On the use of body biasing to improve linearity in low LO-power CMOS active mixers

doi:10.1016/j.mejo.2014.05.001

Microelectronics Journal

Volume 45, Issue 8, August 2014, Pages 1026-1032

https://doi.org/10.1016/j.mejo.2014.05.001 Get rights and content

Abstract

In a radio-frequency (RF) transceiver, the linearity of the mixer has a profound effect on the overall transceiver performance. In many RF transceivers, active mixers are used due to their higher gain which also improves the overall receiver noise figure. In a typical RF active mixer where the transistors in the LO stage switch abruptly, most of the nonlinear distortions come from the transconductance or RF stage and thus the linearity of the mixer can be enhanced by proper design of the RF stage. In low-power receivers, however, to reduce the power consumption of the local oscillator (LO) circuit, the amplitude of LO signal is low and thus the switching of the transistors in the LO stage of the mixer is gradual. In this paper, we propose a technique to improve the linearity of such low-power mixers by enhancing the linearity of the LO stage. In particular, body biasing is utilized in the LO stage to improve the linearity. To verify the effectiveness of the proposed technique, two proof-of-concept double-balanced down-conversion active mixers have been designed and fabricated in 0.13-µm CMOS. The maximum IIP3 of +2.7 dBm and −4.9 dBm at a conversion gain of 13 dB and 16 dB are achieved for the first and second prototype respectively. For a 2.4 GHz RF input signal and an intermediate-Frequency (IF) of 50 MHz, the first prototype consumes 2.4 mW from a 1.2 V supply while the second one consumes only 780 µW from a 0.7 V supply.

Introduction

Down-conversion mixers are one of the important building blocks of wireless communication systems. These mixers are typically used to translate the frequency spectrum of the radio-frequency (RF) received signal of the antenna to a lower or zero intermediate frequency (IF). Important design specifications of a down-conversion mixer include gain, noise figure, linearity, power, and port-to-port isolation. Similar to any other block in a receiver, there is a trade-off among these design parameters. For the building blocks of the receive path, as the blocks get farther from the antenna, their contribution to the noise figure would be lowered by the aggregated gain of their preceding stages (Friis׳ equation [1]). Thus, the higher the gain of the down-conversion mixer, the lower the noise contribution of the stages after the mixer (e.g., baseband amplifiers in a generic integrated receiver) will be. However, the higher the gain of the mixer, the more pronounced the nonlinearity distortions of the subsequent stages would be. This is due to the fact that as the blocks get farther from the antenna, their contribution to the nonlinearity of the receiver would be boosted by the aggregated gain of their preceding stages [2].

Due to the relatively high noise figure of active mixers, the gain of the LNA is typically chosen to be high enough to keep the overall noise figure at an acceptable level. However, this leads to more restrictions on the linearity of the mixer as its IIP3 is scaled down by the gain of the LNA. Thus, it is important to enhance the linearity of the mixer such that the high gain of LNA does not limit the linearity of the whole receiver. Unfortunately, the scaling of CMOS technology which results in lower supply voltages, makes the design of highly linear active mixers challenging.

Power consumption is another performance metric which is of great importance in low-power as well as portable wireless receivers. Thus, in low-power designs, it is desirable to reduce, as much as possible, the power consumption of each single block in the receiver including down-conversion mixers. However, it is challenging to decrease the power without compromising the noise figure and linearity. To keep the power consumption of the receiver to the minimum, it is also desired that the mixer operates with small LO signal levels. This is due to the fact that the lower the amplitude of LO signal, the less power would be required in the VCO or frequency synthesizer to generate the LO signal.

In this paper, we build on our previous work in [3] and propose the idea of enhancing the linearity of down-conversion mixers through incorporating body biasing in the LO-stage transistors. Conventionally, for large LO systems, linearization techniques are commonly applied to the RF stage of the mixer (e.g., see [4], [5], [6], [7], [8]) due to the fact that with the abrupt switching of LO stage, most of the distortion of the mixer is from the RF stage [6], [9], [10]. However, if the LO power is low and the transistors in the LO stage are not instantaneously switched on and off, the LO stage remains in active region for a considerable amount of time during the LO period, and thus the nonlinearity contribution of the LO stage on the mixer distortion should be investigated. To do this, we first overview the structure of the conventional current-commutating mixers in Section 2. Section 3 discusses the linearity of mixers with low-power LO which in turn have non-ideal switching in their LO stage. 5 Using body biasing for linearity enhancement, 6 Effect of body biasing on conversion gain present the application of body biasing to improve linearity and conversion gain in this type of mixers. In order to validate the proposed idea, two proof-of-concept prototypes have been designed and fabricated in 0.13-µm CMOS; one is a standard double-balanced mixer and the other one is a current-bleeding double-balanced mixer. Measurement results of these two prototypes are presented in Section 7. Conclusion remarks as well as comparison with the state-of-the-art mixers are presented in Section 8. Note that while the focus of this work is on the linearity improvement of down-conversion mixers; the results can also be extended to up-conversion mixers as well.

