ECE 342 Electronic Circuits Lecture 26 Response of ...emlab.illinois.edu/ece342/notes/Lec_26.pdfECE...
Transcript of ECE 342 Electronic Circuits Lecture 26 Response of ...emlab.illinois.edu/ece342/notes/Lec_26.pdfECE...
ECE 342 – Jose Schutt‐Aine 1ECE 342 – Jose Schutt‐Aine 1
Jose E. Schutt-AineElectrical & Computer Engineering
University of [email protected]
ECE 342Electronic Circuits
Lecture 26Frequency Response of Cascaded
Amplifiers
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CE Cascade Amplifier
Exact analysis too tedious use computerCE cascade has low upper‐cutoff frequency
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11
1o
o
Define gr
Common source amplifier, followed by common gate stage – G2 is an incremental ground
MOS Cascode Amplifier
22
1o
o
gr
LR current source impedance
1L
L
GR
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• CS cacaded with CG CascodeVery popular configurationOften considered as a single stage amplifier
• Combine high input impedance and large transconductance in CS with current buffering and superior high frequency response of CG
• Can be used to achieve equal gain but wider bandwidth than CS
• Can be used to achieve higher gain with same GBW as CS
MOS Cascode Amplifier
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MOS Cascode Incremental Model
Lo
L
ivG
2 2 2 2 2 2L mb s m s s o oi g v g v v v g
2 2 2 2 2 2L
L s mb m s o oL
ii v g g v g gG
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22 2 2 21 o
L s mb m oL
gi v g g gG
22
2 2 2
1 /L o Ls
mb m o
i g Gv
g g g
KCL at vs2
1 2 1 2 2 2 2 2 2( ) 0m gs s o m s mb s o s og v v g g v g v g v v
MOS Cascode Analysis
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1 21 1
2 2 2
1 /1 0o o L
m gs Lo m mb
g g Gg v i
g g g
1 1
1 2
2 2 2
1 /1
m gsL
o o L
o m mb
g vi
g g Gg g g
1
1 2
2 2 2
o m
in o L oL
o m mb
v gv g G g
Gg g g
Two cases
MOS Cascode Analysis
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1 2 2 2 1 2 om o m mb o o
in
v g g g g r rv
1 2 1 2 1 2 1 2 MB m o m m m mb o oA g g g g g g r r
1 1 2 2MB m o m oA g r g r
CASE 1
The voltage gain becomes
1: 0LCase If G
MOS Cascode Analysis
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1 2
2 2 2
2 :
o L o
Lo m mb
g G gCase If G
g g g
1 o m
MBin L
v gAv G
CASE 2
The voltage gain becomes
MOS Cascode Analysis
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MOS Cascode at High Frequency
23 2
12o
ds
fr C
The upper corner frequency of the cascode can be approximated as:
2 2 2 3 3where db gd db gdC C C C C
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• Capacitance Cgs1 sees a resistance Rsig• Capacitance Cgd1 sees a resistance Rgd1• Capacitance (Cdb1+Cgs2) sees resistance Rd1• Capacitance (CL+Cgd2) sees resistance (RL||Rout)
gd1 m1 d1 sig d1R 1 g R R R Ld1 o1
m2 mb2 vo2
R1R rg g A
H gs1 sig gd1 m1 d1 sig d1C R C 1 g R R R
db1 gs2 d1 L gds2 L outC C R C C R R
HH
1f2
MOS Cascode at High Frequency
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If Rsig is large, to extend the bandwidth we must lower RL. This lowers Rd1 and makes the Miller effect insignificant
If Rsig is small, there is no Miller effect. A large value of RL will give high gain
MOS Cascode at High Frequency
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Effect of Cascoding(when Rsig=0)
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Common emitter amplifier, followed by common base stage – Base of Q2 is an incremental ground
BJT Cascode Amplifier
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Cascode Amplifier
First stage is CE and second stage is CB
1 2v m LA g R
If Rs << r1, the voltage gain can be approximated by
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BJT Cascode Incremental Model
2 2o m Lv g R v
2 22 2
2 2 2
1x xx
x
v r rv v vr r r
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BJT Cascode Analysis
1 1 2 22 2
xm m
x
v g v g vr r
Ignoring rx2
21 1 2 2
2m m
v g v g vr
1 1 2 22
1m mg v v g
r
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BJT Cascode Analysis
11
1 1in
s x
rv vR r r
1 12
1 12
2
11
m in
s xm
g r vvR r r
gr
2 1 1
1 12
2
11
m L m ino
s xm
g R g r vvR r r
gr
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BJT Cascode Analysis
2 2 1 1
1 1 2 21o m m L
in s x m
v g r g r Rv R r r g r
1 2
1 1 2 1o L
in s x
v Rv R r r
1 2v m LA g R
If Rs << rx1+r1, the voltage gain can be approximated by
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BJT Cascode at High Frequency
Define rcs as the internal impedance of the current source. R includes generator resistance and base resistance rx1 base of Q1
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There is no Miller multiplication from the input of Q2 (emitter) to the output (collector).
BJT Cascode at High Frequency
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1 1 2 2 3
1 1 11 1 1MB
e out
A Aj C r R j r C j R C
22 2
12 e
fr C
1 1
12in highf
r R C
1 1
1 1MB
in high out high
A A f fj jf f
BJT Cascode at High Frequency
3 2 out csDefine R r r
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BJT Cascode at High Frequency
3
12out high
out
fC R
2out out outcsC C C
current source output capacitanceoutcsC
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Cascode Amplifier – High Frequency
High‐frequency incremental model
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' 's i s 1 1 1 a 1 bG V G g s C C V sC V
m1 1 a 2 m2 2 1 b 2 m2 2 d0 g sC V g g s C C V g g sC V
2 2 b x2 2 2 2 d 2 o0 g sC V g g s C C V sC V
m2 b m2 2 d L 2 o0 g V g sC V G sC V
Applying Kirchoff’s current law to each node:
Find solution using a computer
Cascode Amplifier – High Frequency
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gm=0.4 mhos =100r=250 ohms rx=20 ohmsC=100 pF C=5 pfGL=5 mmhos GS’=4.5 mmhos
sa=8.0 sd=‐0.0806sb=‐2.02 + j5.99 se=‐0.644sc=‐2.02 – j5.99 sf=‐4.05
sg=‐16.45
POLES (nsec‐1)ZEROS (nsec‐1)
As an example use:Cascode Amplifier – High Frequency
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If one pole is at a much lower frequency than the zeros and the other poles, (dominant pole) we can approximate 3dB
93dB 0.0806 10 rad / sec
3dBf 12.9 MHz
For the same gain, a single stage amplifier would yield:
93dB 0.0169 10 rad / sec
3dBf 2.7 MHz
Second stage in cascode increases bandwidth
Cascode Amplifier – High Frequency