1. 5.5.7 Noise analysis of a CE amplifier
5. SOURCES OF ERRORS Fundamentals of low-noise design Example circuit
5.5.7 Noise analysis of a CE amplifier
VCC RC RS vs RE
VBB rp io
hfe ip ro rb RE RC B E C
icsh vs RS
ibf
ibsh ip
vet
vbt
vst
vct
ro  
hgygygygo

2. Our final aim is to find and minimize the total input noise vn s.
5. SOURCES OF ERRORS Fundamentals of low-noise design Example circuit
Our final aim is to find and minimize the total input noise vn s. rb
vbt B C
hfe ip ip io
icsh rp
vst
ibf
ibsh RS E
vet
vct vs
vn s ? RE RC
hgygygygo
Let us first find vn s by applying superposition.

3. 1+hfe RE/(RE +RS+rb+rp)
5. SOURCES OF ERRORS Fundamentals of low-noise design Example circuit
1) Signal gain As for vs, vst, vbt, and vet. rb
vbt B C
hfe ip ip io rp
vst RS E
vet vs RE RC
hgygygygo
As  = Gs Gs bs fwd
AOL
1+AOLb
_______ io vs
___
As = 1
RS+rb+rp+RE
___________
-hfe
1+hfe RE/(RE +RS+rb+rp)
____________________
+ 0

4. 1+hfe RE/(RE +RS+rb+rp)
5. SOURCES OF ERRORS Fundamentals of low-noise design Example circuit
2) Noise gain Abf for ibf and ibsh. rb B C
hfe ip ip io rp
ibf
ibsh RS E vs RE RC
hgygygygo
Abf  = Gbf Gbf bbf fwd
AOL
1+AOLb
_______ io
ibf
___
Abf =
RS+rb+RE
RS+rb+RE +rp
___________
hfe
1+hfe RE/(RE +RS+rb+rp)
____________________
+ 0

5. 1+hfe RE/(RE +RS+rb+rp)
5. SOURCES OF ERRORS Fundamentals of low-noise design Example circuit
3) Noise gain Acsh for icsh. rb B C
hfe ip ip io
icsh rp RS E vs RE RC
hgygygygo
Acsh  = Gcsh Gcsh bcsh fwd
AOL
1+AOLb
_______ io
icsh
___
Acsh = RE
RE +RS+rb+rp
- ___________
hfe
1+hfe RE/(RE +RS+rb+rp)
____________________
- 1

6. ___ ___ io vct Act  = Dct 1 Act = - RC 4) Noise gain Act for vct. rb
5. SOURCES OF ERRORS Fundamentals of low-noise design Example circuit
4) Noise gain Act for vct. rb B C
hfe ip ip rp RS E io vs
vct /RC RE RC
hgygygygo
Act  = Dct io
vct
___
Act = - 1 RC
___

7. __________ _______ _____ ________ _____ RSbE = RS +rb+RE
5. SOURCES OF ERRORS Fundamentals of low-noise design Example circuit
5) Total input noise vs. time, vn s. rb B C
hfe ip ip io
icsh rp RS
ibf
ibsh
vn s E vs RE RC
RSbE = RS +rb+RE
hgygygygo
vn s(t) = vst +vbt +vet
(ibf +ibsh) Abf As
__________ +
icsh Acsh
_______
vct Act
_____
vn s2( f ) = 4kT RSbE+(ibf 2+ibsh2) RSbE2
(RSbE+rp)2
hfe2
________
+ icsh2
+ 4kT 1
RC As2
_____
 0