Section snippets

Current-commutating active down-conversion mixers

The most commonly used structure for active CMOS mixers is the current-commutating architecture [2], [11]. The simplified single-balanced version of such mixer is shown in Fig. 1(a). In this mixer, transistor M₁, which is typically referred to as the RF stage transistor, operates as a transconductance stage (g_m stage) which converts the RF input voltage signal to a current signal. Assuming that LO signal has a large enough amplitude, transistors M₂ and M₃ act as differential current switches

Non-ideal switching in active down-conversion mixers

For the mixer shown in Fig. 1, ideal current commutation happens if the LO signal is large enough to fully divert the tail current between the branches. Under this condition, LO-stage transistors operate in either off or triode region and produce a current with a square-shape waveform. However, in practice in many applications, the LO signal does not abruptly switch the entire tail current between the differential branches.

For example, in low-power wireless transceivers where the power level of

Nonlinearity analysis in active mixers with non-ideal switching

As mentioned before, in many practical cases, especially in mixers with low LO power, the LO-stage transistors do not switch abruptly and are operating in their active region for a considerable portion of the LO period. The larger the value of Δ in Fig. 2, the longer the LO-stage transistors operate in their active region and the lower will be the conversion gain of the mixer due to the saturation of conversion gain imposed by the nonlinearities shown in Eq. (1). During the time that LO-stage

Using body biasing for linearity enhancement

From Eq. (9), to decrease the nonlinearity in a mixer with a realistic (non-ideal) switching signal, the third-order nonlinear term of the current of transistors in both RF and LO stages, namely, g_m3,RF and β₃, should be minimized. Moreover, as the contribution of the nonlinearity of the LO stage is magnified by the gain of the RF stage, special attention should be paid to the ratio of third-order and first-order terms of the current of the transistors of the LO stage, namely, $β_{3} / β_{1}$ . Note that

Effect of body biasing on conversion gain

In addition to linearity enhancement, depending on the type of the mixer, body biasing at the LO stage can also improve the conversion gain of the mixer. To further explain this potential conversion-gain enhancement, we first review one of the gain degradation sources in mixers, namely, the total capacitance seen at the drain of the RF stage transistor(s). Fig. 4 depicts this capacitance, namely, C_X, which is the sum of gate-source capacitance (C_gs) and source-bulk capacitance (C_sb) of LO-stage

Experimental results

To verify the validity of the proposed body-biasing technique, two proof-of-concept double-balanced mixers are designed and implemented in IBM CMRF8SF 0.13-µm CMOS process (8 metal layers). The micrograph of the fabricated body-biased mixers are shown in Fig. 5. The schematic of the first prototype mixer (Fig. 5(a)) is presented in Fig. 6. The mixer uses degeneration inductors (L_s) in order to improve the linearity of the RF stage. Note that the mixer is intended to operate with a low LO power.

Conclusion

In this paper, a method to enhance the linearity of active mixers through body biasing of LO stage transistors is presented. The technique is particularly suitable for low-power mixers with low power LO drive signals and gradual LO switching. To verify the effectiveness of the proposed technique, two proof-of-concept prototypes are fabricated in a 0.13-µm CMOS technology. The first prototype is a standard double-balanced mixer structure while the second one is a mixer that uses a

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On the use of body biasing to improve linearity in low LO-power CMOS active mixers

Abstract

Introduction

Section snippets

Current-commutating active down-conversion mixers

Non-ideal switching in active down-conversion mixers

Nonlinearity analysis in active mixers with non-ideal switching

Using body biasing for linearity enhancement

Effect of body biasing on conversion gain

Experimental results

Conclusion

Noise gures of radio receivers

Proc.Inst. Radio Eng. (IRE)

RF Microelectronics

A new linearization technique for CMOS RF mixer using third-order transconductance cancellation

IEEE Microw. Wirel. Compon. Lett.

Improved Gilbert mixer with high-precision automatic sweet-spot biasing and active-inductor-based harmonic suppression

Electron. Lett.

An ultra-wideband high-linearity CMOS mixer with new wideband active baluns

IEEE Trans. Microw. Theory Tech.

The micromixer: a highly linear variant of the Gilbert mixer using a bisymmetric class-AB input stage

IEEE J. Solid-State Circuits

A new RF CMOS Gilbert mixer with improved noise figure and linearity

IEEE Trans. Microw. Theory Tech.