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Journal of Circuits, Systems, and Computers Vol. 24, No. 7 (2015) 1550098 (9 pages) # .c World Scienti¯c Publishing Company DOI: 10.1142/S021812661550098X

A Low Voltage, Low Power and Highly Linear CMOS Down-Conversion Gilbert Cell Mixer Using MGTR Method¤

E. Morad† Department of Electrical Engineering, Islamic Azad University, SHADEGAN Branch, Shadegan, Iran [email protected]

S. Z. Moussavi Electrical & Computer Engineering Faculty, Shahid Rajaee Teacher Training University, Lavizan, Tehran, Iran [email protected]

M. Alasvandi Department of Electrical Engineering, Islamic Azad University, Central Tehran Branch, Poonak, Tehran, Iran [email protected]

E. Rasouli Department of Electrical Engineering, Amirkabir University of Technology, Tehran, Iran [email protected] Received 25 October 2014 Accepted 6 April 2015 Published 22 May 2015 A radio frequency (RF) low voltage and low power down conversion mixer with high linearity using TSMC 0.18-
m technology is presented which operates in 2.4 GHz Industrial Scienti¯c and Medical (ISM) band. The local oscillator (LO) frequency is 2.1 GHz with an input power of 5 dBm, whereas IF frequency is 300 MHz. Multiple gated transistors (MGTRs) method is used to increase the linearity of Gilbert cell mixer. In this method an auxiliary transistor is used parallel to the transconductance stage transistor. This increases linearity by decreasing of transconductance stage transistor. The simulation results show an IIP3 improvement of 16.55 dBm. The proposed low power and highly linear mixer consumes a power of 4.46 mW from *This paper was recommended †Corresponding author.

by Regional Editor Piero Malcovati.

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E. Morad et al. 1.8 V a supply voltage. The noise ¯gure (NF) and gain conversion are about 13.8 dB and 9.11 dB, respectively. Keywords: Radio frequency; high linearity; low power; MGTR method; Gilbert cell mixer; local oscillator.

1. Introduction The semiconductor industries proceed towards total integration and implementation of a complete transceiver (analog and digital signal processing) on a single chip, because of high demand for wireless products in mass consumer markets.1 Moreover, demand for wireless services working in Industrial Scienti¯c and Medical (ISM) band has increased due to the fact that, this band is free for ISM applications. In most countries, this radio spectrum can be used without any license. Therefore, ISM band is very attractive and lucrative for chip design companies. Furthermore, use of CMOS technology in chip design provides the possibility of low power mixer at reasonable price, this makes designing CMOS mixers in ISM band an important research topic. On the other hand, with rapid progress of deep submicron radio frequency (RF) CMOS technologies, it is no longer impossible to have exceptionally compact, high performance and low-cost RF integrated circuits and systems.1 In RF transceivers, the mixer is an important component which provides the frequency translation from RF to intermediate frequency. The mixer linearity is very important because it improves the overall system linearity. Many methods and structures have been used to improve mixers linearity but they result in higher power consumption or more complicated circuits.2–6 There are many di®erent methods for circuit's linearization such as feedback,7 feedforward,8 predistortion,9 envelope elimination and restoration (EER),10 multiple gated transistor (MGTR)11 linearization methods, etc. .

Feedback linearization: The feedback linearization method can be applied to RF signal or indirectly to the modulation, i.e., envelope, phase or I (in-phase) and Q (quadrature) components. For example, the output is fed back without detection or down-conversion in RF feedback.12 . Feedforward linearization: This technique that is based on present events is independent of ampli¯er delays.12 . Predistortion: Predistortion is based on the idea of inserting a nonlinear element prior to a RF PA such that the combined transfer characteristic of the two devices is linear.12 . EER: The basic method of EER is to disassemble the RF input signal into phase and envelope components and combine both after ampli¯cation.12

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MGTR: The semiconductor transistors o® currents that have increased as technology scales, is reduced by multi-gate transistors.11

The MGTR method, however, does not need a complicated circuit or any extra power. This method has been used for highly linear low noise ampli¯er (LNA) design in recent years.13–16 Therefore, we use this method to design a highly linear Gilbert cell mixer in this paper; this mixer can work in ISM band using MGTR method. The rest of this paper is organized as follows. Section 2 presents the theory of MGTR method for designing of the mixer. The highly linear Gilbert cell mixer will be proposed in Sec. 3. Section 4 covers the simulation results obtained for the proposed mixer. Finally, we conclude this paper in Sec. 5.

