output impedance.

Coupling capacitors

Bypass capacitors

DC voltage sourcesAC Analysis

Coupling capacitors

Bypass capacitors

DC voltage sources

SMALL-SIGNAL EQUIVALENT CIRCUITS

(12.4)

A more complex equivalent circuit consider drain resistance rd

)()( tvgti gsmd

ddsgsmd rvtvgti /)()(

point

1

QDS

Dd v

ir

Common Source Amplifiers: FET source 端接ground

Common Source Amplifiers:

LDdL RRr

R111

1

Lmo

v Rgv

vA

in

Equivalent load resistance

Input voltage & output voltage

gsin

Lgsmo

vv

Rvgv

Voltage Gain

Common Source Amplifiers:

Input Resistance

Output resistance •disconnect the load,•replace the signal source by the internal resistance,• find the resistance by looking into the output terminals.

21in

inin RRR

i

vR G

dDo rR

R11

1

Example 12.4

Analyze the following circuit. KP=50uA/V2, Vto=2 V, L=10um, W=400um (identical to example 12.2).Assume

Find

•midband voltage gain

•input resistance

•output resistance

•output voltage

dr

ttv mV)2000sin(100)(

Example 12.4

Analyze the following circuit. KP=50uA/V2, Vto=2 V, L=10um, W=400um (identical to example 12.2).

DC Analysis

Fine Q point (see example 12.2)

mAVVKI toGSQDQ 784.0)( 2

Example 12.4

Analyze the following circuit. KP=50uA/V2, Vto=2 V, L=10um, W=400um (identical to example 12.2).

AC Analysis

Fine gm(see Ch 12.4)

mSILWKPVVKg DQtoGSQm 77.1/2)(2

3197111

1

LDdL RRr

R

Equivalent load resistance

dr( )

66.5in

Lmo

v Rgv

vA

Voltage Gain

Example 12.4

Analyze the following circuit. KP=50uA/V2, Vto=2 V, L=10um, W=400um (identical to example 12.2).

Input Resistance

kRRRi

vR G 75021

in

inin

Output Resistance

kRrR

R DdD

o 7.411

1

Input voltage mV)2000sin(23.88)()()( tRR

Rtvtvtv

in

ingsin

Output voltage mV)2000sin(500)()( ttvAtv invo