ECE137A class notes, UCSB, Mark Rodwell, copyright 2019
ECE137A, Notes Set 11:
MOS Multi-stage example
Mark Rodwell, Doluca Family Chair, ECE DepartmentUniversity of California, Santa [email protected]
ECE137A class notes, UCSB, Mark Rodwell, copyright 2019
Multistage CMOS example
2
FETs:
10mA/V ( /1 m)
2mA/V( /1 m)
0.1V
0.3V
1/ =4V
g
v g
th
K W
K W
V
V
DC bias: =0.4V for all FETs boundary of velocity- and mobility-limited regions.gsV
Differential pair with single-ended folded cascode.
All FETs except M5A, M5B, M8: | |=0.4V.DSV
M5A, M5B, M8: | |=0.8V.DSV
M1 forces an equal for M5A, M5BDSV
ECE137A class notes, UCSB, Mark Rodwell, copyright 2019
DC bias analysis
2
2
FET widths are calculated from
( ) (1 )
where =10mA/V ( /1 m)
D gs th DS
g
I K V V V
K W
FET small-signal parameters from | 2 / ( ), 1/
M5A,B: 2mS, 40k . M2: 20k
M6 : 2mS, 40k . M8: 40k
m mobility D gs th DS D
m DS DS
m DS DS
g I V V R I
g R R
g R R
ECE137A class notes, UCSB, Mark Rodwell, copyright 2019
Small signal analysis
6 6 8
6 6 6 6
6 6 6
5 6 2 5
5
6 : Common-gate
|| 10k ||40k 8k
(1/ )(1 / )
500 (1 8k / 40k ) 600
/ 8k / 600 13.333
5 : Common-source
|| || 600 || 20k || 40k 574
Leq D DS
in m Leq DS
v Leq in
Leq B in DS DS B
v B
M
R R R
R g R R
A R R
M B
R R R R
A
5 5
6 5
2mS 574 1.15
/ / 2 15.333 / 2 7.6666
m B Leq B
out d v v B
g R
V V A A
2 56 5 5 6
2 5 6
Alternatively, tracking currents:
|| 13.33k2mS 8k
|| 13.33k 600
= 2mS 0.957 8k 15.333
DS DS Bv v B m B Leq
DS DS B in
R RA A g R
R R R
ECE137A class notes, UCSB, Mark Rodwell, copyright 2019
Maximum signal swings
0.4V0V
+
0.3V+0.3V
0.1mA
0mA
Knee voltage of M6:
Transistor is biased at 0.4V and 0.0V
Knee (saturation): when 0.3V.
Voltages as indicated on drawing
Output can swing to +0.3V.
G S
DG th
V V
V V
6
6 8
8 8
8
M6 cutoff:
We can decrease from 100 A to 0 A.
=10k || 8k
100 A 8k 0.8V.
Output can swing to 0.8V.
note: accounts for the variation of with ;
as goes negative,
D
Leq DS
out
DS D out
out D
I
R R
V
R I V
V I
decreases due to (1+ ).DSV
ECE137A class notes, UCSB, Mark Rodwell, copyright 2019
Maximum signal swings
Knee voltage of M8:
Transistor is biased at 0.4V and 0.8V
Knee (saturation): when 0.3V.
Voltages as indicated on drawing
Output can swing to 0.7V.
G S
DG th
V V
V V
-0.4V
+0.3V
-0.7V
ECE137A class notes, UCSB, Mark Rodwell, copyright 2019
Maximum signal swings: M5A cutoff
5 5
2 56 5
2 5 6
6 6
More carefully:
=100 A to 0 A= 100 A 100 A
||= 100 A 0.957 95.7 A
||
= 96 A 8k 0.768V. can swing to 0.77V.
D A D B
DS DS BD D A
DS DS B in
out D Leq out
I I
R RI I
R R R
V I R V
0mA0.2mA
0.2mA
0.2mA
0mA
0mA
0.1mA
8 6 2 5
Can be calculated approximately
by tracking the currents as shown
to the right.
0.1mA 10k 1V?
We are ignoring the effect
of , , ,
out
DS DS DS DS B
V
R R R R
ECE137A class notes, UCSB, Mark Rodwell, copyright 2019
Maximum signal swings: M5B cutoff
5
2 56 5
2 5 6
6 6
More carefully:
100 A
||= 100 A 0.957 95.7 A
||
=96 A 8k 0.768V. Output can swing to 0.77V.
D B
DS DS BD D A
DS DS B in
out D Leq
I
R RI I
R R R
V I R
0.2mA
0.2mA
0.2mA
0.2mA
0.2mA
0.1mA
8 6 2 5
Can be calculated approximately
by tracking the currents as shown
to the right.
0.1mA 10k 1V?
We are ignoring the effect
of , , ,
out
DS DS DS DS B
V
R R R R
0mA
