A CMOS 2.4 GHz tunnable RF Bandpass Filter in 0.35μm Technology

Aymen Ben Hammadi, Mongia Mhiri, Sehmi Saâd and Kamel Besbes Microelectronic and Instrumentation LR

FSM, University of Monastir Monastir, Tunisia

benhammadiaymen@gmail.com, Mongia.Mhiri@fsm.rnu.tn, sehmi002@yahoo.fr, Kamel.Besbes@fsm.rnu.tn

Abstract—A tunable Q-enhanced bandpass filter is presented. The Q of the passive inductors that form the filter resonators is enhanced using a cross-coupled differential pair of transistors which is degenerated by a negative resistance. This technique allows compensation of frequency dependent inductor losses and ensures the Q-enhanced LC resonators to have frequency behaviour close perfectly ideal in the pass band of the filter. The filter centered at 2.4 GHz with a 31.5 MHz bandwidth is tunable in frequency by 3.75%, exhibits a -33 dBm for 1-dB compression point and a 16.64 dB noise figure while consuming 4 mW of power. The circuit was simulated in AMS 0.35 μm CMOS technology.

Index Terms— Bandpass filters, negative resistance, LC tank varactors, Q enhancement.

I. INTRODUCTION In a fast-growing market, operators of cellular provide new

applications that require high-speed communication between mobile terminals and base station. These applications, such as Bluetooth, wireless LANs, cellular phones, and the global positioning systems (GPS) as well as the portable devices [1], are made possible by the development of communications standards and the emergence of new technologies in the field of microelectronics.

A high integration reduces not only the cost of the products by minimizing the number of off-chip elements but also, to some extents, power consumption by eliminating the need for driving low-impendence off-chip components.

Although much research has been done and reported to achieve the filter with CMOS technology, low voltage, low power, high linearity and high programmable Q-factor, the filter is yet to be investigated.

This paper describes a circuit design based on AMS 0.35 μm CMOS process for a widely tunable filter.

This filter has an application of the passive inductors and negative resistance technology to realize a high Q-factor tuning bandpass filter for ISM band (Bluetooth, Home RF, WiFi 802.11b and 802.11g and Zigbee). These and other issues are addressed in this paper [1-2].

Section II discusses the issue of the Q-enhancement methods.

Section III discusses the synthesis of the above parts and the prototype of a LC-tank resonator and gives a design of a 2nd order tunable RF CMOS LC bandpass filter. Section IV analyses the design and feasibility of automatic tuning, which covers the range from 2.4 GHz to 2.49 GHz with a comparatively steady peak gain. In the last section, the simulation results and the conclusion are presented.

II. Q-ENHANCED LC FILTER DESIGN

A. Frequency tunning The tuning element used in this filter was a varactor diode.

Varactor diode is a semiconductor diode that, when reverse biased below its breakdown voltage, acts as a voltage-controlled variable capacitance. The reverse voltage changes the width of its depletion layer, changing the diode junction capacitance. There is a characteristic curve of the junction capacitance Cj versus the reverse bias voltage Vctr specific to each type of diode, following eq. (1).

�� � �����

��� (1)

Where:

Cj is the reverse biased variable junction capacitance; Cj0 is the junction capacitance without an external bias; V is the applied reverse bias voltage; Vj is the built-in junction potential; � “gamma” is a coefficient relating Cj and V. It depends on the junction doping profile.

When modeling a varactor diode, one should take into account the parasitic elements that are mostly due to the package, in order to obtain an accurate equivalent electrical circuit model at the frequency band at which the diode will operate. For the filters developed in this work, the classical model used is shown in Figure 1, where Rs represents the parasitic series resistance of the diode die, Ls and Cp are the parasitic inductance and capacitance due to the package.

Integrated varactors can be implemented in several ways. This paper focuses on the following type: inversion mode PMOS [3]. Each variable capacitor is replaced by two varactors; the drain and source of the first are controlled by

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the control voltage Vctr for the second they are connected to VDD. (Figure 2 (b)).

B. LC Tank The LC tank is composed of L1, L2, C1 and C2 as

showing Figure 2 (a). The non idealities of the inductor have to be taken into consideration, assuming that the quality factor of the tank capacitance is much larger than the inductor, which is presented by series of resistance. The impedance transformation is performed [4] using the following equations:

�� � �� � ������ ��� �� � �� �

���� �� ��� �� � � � �

It is obvious from equation (2) that by increasing the parallel resistance, the quality factor increases. A negative resistance is added in parallel to the parallel loss resistance in the LC tank so as to increase the equivalent parallel resistance as much as possible to decrease losses.

