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