Active Filter Design

Gm-C, OTA-C filter

Class#5

Outline

Transconductor OTA Gm circuits

Gm-C filter building block Two integrator biquads

Design example Automatic Frequency Tuning

Frequency control Q control

Transconductor What is needed?

An accurate voltage to current conversion A clean transfer of current to output terminals

Attributes of high performance gm Gm value should be stable and well controlled over process, supply and

temperature Large signal handling capability, at both input and output terminals,

along with low distortion, and low noise. Good high frequency characteristics for both magnitude and phase

response. High input impedance. High output impedance. Availability of fully differential and balanced structures. Gm tunability.

Operational Transconductance Amplifier

Simple gm circuits

Simple gm circuits Properties

Gm can be tuned via I Reasonably high input impedance Moderate output impedance Moderate bandwidth

Problems Gm value is not well controlled, depends on process and

temp. Square-root dependence on I limits the tuning range, Large

gm requires a large I, and tuning directly affects supply I. The gm linearity is restricted to a small vin.

Gm linearization schemes-source degeneration

Gm-C-OTA integrator

Building Blocks

Building Blocks

First order circuits

First order circuits

Lossy integrator

Two integrator loop

Two-integrator biquads

Two-integrator biquads

Two-integrator biquads

Two-integrator biquads

Two-integrator biquads

Design Example 1 : Very small gm circuit

[ref] A. Veeravalli, E. Sanchez-Sinencio and J. Silva-Martinez, "Transconductance amplifier structures with very small transconductances: a comparative design approach", IEEE JSSC, Vol. 37 Issue: 6, Jun 2002, pp. 770 -775

Design Example : Very Small Transconductances

Design Example :Output results

Design Example 2 : Pacemaker system

Design Example 2 : 4th order gm-C filter

V im V ip

V bfl

I ss

C A

C B

C A

C B

MP 1 MP 2 MP 2MMP 1M

MP 3 MP 4

MP 6 MP 5

MN 1 MN 3MN 2 MN 4

V o

VDD

VSS

V rst V rstMN 5 MN 6

M:1 1:M

Design Example 2 :Output results

@ vin 100mV ( -23.01 dBV)

- 70

- 20

- 60

- 50

- 40

- 30

10 5k20 50 100 200 500 1k 2k

TTTTT

70Hz10Hz

- 12 dB/octave

Frequency [Hz]

Passband: 10 Hz and 70Hz. The harmonic distortion is below -70 dB at a 30-Hz, 75-mVp-p input signal. The measured power dissipation of the filter is 1.8W.

Automatic Frequency Tuning

By adding additional circuitry to the main filter circuit Have the filter critical frequency automatically

tuned Expensive trimming avoided Accounts for critical frequency variations due to

temperature, supply voltage, and effect of aging Additional hardware, increased Si area & power

Master-Slave Automatic Frequency Tuning

Following facts used in this scheme: Use a replica of the main filter or its building block in the

tuning circuitry The replica is called the master and the main filter is named

the slave Place the replica in close proximity of the main filter to

ensure good matching Use the tuning signal generated to tune the replica, to also

tune the main filter In the literature, this scheme is called master-slave tuning!

Master-Slave Automatic Frequency Tuning

Tuning Methods For CT Filters

Voltage Controlled Filter approach

Problem :VCF method requires a fundamental sine wave, and suffers from offset error.

Voltage controlled oscillator approach

Instead of VCF a voltage-controlled oscillator (VCO) is used

VCO made of replica integrator used in main filter

Tuning circuit operates exactly as a conventional phase-locked loop (PLL)

Tuning signal used to tune main filter

Voltage controlled oscillator approach

Phase detector

gmt

Fclk Loopfilter

gm gmgmIn Out

Slave filter

VCO

Limiter

This is by far the most widely used automatic tuning method and considered the most reliable method. This method made to achieve up to 1 % frequency accuracy.

Digitally Assisted Filter Tuning

DSP, A/D converter and input source are required. Its very difficult !!.

Q-tuning

Mismatch among similar gms or Cs Phase errors are the limiting factors for high Q

circuits and high frequency applications Q tuning should always be performed after

frequency has been tuned, and without affecting frequency tuning.

Some ways of improving Q-tuning Measuring Q directly. Monitor the gain-magnitude at different frequencies and

correct the magnitude frequency response [R. schaumann]. Combine Q-tuning with frequency tuning using a VCO [J.M.

Khoury]. Envelope detection instead of peak detection [J. Silva-

Martinez]. Adaptive technique [J.M. Stevenson].

Gm-C Filter Design Challenges

Power vs. Linearity Dynamic range Tuning scheme Size of Capacitors / Noise

Summary

Gm-C, OTA-C filter Gm circuit Building block Biquad examples Automatic frequency tuning

Despite a rich history, new circuit implementations, tuning schemes are still being explored.

Current mode filter

Class#6

Outline

Current mode circuits Primitive CM circuits Lossy integrator

Building block Integrator Single ended Improved balanced

Current mode circuit

Input Signal: Current Output Signal: Current Basic Building Blocks are:

Inverting Integrators Inverting (Current Amplifiers)

Primitive Circuit Implementations: Single Transistor Inverting Amplifier Simple Current Mirror Capacitor

Primitive CM Circuits

In order to fully obtain the benefits of current-mode techniques simpler circuits with reduced parasitics are desirable.

Primitive CM Circuits

Primitive CM Circuits Low Power supply High frequency Low area Suitable for digital process Good PSR Poor linearity, efficiency (THD

< 4%) Poor voltage gain

Primitive CM Circuits Low power supply (3.3V) High frequency Low area Suitable for digital process Very good PSR Good Linearity (differential) Excellent efficiency ( 100%) Poor common mode rejection

Primitive CM Circuits Linearity sufficient Very high efficiency (100%)

AB, low power Very high frequency Small area Low Power Supply Linearity dep. on process

variations PSR poor

Lossy Integrator

Current mode lossy integrator

Current mode lossy integrator

Differential Integrator

Current mode basic cells-single ended

Integrator-single ended

Improved Balanced Integrators

X. Quan, S. Embabi and E. Snchez-Sinencio, Improved Fully Balanced Current-Mirror Integrator, Electronic Letters, vol. 34, No. 1, pp. 1-3, January 1998.

Improved Balanced Integrators

Improved Balanced Integrators

Summary

CM filters can result in the lowest power dissipation and smallest area Transistor matching and bias current source

matching are main problems Single ended and differential configuration Improved balanced integrators