Abstract:
An Extremely High Frequency (EHF) dual-mode PA with a power combiner is designed using 40-nm bulk CMOS technology. One of the unit PAs can be switched off for the low power applications. In the design, circuit level optimization and trade-off are performed to ensure the good performance in both modes. The PA achieves a P SAT  of 17.4 dBm with 29.3% PAE in high power mode and a P SAT  of 12.6 dBm with 19.6% PAE in low power mode. The reliability measurements are also conducted and a lifetime of 80613 hours is estimated based on a commonly used empirical model. The excellent performance (e.g., highest reported PAE) achieved in this design further confirms the scaling of CMOS technology will continue to benefit the mm-wave transceiver design.

Description:
[0001]    This application claims priority to U.S. Provisional Patent Application Ser. No. 61/620,732, Titled “60 GHz Dual-Mode Class AB Power Amplifier Implementation,” filed Apr. 5, 2012, and also to U.S. Provisional Patent Application Ser. No. 61/625,168, Titled “Extremely High Frequency Dual-Mode Class AB Power Amplifier,” filed Apr. 17, 2012, the disclosure of both of which are incorporated herein by reference in their entireties. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates generally to amplifier circuits, and in particular to a dual-mode, extremely high frequency power amplifier featuring high efficiency in both high-power and low-power modes, owing to a re-configurable output power combiner. 
       BACKGROUND 
       [0003]    Radios operating in the extremely high frequency (EHF) band of the electromagnetic (EM) spectrum exhibit numerous advantages, and are anticipated to play a significant role in communication technology—particularly wireless, mobile communication devices. For example, radios operating in EHF exhibit numerous advantages over radios operating in other frequency bands of the spectrum, including license-free spectrum, relatively narrow beam antennas, and inherent security due to oxygen absorption and the narrow beam width. 
         [0004]    As used herein, the extremely high frequency (EHF) band of the EM spectrum includes frequencies from approximately 30 to 300 GHz. This is the highest frequency range of what is considered to be Radio Frequency (RF) EM radiation. Above this frequency band, EM radiation is considered to be in the low infrared light spectrum (also referred to as terahertz radiation). EM energy in the EHF band has a wavelength in the range of approximately 10 mm to 1 mm. Hence, EHF EM radiation is also generally referred to as millimeter wave RF (mm-wave). Accordingly, the terms EHF and mm-wave are used synonymously herein when referring to a frequency band. 
         [0005]    In the U.S., the Federal Communication Commission (FCC) has allocated an unprecedented 7 GHz of un-channelized spectrum for license-free operation between 57-64 GHz. In contrast, less than 0.5 GHz of spectrum is allocated between 2-6 GHz for WiFi and other license-free applications. The portion of the EHF band near 60 GHz thus represents a significant opportunity to implement multi-gigabit RF communication links. Standardization efforts in this area include WiGig and WirelessHD. 
         [0006]    EHF radios utilize very narrow RF beams, enabling multiple EHF radio base stations or other transceivers to be installed on the same tower, rooftop, or the like, even if they are all operating at the same transmit and receive frequencies. Co-located radios operating in the same transmit and receive frequency ranges can easily be isolated from one another based on small lateral or angular antenna separations, and/or the use of cross-polarized antennas. While the RF beams are relatively narrow, however, they are sufficiently wide, e.g., compared to optical signals, such that fixed antennas may be accurately aligned by a non-expert installer with the use of a simple visual alignment tool, and communications are unaffected by minor antenna movement such as tower or building sway due to wind. 
         [0007]    Oxygen attenuates RF signals near 60 GHz (e.g., ˜57-64 GHz) due to a resonance of the oxygen molecule, a property that is unique to the near-60 GHz portion of the EM spectrum. While this property limits the distances that radio links at this frequency can cover, it also makes these links highly immune to interference from other radios at the same or near frequencies. For example, oxygen absorption ensures that a near-60 GHz signal will not extend far beyond its intended target. 
         [0008]    The combination of narrow beam width and oxygen attenuation provides an inherent degree of security to near-60 GHz link communications. Due to the narrow beam width, an interceptor receiver must be placed directly in the main beam (and tuned to its carrier frequency) to receive a useful signal. In this position, it is likely to degrade the signal at the intended receiver sufficiently to allow for its detection. Due to oxygen attenuation, there is a limited distance beyond an intended receiver, along the main beam, at which a useful signal may be obtained by an interceptor receiver. 
         [0009]    Accordingly, the demand is increasing for EHF capability in mobile communication devices, particularly near 60 GHz, to allow them to engage in communication channels supplemental to their primary channels (e.g., GSM, CDMA, LTE, and similar systems). However, high frequency electronics consume significant amounts of power, and hence are a major factor in depleting useful battery life. In particular, the millimeter-wave power amplifier (PA) is the most power-hungry block in an EHF transceiver. A typical requirement of an EHF PA is to deliver at least 10 dBm output power to set up a communication range of 1 m. 
