ABSTRACT

This paper presents a low voltage programmablecontinuous-time filter for hearing aids. The filter uses ananalog circuit technique employing on-chip charge pumpscalled Dynamic Gate Biasing (DGB). A simple method forreducing the ripple from the DGB charge pumps ispresented. The principle of DGB is experimentally verifiedthrough the implementation of a programmable gm-Cbiquadratic filter. Designed in 0.35 m CMOS, the filteroperates from 1.2 V and dissipates 16 W, provides 62.3 dBdynamic range at -45 dB THD and can realize lowpass,bandpass and highpass filter responses.

I. INTRODUCTION

Low power consumption and filter programmability aretwo important design goals for modern hearing aids. The useof micropower and low-voltage techniques in hearing aidsallows miniaturization of the supply systems and increasesbattery life. In this paper, we investigate an experimentalanalog circuit technique called dynamic gate biasing (DGB)which can be used in digitally controlled analog (DCA)hearing aids to achieve both low-voltage/low-poweroperation and filter programmability. We begin with anoverview of DGB, and illustrate how DGB can be applied toa programmable gm-C filter for hearing aids. Finally, wepresent experimental results of a programmable gm-Cbiquadratic filter fabricated in 0.35 m CMOS.

II. DYNAMIC GATE BIASING

While the use of charge pumps for generating enhancedpotentials to program an EEPROM device is quiteestablished for digital circuits, there has, until recently, beenlittle reported work in applying this technique to the stablebiasing of analog circuits. The earliest analog application ofDGB seems to have been proposed by Monna[1] inconnection with optimizing the dynamic range of acontinuous-time filter. A charge-pump assisted CMOSop-amp was also proposed by Zhou[2]. However, noexperimental results were presented in either work. Morerecently, experimental results have been reported forcontinuous-time MOSFET-C filters utilizing DGB [3][4].

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While the limitations imposed by the charge pump outputripple on the performance of the application were notaddressed in [3], in [4] the charge pump used in the filterhad a reported voltage ripple of less than 1mV. However,the supply voltage used in [4] was relatively large (5V) andthe design is not directly applicable to low voltageoperation. In this work, we present an alternativecontinuous-time filter based on a transconductance( )-Cstructure that is suitable for 1.2V operation. The chargepump operates off the low supply voltage, but still providesa low output ripple.

III CIRCUIT DESIGN

In this section, we describe the circuit elements of thefilter, including the tunable transconductor cell, the chargepump and the filter structure. We also discuss the layout ofthe fabricated test chip.

A. TransconductorIn tunable transconductors, MOS transistors are operated

in the triode region and are used to realize variable resistors.However, as supply voltages decrease, the tuning rangebecomes increasingly restricted, and in extreme cases, eventhe supply voltage itself becomes insufficient to induce aconductive channel in the transistor. To illustrate, considerthe transconductor circuit shown in Fig. 1. The overalltransconductance of this cell is varied using control voltage

on NMOS transistor Q7. However, the source and drainterminals of the device are already biased by the gate-sourcevoltage of input devices and the gate of must bean additional gate-source voltage higher. Consequently, witha low supply voltage, a charge pump is likely required toprovide some degree of tunability.

B. Charge PumpThe observation that the desired gate bias voltage is

essentially twice the gate-source voltage of a MOSFETsuggests that a voltage doubler can be used to double thegate-source voltage of a single transistor. By directly drivingthe MOSFET gate, no current is drawn from the chargepump, and so no ripple is generated due to loading.

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Vc

Q1 2, Q7

Low-Voltage Programmable gm-C Filter for Hearing Aids using Dynamic Gate Biasing

Louie Pylarinos and Khoman Phang

{louie, kphang}@eecg.utoronto.caThe Edward S. Rogers Sr. Dept. of Electrical and Computer Engineering, University of Toronto

Toronto, Ontario, M5S 3G4, CANADA

19840-7803-8834-8/05/$20.00 ©2005 IEEE.

The charge pump circuit used is shown in Fig. 3. Thecircuit is essentially a voltage shifter, producing

with two unique characteristics[5].First, it has a separate input, Vin, that is variable andseparate from the supply. Second, the doubler can acceptinput levels that lie near the threshold voltage. This ability isimportant in low-voltage applications where the biasvoltages are often only slightly higher than the devicethreshold voltage. The charge pump is driven bynon-overlapping clock signals .

