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Sam PalermoAnalog & Mixed-Signal Center
Texas A&M University
ECEN474: (Analog) VLSI Circuit Design Fall 2010
Lecture 24: OTA-C Filters
Announcements
• Project• Preliminary report due Nov 19• No layout• Focus is on good circuit design
• No Class on Monday 11/15
2
Agenda
• OTA-C Filters• Material is related primarily to Project #3• Full class on filters offered (458, 622)
3
OTA-C Filter Applications
• Hard-disk drives require linear phase filters (>100MHz)• RF systems require filters in the GHz range• Wireless xcvrs intermediate frequency (IF) filters (>100MHz)• Often used with variable gain amplifiers (VGAs) for
automatic-gain control (AGC) to maximize dynamic range• Low noise, low power, and high linearity are required
4Hard-Disk Drive Receiver Front-End
OTA-C Filter [Pandey]
OTA-Based Active Resistor
5
mi
i
imoi
gIVR
VgII1
==
==
[Schaumann]
OTA-Based Active Resistors
6
[Schaumann]
OTA-Based Integrator
7
sCg
VV m=
1
2 Ideally
• Finite OTA ro causes a non-zero pole• OTA Co reduces integration constant
( ) oo
m
gCCsg
VV
++=
1
2
[Schaumann]
Lossy gm-C Integrator (1st-Order LPF)
8
( ) omio
m
m
m
ggCCCsg
VV
gsCg
VV
22 2
1
1
2
2
1
1
2
++++−=
+−=
ecapacitancoutput andinput zero-non and resistanceoutput OTA finite gConsiderin
Ideally
[Schaumann]
Fully Differential Lossy gm-C Integrator
9
• Pseudo-Differential
• Fully Differential• 2C because full gm
current goes to each side
• Why just C here?
[Schaumann]
OTA-Based Inductor
10
21
211
2
221
1
2
2
211
1
112
221
1
1
mmeff
effmm
mm
mm
m
m
ggCL
sLgg
sCZ
sCY
Ygg
Z
VI
ggIV
VgIVgI
=
==
=
=
=
==
If
equations two these From
[Schaumann]
Differential Grounded Inductor
11
[Schaumann]
Second-Order Filter
12
22
21
1min
21
21
1
2
21
1min
22
1
min
21
21
1
2
1
min
1
1
oo
m
mm
Q
m
in
LP
oomm
Q
in
BP
Qss
CCgg
CCgg
CRss
CCgg
VV
Qss
Cgs
CCgg
CRss
Cgs
VV
ωω
ωω
+
+
−=
+
+
−=
+
+
=
+
+
=
[Silva]
Differential Second-Order Filter
13
[Mohieldin]
OTA Output Resistance Effects
14
[Silva]
OTA Non-Dominant Pole Effects
15
[Silva]
OTA Parasitic Capacitor Effects
16
[Silva]
OTA-C BPF Modeling Simulations
17
4th-Order BPF
[Silva]
4th-Order Filter Example
18
[Mohieldin]
Magnitude, Phase, and Group Delay
19
[Silva]
Optimization: Non-Dominant Pole & DC Gain
20
[Silva]
Useful References
• “Design of Analog Filters” by R. Schauman(Filters Textbook)
21
Next Time
• Analog Applications• Variable-Gain Amplifiers• Switch-Cap Filters, Broadband Amplifiers
• Bandgap Reference Circuits• Distortion
22