2. The Theory of MGTR Method Several factors a®ect the mixer linearity. If the e®ect of nonlinearity of the switching transistor is ignored, the main factor of nonlinearity in mixers is transconductance stage where RF voltage is converted to RF current. This nonlinearity is due to the inherent nonlinearity of transistors in saturation region. The small signal drain current of the common source transconductance MOSFET can be expressed by the following Taylor series expansion15: ID ¼ IDC þ gm vgs þ

g 0m 2 g 00 v gs þ m v 3gs : 2! 3!

ð1Þ

It is well known that the coe±cient of v 3gs in Eq. (1) plays an important role in mixer linearity. sffiffiffiffiffiffiffiffiffiffi 4 1 IIP3 ¼ ; ð2Þ 3 3 1 ¼ g m ; 3 ¼

g 00m 3!

ð3Þ :

ð4Þ

In MGTR circuitry, linearity is improved by using transconductance linearization which can be achieved by cancellation of the negative peak value of g 00m of the main transistor with the positive one. The main transistor is biased in the saturation region and its g 00m has a negative peak. The auxiliary transistor has a di®erent size and is biased in sub-threshold region and its g 00m has a positive peak. This linearization method does not consume any extra power because the auxiliary transistor is biased in sub-threshold region.13–15,17,18 Figure 1(a) shows the variation of g 0m and g 00m with respect to vgs , as can be seen in Fig. 1(b), the addition of the auxiliary transistor parallel to the main transistor for 1550098-3

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(a)

(b)

Fig. 1. (a) g 0m ; g 00m of the main transistors and (b) g 00m cancellation using MGTR method.

improving linearity, has reduced the magnitude of g 00m which is the third-order nonlinearity coe±cient. This reduction improves the linearity of the mixer. Ids ¼ IAUX þ IMain ; IAUX ¼ IMain ¼ gmMain vgs þ Ids ¼ gmMain vgs þ

g 00mAUX v 3gs

;

g 0mMain 2
g 00mMain 3 v gs þ v gs ; 2! 3!

v 3gs g 0mMain 2 v gs þ ð
g 00mMain þ g 00mAUX Þ : 2! 3!

ð5Þ ð6Þ ð7Þ ð8Þ

It can be seen that the coe±cient of v 3gs is decreased. This improves linearity of mixer.

3. Mixer Design The mixer design involves many compromises between conversion gains, linearity, noise ¯gure (NF), port to port isolation, supply voltage and current consumption.2 The schematic of the proposed highly linear mixer is shown in Fig. 3, M1 and M2 are transconductance stage transistors that convert RF voltage to RF current and the transistors M3–M6 act as switches. The chopping action of M3–M6 transistors causes RF signal and local oscillator (LO) signal to get multiplied and converts the RF current to IF current. R1 and R2 act as loads to convert IF current to IF voltage.19,20 If two switching pair transistors contact at the same time, noise will increase.19,21 In the LO stage of the mixer, switch operation for the mixing has an on/ o® transition, and during the transition, a high °icker noise is delivered to the output.5,14 1550098-4

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Third-order intercept point and the conversion gain of the mixer are proportional to the square root of the bias current.4  1 2 2 2 w GC ¼ gm RL ¼ RL
n cox ID ; ð9Þ   L 0 11 2

AIIP3

ID C B 32 ¼@ A : 3
c w n ox L

ð10Þ

To increase the conversion gain and IIP3, bias current could be increased, however, this would increase the voltage drop across R1;2 , which would a®ect the good operation of the switching transistors.3 This would result in a higher nonlinearity. rffiffiffiffiffiffi 32 ðV
Vth Þ : AIIP3 ¼ ð11Þ 3 gs However, if (Vgs
Vth ) of transconductance transistors are increased without increasing the bias current, IIP3 can be improved without the increasing the power consumption, This would, however, result in a lower conversion gain, and can be achieved by decreasing the (w=L) of transconductance transistors.16,20 The primary mixer circuit before adding proposed strategies is shown in Fig. 2.

