l32 cascode amplifiers - nanohub.org
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1
ECE 255
Cascode Amplifiers (Sedra and Smith, 7th Ed., Sec. 8.5)
Mark Lundstrom School of ECE
Purdue University West Lafayette, IN USA
ECE 255: Fall 2019 Purdue University
Lundstrom: Fall 2019
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CS amplifier with active load
2 Lundstrom: Fall 2019
+
υi
−
+
υ0
−
D S
VDD
G
IO
Ideal current source
RO = ∞ Aυo= υo
υi
= −gmro
Rin = ∞
Ro = ro
But this gain is not large enough
Aυo= −A0
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Solution: cascade amplifiers
3 Lundstrom: Fall 2019
Examples: CE:CE CC:CE CE:CC
CE:CB or CS:CG = “cascode”
Stage 2 Stage 1
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Basic cascode
4 Lundstrom: Fall 2019
+
υi
−
υ0
D S
G
VG2
“cascaded cathode”
grid
plate
cathode
Q1
Q2
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Cascode amplifiers
5
CG CS
1) Discrete transistor version first 2) Then MOS IC cascodes 3) Finally, BJT cascodes
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Outline
6
1) Introduction
2) Discrete cascode!
3) IC MOS cascode
4) Discussion
5) BJT cascode
6) Summary
Lundstrom: Fall 2019
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Discrete CS:CG cascode
7
+
υi
−
υ0
VDD
RD
RSυsig
Rsig
CS
R1
R2
R3
CG
CC1
CC2
RL
Q1
Q2
Ignore ro
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CS:CG discrete cascode (without bias resistors)
8
+υi
−
Aυo= Aυ1
× Aυ2
RD
+υo
−
Aυ1= ?
Aυ2= ?
Rin = ∞
Ro = RD
Q1
Q2
Rin2 =1gm2
*
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CS:CG discrete cascode (without bias resistors)
9
+υi
−
Aυo= Aυ1
× Aυ2
RD
+υo
−
Aυo= −gm1
1gm2
⎛⎝⎜
⎞⎠⎟× gm2RD
Rin = ∞
Ro = RD
Q1
Q2
Aυo= −gm1RD
Aυ1= − gm1 gm2 Aυ2
= +gm2RD
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CS:CG cascode summary
10 Lundstrom: Fall 2019
CS CG
• Input resistance of CS • Output resistance of CS • Voltage gain of CS (not gain squared!) • No Miller effect
(excellent high frequency response)
Aυo≈ −1 Aυo
= +gmRD
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Outline
11
1) Introduction
2) Discrete cascode
3) IC MOS cascode
4) Discussion
5) BJT cascode
6) Summary
Lundstrom: Fall 2019
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CS:CG cascode (IC)
12 Lundstrom: Fall 2019
+υi
−
υo
Rin = ∞
Q1
Q2
Aυo= Aυ1
× Aυ2
Aυ1= −gm1 ro1 || Rin2( )
Rin2 Rout2
Aυ2= +gm2Rout2
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Recall L31
13 Lundstrom: Fall 2019
RL
RS
Rout = ro + 1+ gmro( )Rseries
Rout ≈ ro + gmro( )RS ≈ gmro( )RS
Rin =ro + RL( )1+ gmro( )
Rin ≈1gm
+ RL
gmro
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The easy way
14
+υi
−
Q1
Q2
Rin2Rout2
gm1υiυo = − gm1υi( )Rout2
Rout 2 ≈ gm2ro2( )ro1
Aυi= − gmro( )2
Aυo= −A0
2
gm1υi
Lundstrom: Fall 2019
Rout ≈ gmro( )RS
*
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Aυo= − A0( )2Cascode:
Recall: CS amplifier with active load
15 Lundstrom: Fall 2019
+
υi
−
+
υ0
−
D S
VDD
G
IO
Ideal current source
RO = ∞
Aυo= −gmro
Aυo= −A0
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Outline
16
1) Introduction
2) Discrete cascode
3) IC MOS cascode
4) Discussion
5) BJT cascode
6) Summary
Lundstrom: Fall 2019
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Implementation
17
VDD
R1
Q2
IREF
Q3
cascode amplifier
υo
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Implementation
18 Lundstrom: Fall 2019
+υi
−
υ0
VG2
Q1
Q2
VDD
IO
Aυo= −gm gmr0( )ro⎡⎣ ⎤⎦ || rop
Aυo≈ −gmro = −A0
PNP current mirror
The solution is to use a cascode current course.
(S&S p. 549)
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Include a load resistor
19
+υi
−
υo
Q1
Q2
Rin2
Rout2 RL
Aυo= −gm1Rout2 || RL
Rout2 = gm2ro2( )ro1
Can you find the gain of stage 1 (it depends on RL!). See S&S, Sec. 8.5.3
Aυo≈ −gm1RL
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Double cascode
20
+υi
−
Q2
Q1
+υo
−
Q3
⇐ R02 ⇐ R03
Aυo= −gmRo3
*
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Double cascode
21
+υi
−
Q2
Q1
+υo
−
Q3
⇐ R02 = gm2ro2( )r01 ⇐ R03 = gm3ro3( )R02R03 = gm3ro3( ) gm2ro2( )r01
Aυo= −gmRo3 Aυo
= − gmr0( )3
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Outline
22
1) Introduction
2) Discrete cascode
3) IC MOS cascode
4) Discussion
5) BJT cascode
6) Summary
Lundstrom: Fall 2019
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BJT cascode
23
+υi
−
Q1
Q2
Rout2
gm1υi
υo = − gm1υi( )Rout2gm1υi
+υo
−
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Recall L31
24
RL
RS
Rout = ro2 + 1+ gm2ro2( ) RS || rπ 2( )
Q2 Rout ≈ gm2ro2( ) RS || rπ 2( )
cannot be as larges as for the MOS case
Lundstrom: Fall 2019
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BJT cascode
25
+υi
−
Q1
Q2
Rout2
gm1υi
υo = − gm1υi( )Rout2
Rout ≈ gm2ro2( ) ro2 || rπ 2( )
Aυo≈ −gm1 gm2ro2( )rπ 2
Aυo= −βgmro = −βA0
Lundstrom: Fall 2019
gm1υi
Rout ≈ gm2ro2( ) RS || rπ 2( )
Rout ≈ gm2ro2( )rπ 2
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BJT cascode
26
+υi
−
Q1
Q2
Rout2
gm1υi
Aυo= −βA0
See the discussion in Sec. 8.5.6 of S&S
Lundstrom: Fall 2019
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Outline
27
1) Introduction
2) Discrete cascode
3) IC MOS cascode
4) Discussion
5) BJT cascode
6) Summary
Lundstrom: Fall 2019
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Cascode:
Aυo= − gmro( )2
MOS summary
28 Lundstrom: Fall 2019
+
υi
−
+
υ0
−
D S
VDD
G
Aυo= −gmro
IO
Ideal current source
RO = ∞Common Source:
Double Cascode:
Aυo= − gmro( )3
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Cascode:
Aυo= −β gmro( )
BJT summary
29 Lundstrom: Fall 2019
+
υi
−
+
υ0
−
C E
VDD
B
Aυo= −gmro
IO
Ideal current source
RO = ∞ Common Emitter:
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Questions
Lundstrom: Fall 2019 30
1) Introduction
2) Discrete cascode
3) IC MOS cascode
4) Discussion
5) BJT cascodes
6) Summary