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A 1.2V, 10MHz, low-pass Gm-C filter with Gm-cells based on triode-biased

MOS and passive resistor in 0.13m CMOS technologyJun-Gi Jo

, Changsik Yoo

, Chunseok Jeong

, Chan-Young Jeong

, Mi-Young Lee

, and Jong-Kee Kwon

Department of Electronics and Computer Engineering, Hanyang University, Seoul 133-791, Korea

Electronics and Telecommunications Research Institute (ETRI), Daejeon 305-350, Korea

Abstract

A 1.2V 10MHz low-pass Gm-C filter implemented with

low-voltage Gm-cell based on passive resistor and triode-

region MOSFET is described. The Gm-cell converts the input

voltage to the output current by passive resistor for wider

signal swing. For low-voltage operation, triode-region MOS

transistors are widely used while the output resistance isimproved by regulated gate cascode circuit. The 10MHz low-

pass Gm-C filter was implemented in a 0.13m CMOStechnology and the measured input third order intercept point

is 3dBV and 9.5dBV, respectively for in-band and out-of-

band input.

I. Introduction

The scaling of CMOS technology is driven by the

requirement of lower price-per-performance for digital

circuitry because the area and power consumption of digital

circuitry for a given performance can be much smaller with

scaled CMOS technology. For low-cost system, analog

function would ideally be integrated on the same die as

digital circuitry. Analog design, however, becomes much

more challenging with scaled technology due to the lowered

supply voltage and the decreased allowable signal swing [1].

The performance requirement on analog circuits is ever

increasing with the rapidly improving computing power of

digital circuitry, aggravating the situation.

For these reasons, it is a challenging task to design a low-

voltage and high-frequency analog filter with scaled CMOS

technology. Although Gm-C filter is considered to be most

suited architecture for high-frequency operation among

various types of analog filters, the linearity and frequency

response of transconductance (Gm) cell becomes poor very

rapidly with lowered supply voltage [2-5].In this paper, a low-voltage and high-frequency Gm-cell is

proposed which is developed to maximize the dynamic range

under lowered supply voltage (1.2V for 0.13m standardCMOS technology). The voltage-to-current conversion is

performed by a passive resistor for wider signal swing. The

voltage across the passive resistor is kept to be proportional

to the input signal by connecting one of the terminals of the

resistor to VDD and maintaining the potential of the other

terminal to be equal to the input voltage by a feedback. For

low-voltage operation, MOS transistors operating in the

triode region are widely used while the output resistance of

the Gm-cell is improved by regulated gate cascode circuits.

The detailed description of the proposed Gm-cell will be

given in the following sections. The Gm-cell has been applied

to a third-order Butterworth low-pass Gm-C filter to verify its

performance and the measured results of the filter will also be

given.

(a) (b)

Fig. 1. (a) Conventional low-voltage Gm-cell based on triode-

region MOS transistor and (b) its maximum signal swing.

II. Conventional Triode-MOS Based Gm-Cell

The low-voltage Gm-cell proposed in this paper has been

evolved from the conventional Gm-cell based on MOS

transistors operating in the triode region shown in Fig. 1-(a).

This type of triode MOS based Gm-cells have been widely

used for low-voltage operation because of its large dynamic

range even with low supply voltage [2-3]. The recentlyreported low-pass Gm-C filer with Gm-cells of the same

basic structure as in Fig. 1-(a) has achieved 10MHz cut-off

frequency and 16.3dBm input-third-order intercept point

(iIP3) under 1.8V supply voltage [3].

With the supply voltage of 1.2V (nominal recommended

supply voltage for standard 0.13m CMOS technology),however, the Gm-cell in Fig. 1-(a) has a very narrow

allowable signal swing. As illustrated in Fig. 1-(b), the

common-mode level of the signals is desirably VDD/2 and the

minimum possible input voltage is Vdn+Vtn(Vtn : the thresholdvoltage of nMOS transistors) because the transistor M1 must

Acmfb

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stay in the triode region. Since the signal swing should be

symmetrical along the common-mode level, the maximum

input voltage is VDDVdnVtn.

The threshold voltage of nMOS transistors is about 0.4Vfor the 0.13m CMOS technology used in this work. If weassume Vdn is 0.1V (for low-voltage operation, it is desirable

to set Vdn as low as possible), the maximum and minimum

signal levels of the Gm-cell in Fig. 1-(a) are 0.5V and 0.7V,

respectively and thus the allowed signal swing is only 0.2Vp-

p. Therefore, the conventional triode-MOS based Gm-cell in

Fig. 1 would have very poor linearity performance. For a

given technology (for a given VDD and Vtn), the only way to

have wider signal swing is to have smaller value ofVdn. But

because Vdn directly determines the transconductance of the

Gm-cell, we cannot use arbitrarily small value ofVdn.