Like any clocked circuit, charge pumps introduce someripple at the output voltage. This ripple is generated throughparasitic current leakage, charge injection and clock andpedestal feedthrough, and produces a fluctuating filtercharacteristic and some clock feedthrough. It has beenshown in that a fluctuation of more than 10 Hz in the poleresonant frequency of a hearing aid filter becomesperceptible to the user[7]. This hearing aid requirementtranslates to a maximum tolerable charge pump ripple of a

I1 I1

I2I2

Q1 Q2

Q3 Q4Q5 Q6

Vc

VDD

I1-I2-io1I1-I2+io1

Vi+ Vi-

Q7

io1

Fig. 1 CMOS tunable transconductor cell.

Vout VDD Vin+=

Φ1 2,

Q1 Q2

Q4Q3

Q6Q5

C3

C5C1 C2 C6

C4

Vin

Vout

Cout

VinVin

Q7 Q8

Φ1Φ1

Φ1 Φ2 Φ2 Φ2Fig. 2 Variable output charge pump.

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Φ2

few millivolts. A detailed analysis of the sources of ripple inthis charge pump circuit was presented in [8], whichconcluded that the major source of ripple was due to amismatch in parasitic capacitance. For DGB applications,where the charge pump does not supply a dc current andwhere efficiency is not crucial, the paper suggestedeliminating the mismatch problem altogether by simplyremoving transistor and using the output off of a singlehalf of the circuit.

C. Filter StructureThe general -C biquadratic structure shown in Fig. 3

was used for the design. The structure can be used to realizelow-pass, band-pass, and high-pass characteristics[6]. Acharge pump is associated with each transconductor cell formaximum flexibility.

IV EXPERIMENTAL RESULTS

In this section, we present details of the fabricated chip,the test set-up, and measured frequency response, distortion,and noise characteristics of the filter. A diemicrophotograph of the programmable filter is shown inFig. 4. The chip was fabricated in the TSMC 0.35 mCMOS process. The chip measures 1.2 mm x 1.2 mm. Thetop half of the chip consists of a row of six transconductorswith the common-mode feedback (CMFB) circuitrysandwiched between two capacitor banks. Two sets ofcapacitors of 35 pF each and one set of 30 pF capacitorscomprise the polysilicon capacitor banks. Beneath this issituated a row of six charge pumps. The chip biasingcircuitry is located in the bottom left corner. Thenon-overlapping clock generator and RC-network occupythe bottom and right corner, away from the main gm cells tominimize any noise coupled through parasitics. The gm-C

Q8

gm

gm2gm1 gm3

gm5gm4Vin

Vout

Vc1

Vc2

Vc4

Vc5

Vc3

Clock

Clock

Clock

ClockClock

CP3

CP1

CP4

CP5

CP2

Fig. 3 General -C biquadratic filter.gm

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filter occupies approximately 60% of the active IC area, thecharge pumps occupy approximately 25% and the clockgenerator and related biasing occupy the remaining 15%.

Fig. 5 shows the custom printed circuit board used intesting. A total of five, 8-bit D/A converters (TITLC7528CN) and op-amps (National LM358N) wererequired to set the input voltages of the five on-chip chargepumps.

The lowpass, bandpass, and highpass frequencyresponses of the filter were measured and some typicalresponses are shown in across a range of filter Q’s andcut-off frequencies from 350Hz to 10 kHz.

Fig. 7 shows a plot of the total harmonic distortion as afunction of frequency. The presented results are A-weightedto account for the frequency response of the human ear. Alow-pass filter configuration with a pole resonant frequencyof 50kHz was used so that all harmonic components ofinterest would fall within the passband. THD measurements

ChargePumps

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Biasing Clock RC Network

cells/

Fig. 4 Micrograph of test chip.

CapacitorBanks CMFB

Dip Switch

Dip Switch

TLC7528CN

LM358N

DUT

TLC7528CN

LM358NTLC7528CN

OutputInput

Fig. 5 Printed circuit board for testing.

were recorded for the frequency range between 100 Hz to 20kHz for a 150mVpp input signal. A 150mVpp input signalyields a THD (A-weighted) below -39 dB or less than 1.1%for all audible frequencies. A 140mVpp input signal yieldeda measured THD of approximately -44 dB. The A-weighted,input-referred noise spectrum of the filter is shown in Fig. 8.