C. Negative resistance A primary method for increasing the Q of non-ideal on-

chip LC tank is the use of negative resistance, implemented by active devices, as shown in Figure 2 (a), but at the cost of higher power consumption and noise, presented by the active devices. A differential topology is used for our balanced circuits which is better in high frequency operation [4], using a cross-coupled transistor pair T1/2 (gate inputs are connected to opposite drain Outputs), achieving a positive feedback that compensates for the losses in the LC tank [5]. The voltage to current ratio indicates the effective negative resistance at the terminals M1/2 as shown in Figure 3:

R � Vds1!Vds2i

� !2gm �"�

Negative transconductance of the circuit is generated when the current flows through the transistors. This transconductance also depends on the technology process and the source degradation resistance. At resonance frequency, the negative transconductance brings the tank circuit impedance to a high value, which converts into a high Q of filter [5]. The negative resistance is -2/gm when the two transistors have the same size.

D. Q-Tuning It is clear from Figure 3 and equation 6, that the effective

quality factor can be adjusted by varying the transconductance, gm, of the differential pair. This makes the tuning of the quality factor easier.

�#$% � !&'() �� �*�

Where Qenh is the effective quality factor of the LC tank. Q-tuning is made by changing the tail current of the

negative conductance circuit built with two cross-coupled NMOS.

III. FILTER ARCHITECTURE The 2nd order RF CMOS filter of Figure 4 is composed

mainly of a conventional LC resonator associated with a negative resistance, an input transconductance to provide the current needed to the circuit , two circuit based on the current mirror ( based on MOSFETs) respectively can adjust the quality factor and the gain of the circuit. In the parallel LC resonator, we use two inductors L1 and L2 with a value of 1.79nH and four varactors in parallel to tune the center frequency of filter.

One of the advantages of the circuit is the possibility of granting the center frequency of 2.4 GHz to 2.49 GHz, which corresponds to a relative tunability of 3.75% (Figure 5) by varying the control voltage Vctr between 0.1V and 1.5V.

Figure 1. LC tank with diode varactor.

Figure 2. (a) LC tank with diode varactor, (b) set of varactor diode.

Figure 3. Negative resistance implanted by cross-coupled NMOS transistors.

Rs Cj Ls

Cp

(a)

(b)

i

IQ

Vds2Vds1

M1 M2

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Figure 4. Q-enhanced LC bandpass filter.

The frequency tuning is achieved through the varactors, whereas the center frequency is tuned by changing the tail current of the negative conductance circuit built with two cross-coupled NMOS transistors. Additional programmability in peak gain is incorporated into this design.

Center Frequency �0 of Filter is given by

+, � -./010 (7)

And the magnitude of the voltage gain can be expressed from

2�3� �456789:;: �3 � <=> �

3� � 3 �<=> ? 45@9:;: � ��>9:;: A� ? 45@<=B

�C�

It should be noted that the enhanced quality factor Q is satisfied with the below formula

D � <=9:;:->9:;:A� ? 45@<=B �E�

gmQ is the transconductance of the cross-coupled MOS M5 and M6 and generating the controlled negative resistance to compensate for the losses in the passive components. Rp represents the total losses in the inductance, capacitance and MOS gate resistance. Ctot is the total output capacitance of the filter including the capacitance of the varactor array denoted as C. L is the value of the passive inductors. The input

transconductance, forming a cross-coupled pair, is not used only to perform as negative resistance for compensating the loss of the inductor, but also adjust the linearity of the filter in some circumstances. But as the Q increases, the peak gain is enhanced and hence the output voltage swing will increase. That is one major source of contributors to nonlinearity [6].

IV. SIMULATION RESULTS The proposed filter has been simulated with T-spice

software using AMS 0.35μm CMOS process models. All transistors have the minimum channel length of 0.35-�m. The complete circuit has very promising results. With 1.5 V supply, the circuit consumes less than 2.67 mA (4mW).

Figure 5 shows the tuning center frequency range that is about 2.4GHz~2. 49GHz. The varactor provides the tuning range about 90 MHz when the Vctr varies from 0 to 1.5V. At the frequency of 2.4GHz, the measured - 3dB bandwidth of the filter is about 31.57MHz, which indicates the high Q of about 76.

As shown in Figure 6, the quality factor of the filter is tuned from 65 to 170 by increasing the bias current from 0.5mA to 4mA.

To study the stability of the circuit, transient analysis is performed using the Agilent ADS tools. So, it was applied to the input circuit a sinusoidal signal of amplitude 0.2 V, at a frequency of 2.4 GHz with an offset of -1V. This gives in the output a sinusoidal signal whose stability is reached after 12.5ns. A transient simulation giving the output of the RF bandpass LC filter is shown in Figure 7.

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The noises in the circuit are presented in Figure 8. Using T-spice, we can determine the noise spectral density referred to the input, the spectral density of output noise and the noise factor. From these parameters, we can determine the noise of our circuit, which is about 14.21 dB. The main noise contributions come from the ohmic losses in inductors and followed by the thermal noise of transistors.

The linearity of the circuit can be measured from the output versus input power graphs. In Figure 9 we show the 1-dB compression point which is -33dBm input power.

The three main contributors to the nonlinearity of the filter are the negative conductance generator, the varactor and the input gm stage. The nonlinearity analysis of the circuit demonstrates that the contributions of the negative conductance generator and the varactor are much more pronounced than that of the input stage.

