Gm-C filters Biquad

What is Gm-C filters?

� Definition:

� Active filter controlled by OTA (current gain) and capacitors parameters,

over a CMOS or BiCMOS technology.

� Key Points:

� Introduction

2/9/07 Course Title 2

� Comparative of RC, MOSFET-C and Gm-C filters

� Biquad overview

� Gm-C filter synthesis

08/26/2008 BA01 Analog IC Design

Introduction: Gm-C Filters

� Where / why we do use Continous-Time Filters:

� Pre and post filters for sampled-data systems;

� Mixed analog digital systems;

� Biomedical applications (peacemarker);

� Area and Power efficient solution.


� Gm-C Filters properties:

� Well-suited for high-frequency applications;

� Many transconductance elements required;

� Limited dynamic range (OTA nonlinearities);


Comparison of CT-filters type

2/9/07 Course Title 408/26/2008 BA01 Analog IC Design

The Biquad Filter


The Biquad Filter


The Biquad Filter

� The ressonator Natural Modes


� Active ressonator


The Two-Integrator-Loop Biquad

� Consider first a high pass filter.


Kerwin-Huelsman-Newcomb (KHN) biquad

The Tow-Integrator-Loop Biquad

� Another way to sum the partial integrated signals

Tow-Thomas biquad


Other Techniques Available

� SAB ( Single Amplifier Biquad)

� Simulation of passive ladder network

� Other higher order techniques.


First-Order OTA-C


Basic Blocks

� Basic Building Block

� OTA-C Integrator


Basic Blocks

� Parasitic capacitance effects:

� The Miller effect


� How to reduce this parasitic capacitance


Basic Blocks

� OTA simulated resistor


Basic Blocks

� OTA simulated inductor


Filter Topologies

� Example of an RLC circuit


Two Integrator Loop OTA-C filter

� Very known from opamp realization



� Summed feedback and Distributed feedback



� Designing

� Making t1 = t2 = t ; select k11 and k12

� Or making k11=k12; select t1 and t2


� Or making k11=k12; select t1 and t2



� Biquad


OTA-C Realizations of (DF) Configuration

� How to use inputs and outputs


OTA-C Realizations of (DF) Configuration

� Summarized table


Biquad Realization

� Summarizing

� Distributed Feedback

� Summer feedback (KHN – biquad)

� Lossy integrator (Tow-Thomas)


Final Considerations

� Dynamic Range

� Mismatch transconductance

� Output impedance and load problem

� Component substitution (LC ladder, RLC Ressonator)


References

� Deliyannis, T. Sun, Y. Fidler, J.K. Continuous-Time Active Filter Design. New York:

CRC Press. 1999

� Johns, D. Martin, K. Analog Integrated Circuit Design. New York: Wiley. 1997

� Ficher, G. Transconductance-C Filters at:

http://www.ele.uri.edu/courses/ele539/lectures/chapter9.pdf

� Sedra, A.S., Smith, K.C. Microelectronic circuits. Fifth Ed. New York: Oxford press.

2004

� R. L. Geiger and E. Sánchez-Sinencio , "Active Filter Design Using Operational


� R. L. Geiger and E. Sánchez-Sinencio , "Active Filter Design Using Operational

Transconductance Amplifiers: A Tutorial," IEEE Circuits and Devices Magazine, Vol. 1,

pp.20-32, March 1985.


Pratical example

� Band Stop

� Making t1 = t1 =t

� Or making k12=k22=k


� Or making k12=k22=k

� requirements


Pratical example

� Band Stop

� The integrators and amplifiers transconductance

� Simulated Resistor and Capacitors


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Gm-C filters Biquad

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