multistage amplifiers - rationalepersonal.stevens.edu/~bmcnair/ee359-s13/class13.pdf · 18...
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16
Multistage amplifiers - rationale
High input Z for minimal
loading
Low output Z for minimal impact
from load
High power output
High gain
17
Multistage amplifiers - rationale
High input Z for minimal
loading
Low output Z for minimal impact
from load
High power output
High gain Conflicting requirements
18
Multistage amplifiers - rationale
(differential input for
noise immunity)
Low output Z for minimal impact
from load
High power output
High gain (in multiple stages)
High input Z for minimal
loading
19
Multistage amplifiers (CMOS)
20
Multistage amplifiers (CMOS)
Differential input
21
Multistage amplifiers (CMOS) Current mirror
Differential input
22
Multistage amplifiers (CMOS) Current mirror
Differential input
Active load current mirror
23
Multistage amplifiers (CMOS) Current mirror
Differential input
Output stage
Active load current mirror
24
Multistage amplifiers (CMOS) Current mirror
Differential input
Output stage
Active load current mirror
Current source
25
Multistage amplifiers (BJT)
26
Multistage amplifiers (BJT)
Differential input
27
Multistage amplifiers (BJT)
Differential input
Differential single ended 2nd stage
28
Multistage amplifiers (BJT)
Differential input
3rd stage, plus DC shift Differential single ended 2nd stage
29
Multistage amplifiers (BJT)
Differential input
Emitter follower output stage
3rd stage, plus DC shift Differential single ended 2nd stage
30
Multistage amplifiers (BJT)
Differential input
Current sources
Emitter follower output stage
3rd stage, plus DC shift Differential single ended 2nd stage
2
Feedback
Ch 10
3
Motivation for feedback in systems
A
Typical amplifier
4
Motivation for feedback in systems
A
Gain variation due to component variation
Typical amplifier
in
out
5
Motivation for feedback in systems
A
Gain variation due to component variation
Typical amplifier
System nonlinearity
in
out
in
out
6
Motivation for feedback in systems
A
Gain variation due to component variation
Typical amplifier
System nonlinearity
in
out
in
out
Frequency response f
7
Structure of feedback system
A Input Output
Forward gain stage
8
Structure of feedback system
A Input Output
+
Means to change system input
9
Structure of feedback system
A Input Output
+
Means to sample system output
10
Structure of feedback system
A Input Output
+
β
Means to use output to influence input
11
Structure of feedback system
A Input Output
+
β
Means to use output to influence input
12
Structure of feedback system
A Input Output
+
β
Negative feedback
partially cancels input
-
13
Structure of feedback system
A Input Output
+
β
-
xs xo
xf
xi
14
Structure of feedback system
A Input Output
+
β
-
xs xi=xs-xf xo=Axi
xf=βxo
15
Structure of feedback system
A Input Output
+
β
-
xs xi=xs-xf xo=Axi
xf=βxo
1o
fs
x AAx Aβ
= =+
16
Structure of feedback system
A Input Output
+
β
-
xs xi=xs-xf xo=Axi
xf=βxo
1o
fs
x AAx Aβ
= =+
1fA β≈For large A:
17
Gain desensitization
A Input Output
+
β
-
1o
fs
x AAx Aβ
= =+
( )21
1d
fdA A Aβ=
+
( )1
1f
f
dA dAA A Aβ
=+
18
Gain desensitization
A Input Output
+
β
-
1o
fs
x AAx Aβ
= =+
( )21
1d
fdA A Aβ=
+
( )1
1f
f
dA dAA A Aβ
=+
The variation in the forward amplifier gain is reduced by 1/(1+Ab)
19
Gain/bandwidth tradeoffs
A(s) Input Output
+
β
-
( )1 /
M
H
AA ss ω
=+
20
Gain/bandwidth tradeoffs
A(s) Input Output
+
β
-
( )1 /
M
H
AA ss ω
=+
( )( )1 ( )fA sA sA sβ
=+
21
Gain/bandwidth tradeoffs
A(s) Input Output
+
β
-
( )1 /
M
H
AA ss ω
=+
( )( )1 ( )fA sA sA sβ
=+
( )( )
1( )
1 ( / ) 1M M
fH M
A AA s
s Aβ
ω β+
=+ +
22
Gain/bandwidth tradeoffs
A(s) Input Output
+
β
-
( )1 /
M
H
AA ss ω
=+
( )( )1 ( )fA sA sA sβ
=+
( )( )
1( )
1 ( / ) 1M M
fH M
A AA s
s Aβ
ω β+
=+ +
For closed loop:
Midband gain
( )1M
M
AA β+
Band limits
( )1Hf H MAω ω β= +1
HLf
MAωω
β=
+
23
Gain/bandwidth tradeoffs
24
Reduction in nonlinear distortion