This limited signal swing is due to the fact that the input

signal is directly connected to the gate of the voltage-to-

current converting transistor operating in the triode region.

III. Proposed Triode-Biased MOS and Passive

Resistor Based Low-Voltage Gm-Cell

The proposed low-voltage Gm-cell shown in Fig. 2 has

much wider signal swing than the conventional one by

separating the input signal from the gate of the MOS

transistor (M1) in the triode region. The Gm-cell has two

input pairs to facilitate the design of a filter. The transistors

M1,M3,M5,M7,M10,M12,M13,M14,M15,M17,M19,M21,M23,

andM24 are in the triode region while the other transistors are

in the saturation region.

A. Operation Principle

The voltage-to-current conversion is performed by the

passive resistor R for wider signal swing. *The voltage level

of the lower terminal of the resistor (upper left one) is kept to

be equal to the input voltage by the feedback network

consisting ofM1,M2,A1, andA2. Thus, the voltage across the

resistor is VDDVin1 and the current through the resistor R,M1,and M2 is (VDDVin1)/R. Because the drain voltages of thetransistors M1 and M3 are all equal to Vdn and their gate

voltages are same, the drain current ofM1, (VDDVin1)/R, iscopied to the transistor M3. It can be thought the transistors

M1 and M3 in the triode region constitute a current mirror

with their drain voltages being forced to be equal by thefeedback. The current flowing through the transistor M1 is not

connected to output and therefore wasted. Thus, the (W/L)

ratio of the transistor M3 is made Ntimes larger than that of

M1 in order to minimize the wasted current in M1.

The common-mode level of the differential output is

stabilized by the common-mode feedback network consisting

ofM9-M12,A7,A8, andAcmfb. The capacitors C1 and C2 provide

a high-frequency path in the common-mode feedback

network, improving the common-mode feedback stability.

* The explanation is given for the upper-left part of the Gm-cell, that

is, for the input Vin1, but can be generalized for the other inputs.

The capacitance ofC1 and C2 must be considered as a part of

load capacitance when implementing a filter.

Because the Gm-cell is a pseudo-differential circuit, the

common-mode variation of the input signals is not rejectedand thus the common-mode feedforward network consisting

ofRcmff, M13, M14, M23, and M24 is employed as well. The

current (Vin1Vin1b)/2Rcmff through the feedforward resistors isadded to the output current.

The differential output current is now given as ;

(1)

and therefore the transconductance is N(1/R+1/Rcmff). Byconfiguring the resistor R as a resistor array, variable

transconductance can be obtained, giving tunability to Gm-C

filter.

Fig. 2. Low-voltage Gm-cell where the voltage-to-current

conversion is performed by passive resistor.

B. Allowable Signal Swing

Now, lets derive the allowed signal swing of the proposed

Gm-cell with 1.2V supply voltage. The drain voltage Vdn of

M1 is assumed to be 0.1V as in the conventional Gm-cell of

Fig. 1 for fair comparison. By proper sizing, the minimum

required drain-to-source voltage Vds,sat,M2 for the transistor M2

to stay in the saturation region is set to be 0.2V. Then, the

input signal can swing down to 0.3V (Vds,sat,M2+Vdn) and up to0.9V (VDDVds,sat,M2Vdn). Therefore, the allowed signal swing

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is 0.6Vp-p which is three times larger than that of the

conventional low-voltage Gm-cell in Fig. 1-(a). Another

difference from the conventional one is that the value ofVdn

can now be chosen without any concern on thetransconductance.

The above derivation is assuming the input transistor M1

stays in the triode region. If the (W/L) ratio of the transistor

M1 is sufficiently large so its gm be large enough and thereby

its gate voltage need not be modulated too much for large

input swing, the above assumption is valid. For the proposed

Gm-cell, the (W/L) ratio of the transistorM1 can be chosen to

be sufficiently large without affecting the transconductance

of the Gm-cell because the (W/L) ratio of the transistor has no

effect on the transconductance. The transconductance is

solely determined by the resistorR.

-0.9 -0.6 -0.3 0 0.3 0.6 0.9Differential input voltage [V]

1.2

0.6

0

-0.6

-1.2

Differentialoutputcurrent[mA]

(a)

-0.9 -0.6 -0.3 0 0.3 0.6 0.9Differential input voltage [V]

1.1

0.9

0.7

0.5

0.3

(b)

Fig. 3. Simulated (a) voltage-to-current transfer characteristic

and (b) transconductance.

The Gm-cell was simulated for different values ofR with

the HSPICE. Fig. 3-(a) shows the voltage-to-current transfer

characteristic and the transconductance is plotted in Fig. 3-(b).

The transconductance can be varied from 0.65mA/V to

1.18mA/V.

