A 1.2V, 10MHz, low-pass Gm-C filter with Gm-cells based on triode-biased
MOS and passive resistor in 0.13μm CMOS technology
Jun-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 trioderegion 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 is
improved by regulated gate cascode circuit. The 10MHz lowpass Gm-C filter was implemented in a 0.13μm CMOS
technology and the measured input third order intercept point
is 3dBV and 9.5dBV, respectively for in-band and out-ofband input.
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.
Acmfb
†
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 lowvoltage 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 0.13μm standard
CMOS 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
(a)
(b)
Fig. 1. (a) Conventional low-voltage Gm-cell based on trioderegion 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 recently
reported 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 0.13μm 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 threshold
voltage of nMOS transistors) because the transistor M1 must
stay in the triode region. Since the signal swing should be
symmetrical along the common-mode level, the maximum
input voltage is VDD−Vdn−Vtn.
The threshold voltage of nMOS transistors is about 0.4V
for the 0.13μm CMOS technology used in this work. If we
assume 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 0.2Vpp. 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 of Vdn. But
because Vdn directly determines the transconductance of the
Gm-cell, we cannot use arbitrarily small value of Vdn.
This limited signal swing is due to the fact that the input
signal is directly connected to the gate of the voltage-tocurrent converting transistor operating in the triode region.
The capacitance of C1 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 rejected
and thus the common-mode feedforward network consisting
of Rcmff, M13, M14, M23, and M24 is employed as well. The
current (Vin1−Vin1b)/2Rcmff through the feedforward resistors is
added to the output current.
The differential output current is now given as ;
⎛1
1
iout = N × ⎜ +
⎜R R
cmff
⎝
⎞
⎟ × {(Vin1 − Vin1b ) + (Vin 2 − Vin 2 b )}
⎟
⎠
(1)
and therefore the transconductance is N×(1/R+1/Rcmff). By
configuring the resistor R as a resistor array, variable
transconductance can be obtained, giving tunability to Gm-C
filter.
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,
and M24 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 of M1, M2, A1, and A2. Thus, the voltage across the
resistor is VDD−Vin1 and the current through the resistor R, M1,
and M2 is (VDD−Vin1)/R. Because the drain voltages of the
transistors M1 and M3 are all equal to Vdn and their gate
voltages are same, the drain current of M1, (VDD−Vin1)/R, is
copied 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 the
feedback. 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 N times 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
of M9-M12, A7, A8, and Acmfb. 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.
Fig. 2. Low-voltage Gm-cell where the voltage-to-current
conversion is performed by passive resistor.
B. Allowable Signal Swing
Now, let’s 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 to
0.9V (VDD−Vds,sat,M2−Vdn). Therefore, the allowed signal swing
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 of Vdn
can now be chosen without any concern on the
transconductance.
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 transistor M1 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 resistor R.
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.
0.6
0
-0.6
-0.3
0
0.3
Differential input voltage [V]
0.6
+
+
+
+
+
+
+
-1.2
-0.9
+
+
-0.6
+
+
Vin
+
Differential output current [mA]
1.2
IV. Third-Order Butterworth Filter
Vout
0.9
(a)
Fig. 4. Third-order Butterworth filter implemented with the
Gm-cell in Fig. 2.
1.1
0.5
Capacitors
0.7
Gm-cells
Capacitors
0.9
0.3
-0.9
-0.6
-0.3
0
0.3
Differential input voltage [V]
0.6
0.9
(b)
Fig. 3. Simulated (a) voltage-to-current transfer characteristic
and (b) transconductance.
The Gm-cell was simulated for different values of R 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.
Fig. 5. Microphotograph of the third-order Butterworth filter.
V. Experimental Results
The third-order Butterworth filter built with the lowvoltage Gm-cell of Fig. 2 has been implemented in a 0.13μm
standard CMOS technology whose microphotograph is in Fig.
5. The active area of the filter is 0.9mm×0.6mm. Because the
current design is to verify the low-voltage Gm-cell, automatic
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 under
1.2V 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 is
9.5dBV which is measured with two tones at 20MHz and
35MHz.
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 Circuits
Conf., pp. 49-52, 2004.
[6] U. Yodprasit and C. Enz, “A 1.5V 75dB dynamic range
third-order Gm-C filter integrated in a 0.18μm standard
digital CMOS process,” IEEE J. Solid-State Circuits, Vol.
38, No. 7, pp. 1189-1197, Jul. 2003.
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 0.13μm standard CMOS technology.
Acknowledgments
Fig. 6. Measured frequency response of the filter. The result
shows about 1.6dB loss in the passband due to the active
baluns at input and output of the measurement setup.
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.
In-band iIP3 = 3dBV
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Out-of-band iIP3 = 9.5dBV
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.