Fig. 2. Primary circuit before adding proposed auxiliary circuit.

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Fig. 3. Proposed Gilbert cell mixer.

Figure 3 shows the proposed structure that improves system linearity, does not consume any extra power. On the other hand, the proposed strategy does not need to design new (w=L). L1 ; L2 ; LA1 and LA2 at the source of transconductance and auxiliary transistors are for impedance matching. An inductor (LD ) is interposed between two transconductance stages to tune the parasitic capacitors and improve the conversion gain.22 Addition of LA1 and LA2 reduce the linearity; however, since impedance matching is necessary in RF circuits, they cannot be removed. The value of inductors is selected in such a way that they can be implemented on-chip. MA1 and MA2 are biased in weak inversion in order to generate positive g 00m for cancellation of the third-order nonlinearity of the NMOS transistor.17,18,22 The size of MA1 and MA2 are chosen equal to (16/0.18)*64 on order to cancel the third-order nonlinearity of M1 and M2.3 CB1 and CB2 are applied to the DC-blocking capacitors.

4. Simulation Results The simulation of the proposed mixer was performed using HSPICE RF in a RF CMOS technology of 0.18-
m. Figures 4(a) and 4(b) shows the simulated input return loss and NF of the proposed Gilbert cell mixer, respectively. 1550098-6

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(a)

(b)

(c) Fig. 4. (a) Simulated input return loss (s11), (b) simulated NF of proposed mixer and (c) IIP3 of proposed Gilbert cell mixer.

For linearity analysis, IIP3 improvement is shown in Fig. 4(c) and IIP3 in Fig. 4(c) is obtained by varying two frequency tones with 10 MHz spacing. It is worth noting that the current increase due to MA1 and MA2 are negligible because they are biased under threshold condition.14 Table 1. Simulation results and comparison with prior mixers. References

Process Supply voltage (V) Frf (GHz) Flo (GHz) Power consumption (mW) LO power (dBm) IIP3 (dBm) P
1 (dBm) Conversion gain (dB) SSB NF (dB)

23

2

3

4

5

13

This work

0.18u 1.5 2.4 2.3 5.6
7 5.46
8.98 3.3 14.87

0.25u 2.5 1.9 1.85 13.01 — 5.13
5.83 2.13 18.2

0.35u 3 3.1–4.8 — 18 — 0 — 12 7.7

0.18u 1.8 2.4 2.1 14.3 — 8.032
4.65 10.7 16

0.13u 1.5 2.4 2.39 9 — 4.4 — 11.4 13.2

0.35u 1.8 2.4 — 5.4 — 9 — 16.5 —

0.18u 1.8 2.4 2.1 4.46 þ5 14.2 2.35 9.11 13.82

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To make the comparison between the proposed Gilbert cell mixer and the conventional Gilbert cell mixer easier, we have not changed the bias current, the size of transistors, the LO power or the size of switching transistors. Therefore, this improvement is only due to the addition of the auxiliary transistor and LA1 , LA2 and LD inductors. 5. Conclusion The proposed strategies can be applied in every mixer without any change in circuit's structure. System linearity will be increased without not consuming any extra power. On the other hand, the proposed method does not need to design new transistors sizing (w=L) but other linearization methods do not have these property. The transconductance stage is an important factor which restricts the mixer linearity. The mixer linearity can improve the linearity of the overall front-end system. IIP3 can be improved by using MGTR technique without any extra power consumption. Simulation results show that the IIP3 improvement of the proposed mixer is 16.55 dBm compared to conventional Gilbert cell mixer. The gain conversion of the proposed mixer is 9.11 dB while drawing only 2.48 mA from the 1.8 V power supplies.

Acknowledgment We appreciated our colleagues at Fava Meraj Center. We thank engineers Ali Youse¯ pour, Abd Al Amir Salem pour and Mahmoud Mokhtar Band because for their patience and of course because persuaded us to study about microwave and RF.

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CMOS (2008), http://citeseerx.ist.psu.edu/viewdoc/summary? doi: 10.1.1.157.2104. 6. M. Amiri and A. Abrishamifar, A high-linear CMOS down conversion mixer using adjusting the second and third-order harmonic in transconductance stage, J. Circuits, Syst. Comput. 24 (2015) 1550002.

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