IV. Third-Order Butterworth Filter

In order to verify the functionality and performance of the

proposed triode-MOS and passive resistor based low-voltage

Gm-cell, a fully differential third-order low-pass Butterworth

filter shown in Fig. 4 has been implemented. The filter

topology is derived from a doubly terminated LC-ladder

prototype and all the Gm-cells have the same

transconductance. With the dual-input Gm-cell, the number

of Gm-cells can be reduced, which saves chip area and power

consumption. The very first Gm-cell has two times the

effective transconductance by having two input pairs shorted

together to compensate for the inherent 6dB loss in the

passband of a doubly terminated LC-ladder filter. The

magnitudes of the load capacitors are determined by taking

the parasitic capacitance and common-mode feedback path

stabilizing capacitors (C1 and C2 in Fig. 2) into account.

Fig. 4. Third-order Butterworth filter implemented with the

Gm-cell in Fig. 2.

Fig. 5. Microphotograph of the third-order Butterworth filter.

V. Experimental Results

The third-order Butterworth filter built with the low-

voltage Gm-cell of Fig. 2 has been implemented in a 0.13mstandard CMOS technology whose microphotograph is in Fig.

5. The active area of the filter is 0.9mm0.6mm. Because the

current design is to verify the low-voltage Gm-cell, automatic

+

++

Vin+

++

+

++

+

++

Vout

Capacitors

Gm-cells

Capacitors

Capacitors

Gm-cells

Capacitors

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frequency tuning circuit is not included.

The current design is focusing on low-voltage operation

and not optimized for low-power consumption and therefore

the filter dissipates somewhat large power of 22mW under1.2V supply voltage. The power consumption would be

greatly reduced if the transconductance of the Gm-cells and

load capacitance of the integrators are optimized.

The measured frequency characteristic of the filter is

shown in Fig. 6 when the transconductance of the Gm-cell is

manually set so the 3dB cut-off frequency is 10MHz. The

loss of 1.6dB at passband is due to the non-ideal frequency

response of the active baluns at the input and output of the

filter which are implemented with operational amplifiers.

Fig. 7 shows the two-tone test results for both in-band and

out-of-band inputs. For in-band test, two tones at 2MHz and

3MHz are applied and the measured input third-order

intercept point (iIP3) is 3dBV. The out-of-band iIP3 is9.5dBV which is measured with two tones at 20MHz and

35MHz.

VI. Conclusion

A low-voltage and high-frequency Gm-cell is proposed

whose voltage-to-current conversion is performed by a

passive resistor for good linearity. For low-voltage operation,

MOS transistors operating in the triode region are widely

used while the output resistance of the Gm-cell is improved

by regulated gate cascode circuit. The proposed low-voltage

Gm-cell is applied to a third-order low-pass Butterworth filter

implemented in a 0.13m standard CMOS technology.

Acknowledgments

A part of this work was supported by the System IC 2010

Research Project and the Human Resource Development

Project for IT SoC Key Architect funded by the Ministry of

Information and Communication, Korea. The CAD tools

were provided by IDEC.

References

[1] A. J. Annema, B. Nauta, R. van Langevelde, and H.

Tuinhout, Analog circuits in ultra deep submicron

CMOS,IEEE J. Solid-State Circuits, Vol. 40, No. 1, pp.

132-143, Jan. 2005.[2] C. Yoo, S.-W. Lee, and W. Kim, A 1.5-V, 4-MHz

CMOS continuous-time filter with a single-integrator

based tuning, IEEE J. Solid-State Circuits, Vol. 33, No.

1, pp. 18-27, Jan. 1998.

[3] Y.-H. Kim, J.-W. Park, M.-Y. Park, and H.-K. Yu, A

1.8V triode-type transconductor and its application to a

10MHz 3rd-order Chebyshev low pass filter, Proc.

Custom Integrated Circuits Conf., pp. 53-56, 2004.

[4] S. Hori, T. Maeda, N. Matsuno, and H. Hida, Low-power

widely tunable Gm-C filter with an adaptive DC-blocking

triode-based MOSFET transconductor, Proc. European

Solid-State Circuits Conf., pp. 99-102, 2004.

[5] B. Guthrie, T. Sayers, A. Spencer, and J. Hughes, A

CMOS gyrator low-IF filter for a dual-mode Bluetooth /

ZigBee transceiver, Proc. Custom Integrated CircuitsConf., pp. 49-52, 2004.

[6] U. Yodprasit and C. Enz, A 1.5V 75dB dynamic range

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38, No. 7, pp. 1189-1197, Jul. 2003.

Fig. 6. Measured frequency response of the filter. The result

shows about 1.6dB loss in the passband due to the activebaluns at input and output of the measurement setup.

Fig. 7. Measured third-order input intercept point (iIP3) for

in-band (with two tones at 2MHz and 3MHz) and out-of-band

(with two tones at 20MHz and 35MHz) inputs.

Out-of-band iIP3 = 9.5dBV

In-band iIP3 = 3dBV