         [0010]    A conventional approach to satisfying both the high output power demands of EHF radios and minimizing power consumption (and hence battery depletion) when not transmitting, is a dual-mode PA in which two or more unit PAs are coupled together to achieve high output power. When not transmitting, one or more of the PA units may be disabled to reduce power consumption. The output signals of the units are typically combined with a transformer-based combiner. Such a combiner achieves an insertion loss of as little as 1.2 dB. However, the combiner loss can be as high as 5 dB in low-power mode, due to the extra loss introduced by the parasitic loading of the unit PA(s) in an off state. 
         [0011]    The Background section of this document is provided to place embodiments of the present invention in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section. 
       SUMMARY 
       [0012]    The following presents a simplified summary of the disclosure in order to provide a basic understanding to those of skill in the art. This summary is not an extensive overview of the disclosure and is not intended to identify key/critical elements of embodiments of the invention or delineate the scope of the invention. The sole purpose of this summary is to present some concepts disclosed herein in a simplified form as a prelude to the more detailed description that is presented later. 
         [0013]    According to one or more embodiments described and claimed herein, the insertion loss of a coupling transformer in low-power mode of an extremely high frequency (EHF) power amplifier (PA) is reduced by inserting a switch at the primary side of the transformer for the unit(s) to be turned off. The switch is biased to eliminate reliability problems. For example, the PA may be switched to low-power mode when short-range communication (e.g., within about 1 m) is requested. In the low-power mode, power consumption is reduced by approximately 50% compared to high-power mode, thus affording significant savings of battery power for mobile radio devices. 
         [0014]    One embodiment relates to a dual-mode PA operative to amplify signals having frequencies in the EHF band. The PA includes a plurality of separately-enabled, differential PA units arranged in parallel. Each PA unit includes an input stage comprising a pair of transistors arranged in a common-gate configuration; and a plurality of serially-connected output stages, each comprising a pair of transistors arranged in a common-source configuration. The PA also includes a transformer-based coupling circuit combining the outputs of the PA units. One or more PA units further includes a switch operative to short the PA unit outputs at the interface to the transformer-based coupling circuit in a low power mode, and to allow the PA unit outputs to couple to the transformer-based coupling circuit in a high power mode. 
         [0015]    Another embodiment relates to a method of operating a dual-mode PA operative to amplify RF signals in the EHF band. The PA comprises a plurality of separately-enabled, differential PA units arranged in parallel, wherein each PA unit comprises an input stage comprising a pair of transistors arranged in a common-gate configuration and a plurality of serially-connected output stages, each comprising a pair of transistors arranged in a common-source configuration. A transformer-based coupling circuit combines the outputs of the PA units. According to the method, in a high power mode, two or more of the plurality of differential PA units are enabled by allowing the outputs of each PA unit to couple to the transformer-based coupling circuit. In a low power mode, at least one of the plurality of differential PA units is disabled by shorting the outputs of the disabled PA unit at the interface to the transformer-based coupling circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. However, this invention should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. 
           [0017]      FIG. 1  is a schematic diagram of a power amplifier according to one embodiment. 
           [0018]      FIG. 2  is a graph of the insertion loss of the combiner in the circuit of  FIG. 1  as a function of the on-resistance (R ON ) of the switch. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    It should be understood at the outset that although illustrative implementations of one or more embodiments of the present disclosure are provided below, the disclosed systems and/or methods may be implemented using any number of techniques, whether currently known or in existence. The disclosure should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, including the exemplary designs and implementations illustrated and described herein, but may be modified within the scope of the appended claims along with their full scope of equivalents. 
         [0020]    An effective dual-mode PA—operative in a high-power mode and a low-power mode—at mm-wave frequencies, with low insertion loss in low-power mode, has not been achieved in the prior art, due to implementation challenges in both the combiner design and the layout floor plan.  FIG. 1  depicts a schematic diagram of an EHF dual-mode Class AB power amplifier (PA)  10  according to one embodiment of the present invention. The design incorporates two unit PAs  12 ,  14  with a transformer-based power combiner  16  at the output. Each PA  12 ,  14  is a three-stage differential amplifier, wherein the transistor sizes of the three gain stages  18 ,  20 ,  22  are scaled by a factor of two. The last two amplifier stages  20 ,  22  are configured as common-source (CS) amplifiers, for superior power gain at mm-wave frequencies (i.e., 2 dB higher in maximum power gain at 60 GHz compared with common-gate configuration). The common-gate (CG) stage functions as the input stage  18 , partially because its low input impedance simplifies the design of the input matching network  24 . 
         [0021]    The neutralization technique is adopted by cross-connecting the interdigitated metal-oxide-metal (MOM) capacitors between the drain and gate terminals to improve the stability and reverse isolation at mm-wave frequencies. Advanced CMOS technology provides sufficient power gain (i.e., 10 dB) for a single amplifier stage for the PA  10  to be biased in Class AB mode to ensure high efficiency. In low-power mode, the gate bias of one unit PA  12  is switched off to reduce power consumption. 