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Fig. 6 Sample measured frequency responses.

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Fig. 7 THD (A-weighted) vs. frequency.

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eigh

ted

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THD(dB)

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Fig. 8 Input-referred noise (A-weighted) vs. frequency.

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Fig. 9 shows the measured clock feedthrough tonepresent at the output. A 20kHz clock was used to ensure thatthe tone and its harmonics are sufficient high to beinaudible. Fig. 10 confirms the measured output ripple ofthe charge pump to be about 1mV as required. The filterperformance is summarized in the table below.

Parameter Result

Technology 0.35 m CMOS

Supply voltage 1.2 V

*Power dissipation < 16 W

Frequency range 100 Hz - 30 kHz

THD (Vin = 150 mVpp) < 1.1%

Multiplier output ripple < 1.0 mV

Input referred noise (100 Hz - 10 kHz)

38 Vrms

SNR (at -45 dB THD) 62.3 dB

PSRR (at 1 kHz) 15 dB

CMRR (at 1 kHz) 27 dB

Max. input signal 350 mVpp

Active Area 1.0 mm2

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Frequency (Hz)

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oise

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ctru

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ot H

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20 kHz clock signal

Fig. 9 Measured clock feedthrough tone.

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(dB)

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Fig. 10 Output charge pump ripple (1mV/div).

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The overall power consumption was 16 W, althoughthis figure excludes the power dissipation of the oscillatorand more importantly, the off-chip data converters. Ifimplemented on-chip, the DACs more than likely woulddouble the total power consumption. One way around thisproblem is to use a switchable capacitor bank for tuning andeliminate the need for the DACs altogether [9].

V CONCLUSIONS

We have successfully applied dynamic gate biasing inthe design of a micropower, continuous-time -C filter forhearing aid applications. The application of DGB allowedvoltage swings that exceeded the supply voltage, enhancedthe filter’s tuning range and helped reduce distortion. Theexperimental results from this work will help establish DGBas a general technique for realizing low-voltage analogcircuits.

Acknowledgements

The authors would like to thank Steve Armstrong ofGennum Corp. for his technical support on hearing aids. Thechip was fabricated with the support of the CanadianMicroelectronics Corporation.

REFERENCES

[1] G.L.E. Monna et al., “Charge pump for optimal dynamic rangefilters,” IEEE ISCAS, vol. 5, pp. 747-750, 1994.[2] J. Zhou et al., “Charge-Pump Assisted Low-Power/Low-Voltage CMOS Opamp Design,” IEEE Proc. Int. Symp. LowPower Electronics and Design, pp. 108-109, 1997.[3] H. Schmid and G. S. Moschytz, “A Charge-Pump-ControlledMOSFET-C Single-Amplifier Biquad,” IEEE ISCAS, pp. 677-680.May 2000.[4] A. Yoshizawa and Y. Tsividis, “Anti-Blocker DesignTechniques for MOSFET-C Filters for Direct ConversionReceivers,” IEEE J. of Solid-State Circuits., vol. 37, no. 3, pp.357-364, March 2002.[5] K. Phang and D. Johns, “A 1V 1mW CMOS Front-End withOn-chip Dynamic Gate Biasing for a 75Mb/s Optical Receiver,”IEEE ISSCC, pp. 218-219, Feb. 2001.[6] D.A. Johns, K. Martin, Analog Integrated Circuit Design, NewYork: Wiley, pp. 582-584, 1997.[7] R. E. Sandlin, Textbook of Hearing Aid Amplification, SingularPublishing Group, San Diego, CA, 2000.[8] L. Pylarinos and K. Phang, “Analysis of Output Ripple inMulti-Phase Clocked Charge Pumps,” IEEE ISCAS, pp. 285-288,May 2003.[9] J. K. Mahabadi, “A Cell Based Super Low Power CMOSCrystal Oscillator with ‘ON CHIP’ Tuning Capacitors,” IEEEInternational Proceedings of Fourth Annual ASIC Conference andExhibit, pp. P5, 1991.

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