A summary of the simulated results is given in Table I.

TABLE I. PERFORMANCE SUMMARY OF RF BANDPASS LC FILTER

Measured value Parameter This work [7] [8] Process AMS 0.35μm

CMOS 0.18 μm CMOS

0.35μm CMOS

Filter order 2 2 2 Center frequency

2.4 --2. 49GHz 2.33 --2.57 GHz 1.93--2.19GHz

Quality factor 65-170 15-107 20-170 Average -3dB Bandwidth

31.57--32.76MHz -- --

Peaking passband gain

10.5 dB - --

Supply voltage 1. 5V 1.8V 1.3V Power 4 mW 12.69 mW 5.2 mW Noise figure 14.21dB 15.38 dB 26 dB IP1 dB -33 dBm -2.73 dBm -33.5 dBm

V. CONCLUSION In this work, the detailed design consideration including

stability, Q enhancement, linearity and noise of the proposed Q-enhancement RF bandpass LC filter in AMS 0.35um CMOS have been presented and incorporated into the design process. Results have confirmed Q enhancement with negative resistance and frequency tuning with PMOS varactor using a lossy LC resonant circuit

The frequency center is tunable from 2.4 to 2.49 GHz with a gain of 10.5 dB and a bandwidth between 31.57 MHz and 35.23 MHz. The frequency range is tunable up to 3.75% . The input referred compression point is about -33 dBm. This structure has a frequency drift of 90 MHz tuning the quality factor from 65 to 170. The noise figure is 14.21 dB.

Figure 5. Frequency tunning of the RF filter.

Figure 6. Quality factor (Q) tuning of the bandpass filter by IQ

Figure 7. Differential output of the filter

Figure 8. Noise spectral density of the filter.

2.30 2.35 2.40 2.45 2.50 2.55

Frequency (GHz)

-5

0

5

Volta

ge M

agni

tude

(dB)

vdb(8,2)

2.20 2.25 2.30 2.35 2.40 2.45 2.50

Frequency (GHz)

-5

0

5 Vo

ltage

Mag

nitu

de (d

B)

vdb(8,2)

2.25 2.30 2.35 2.40 2.45 2.50

Frequency (GHz)

0

5

10

Noi

se S

pect

ral D

ensi

ty (n

V/H

z)̂

onoise(mag)inoise(mag)

Output noise

Input noise

t(s)

Vout+ - Vout- (V)

Gai

n (d

B)

Gai

n (d

B)

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-55 -50 -45 -40 -35 -30 -25 -20-15

-10

-5

0

5

10

15

Out

put p

ower

(dBm

)

Input power (dBm)

Figure 9. 1-dB compression point

REFERENCES [1] R. A. Duncan, K. W. Martin, and A. S. Sedra, “A Q-enhanced active-

RLC bandpass filter,” in Proc. IEEE Int. Symp. Circuits and Systems, 1993, pp. 1416–1419.

[2] T. Soorapanth and S. S. Wong, “A 0 dB-IL 2140 _ 30 MHz bandpass filter utilizing Q-enhanced spiral inductors in standard CMOS,” in Symp. VLSI Circuits Dig. Tech. Papers, June 2001, pp. 15–18.

[3] O.H.Kassem and S. Mahmoud, “Low voltage, low power CMOS Front-end for Bluetooth Applications,” International Conference on Microelectronics, 2008.

[4] W. B. Kuhn, F. W. Stephenson, and A. Elshabini-Riad, “A 200- MHz CMOS Q-enhanced LC bandpass filter,” IEEE J. Solid-State Circuits, vol. 31, no. 8, pp. 1112–1122, Aug. 1996.

[5] A.W. Orsborn, “Noise analysis and automatic tuning of Q-enhanced LC bandpass filters,” M.S. thesis, Dept. Electr. Comput. Eng., Kansas State Univ., Manhattan, KS, 2001.

[6] Jiangdong Ge and Anh Dinh, "A 3.6GHz Tunable CMOS Bandpass Filter Using Q-Enhanced Circuit," Communications, Computers and signal Processing, 2003. PACRIM. 2003 IEEE Pacific Rim Conference on Volume: I Page(s) 57 - 61, 2003.

[7] Joshua K. Nakaska, Student and James W. Haslett, Fellow, “A CMOS Quality Factor Enhanced Parallel Resonant LC-Tank with Independent Q and Frequency Tuning for RF Integrated Filters” , IEEE Trans. on Circuits and Systems-II, 2005 .

[8] F. Dulger, E. S. Sinencio, J. S. Martinez, “A 1.3-V 5-mW fully integrated tunable bandpass filter at 2.1 GHz in 0.35 !m CMOS” IEEE Journal of Solid-State Circuits, vol. 38, No. 6, pp. 918-928, Juin. 2003.

P1dB= -33dBm

Input power (dBm)

Out

put p

ower

(dB

m)

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