1

5.5.7 Noise analysis of a CE amplifier

RS

RC

RE

VCC

VBB

vs

5. SOURCES OF ERRORS. 5.5. Fundamentals of low-noise design. 5.5.7. Example circuit

r

io

hfe i

ro

rb

RE RC

B

E

C

icsh

vs

RS

ibf ibsh

i

vet

vbt

vst

vct

ro

2

Our final aim is to find and minimize the total input noise vn s.

5. SOURCES OF ERRORS. 5.5. Fundamentals of low-noise design. 5.5.7. Example circuit

r

io

hfe i

rb

RE RC

B

E

C

icsh

vs

RS

ibf ibsh

i

vet

vbt

vst

vn s

?vct

Let us first find vn s by applying superposition.

3

As Gs Gs s fwd AOL

1AOL

io

vs

1) Signal gain As for vs, vst, vbt, and vet.

5. SOURCES OF ERRORS. 5.5. Fundamentals of low-noise design. 5.5.7. Example circuit

r

io

hfe i

rb

RE RC

B

E

C

vs

RS

i

As 1

RSrbrRE

hfe

1hfe RE/(RE RSrbr) 0

vet

vbt

vst

4

Abf Gbf Gbf bf fwd AOL

1AOL

io

ibf

2) Noise gain Abf for ibf and ibsh.

5. SOURCES OF ERRORS. 5.5. Fundamentals of low-noise design. 5.5.7. Example circuit

r

io

hfe i

rb

RE RC

B

E

C

vs

RS

ibf ibsh

i

Abf RSrbRE

RSrbRE r

hfe

1hfe RE/(RE RSrbr) 0

5

Acsh Gcsh Gcsh csh fwd AOL

1AOL

io

icsh

r

io

hfe i

rb

RE RC

B

E

C

icsh

vs

RS

i

3) Noise gain Acsh for icsh.

5. SOURCES OF ERRORS. 5.5. Fundamentals of low-noise design. 5.5.7. Example circuit

Acsh RE

RE RSrbr

hfe

1hfe RE/(RE RSrbr) 1

6

Act Dct

io

vct

4) Noise gain Act for vct.

5. SOURCES OF ERRORS. 5.5. Fundamentals of low-noise design. 5.5.7. Example circuit

r

io

hfe i

RE RC

B

E

C

vs

RS

i

vct /RC

rb

Act 1

RC

7

5) Total input noise vs. time, vn s.

5. SOURCES OF ERRORS. 5.5. Fundamentals of low-noise design. 5.5.7. Example circuit

r

io

hfe i

RE RC

B

E

C

vs

RS

i

vn s

vn s(t) vst vbt vet

)ibf ibsh( Abf

As

icsh Acsh

As

vct Act

As

rb

vn s2( f ) 4kT RSbE+(ibf

2ibsh2) RSbE

2)RSbEr(2

hfe2

icsh2 4kT

1

RC As2

0

RSbE RS rbRE

icsh

ibf ibsh