         [0022]    The transformer-based combiner  16  is employed for power combining and ensures a compact layout. It is implemented in an overlay structure with the top two metal layers. Both metal layers have a thickness of only 0.9 μm, and the combiner  16  achieves an insertion loss of 1.2 dB. However, the combiner loss could be as high as 5 dB in low-power mode due to the detuning and the extra loss introduced by the parasitic loading of the unit PA  12  in off state. 
         [0023]    According to embodiments of the present invention, this loss is alleviated by shorting the output stage  22  of the off-state amplifier unit  12  with a MOS transistor switch  26 .  FIG. 2  depicts the insertion loss of the combiner  16  as a function of the on-resistance (R ON ) of the switch  26 . In one embodiment, an R ON  of 10Ω is used, which reduces the insertion loss to 2.3 dB. 
         [0024]    To minimize the parasitic capacitance introduced by the switch  26 , a thin-oxide transistor with gate length of 40 nm is used. To ensure reliable operations, the gate of the switch transistor  26  is biased at 0.5 V in the “on” state and 2 V in “off” state. The maximum voltage across the gate oxide of the switch  26  in both states is approximately 1V, which leaves sufficient margin to avoid gate oxide breakdown. 
         [0025]    To ensure proper operation of the dual-mode PA  10 , the design of the first amplifier stage  18  and the input matching network  24  is also important. A common-gate amplifier configuration is used for the input stage  18 , which has ten times lower input impedance compared with a common-source configuration at ˜60 GHz. Note that the insertion loss of the input matching network  24  is limited by the passives&#39;  0 -factor and its load impedance. By employing an input stage  18  with low input impedance, the PA  10  can draw nearly the same amount of signal current from the input matching network  24  (i.e., the matching network  24  has similar insertion loss) in both modes. In addition, to compensate the 1 dB difference in the insertion loss of the power combiner  16  between different modes, the input matching network  24  (i.e., a transformer  28  and differential transmission lines  30 ) is optimized for the low-power mode so that the PA  10  can provide similar power gain in both modes. 
         [0026]    Table 1 depicts several technical characteristics of one embodiment of the present invention, as compared to several prior art EHF dual-mode power amplifiers. 
         [0000]                                                                    TABLE 1                   Comparison of Technical Specifications                This Work                       (High/Low)   [3]   [4]   [6]                            Tech. [nm]   40   65   65   45           V DD  [V]   1   1.2   1   7           P SAT [dBm]   17.4/12.6   19.9   18.6   14.5           P 1 dB  [dBm]   14.0/9.5    18.2   15.0   11.2           PAE MAX [%]   29.3/19.6   14.2   15.1   14.4           P DC  [mW]   153/80    640   480   180           Area [mm 2 ]   0.074   1.4*   0.12*   0.04                       *Estimated from the chip micrograph.            
wherein
   Tech=CMOS Technology;   V DD =Drain voltage;   P SAT =Saturation Power—the power for which output power is not increased with further increase in input power;   P 1dB =1 dB compression point;   PAE MAX =Maximum Power Added Efficiency;   P DC =DC power consumption; and   Area=silicon die area.   
 
         [0034]    The prior art designs compared against in Table 1 are included in the following list, the disclosures of which are all incorporated herein by reference in their entireties:
   [1] IEEE 802.15 Working Group: Wireless PAN Task Group 3c. Millimeter wave alternative PHY, 2009. Online available: http://www.ieee802.org/15/pub/TG3c.html.   [2] WiGig MAC and PHY specification, version 1.0, Wireless Gigabit Alliance.   [3] C. Y. Law and A. Pham, “A high-gain 60 GHz power amplifier with 20 dBm output power in 90 nm CMOS,” ISSCC Dig. Tech. Papers, pp. 426-427, February 2010.   J. Chen and A. M. Niknejad, “A compact 1V 18.6 dBm 60 GHz power amplifier in 65 nm CMOS,” ISSCC Dig. Tech. Papers, pp. 432-433, February 2011.   D. Chowdhury et al., “A fully integrated dual-mode highly linear 2.4 GHz CMOS power amplifier for 4G WiMax applications,” IEEE J. Solid-State Circuits, vol. 44, no.12, pp.3393-3402, September 2009.   M. Abbasi et al., “A broadband differential cascode power amplifier in 45 nm CMOS for high-speed 60 GHz system-on-chip,” RFIC Symposium, pp. 533-536, May 2010.   T. Kjellberg et al., “A compact cascode power amplifier in 45-nm CMOS for 60-GHz wireless system,” CISC Symposium, November 2009.   D. Stephens et al., “RF reliability of short channel NMOS devices,” RFIC Symposium, pp. 343-346, May 2009   [9] B. Koo et al., “A fully integrated dual-mode CMOS power amplifier for WCDMA applications,” ISSCC Dig. Tech. Papers, pp. 82-83, February 2012. The full disclosure of a draft of a paper to be published by Zhao, et al., “A 60 GHz Dual-Mode Class AB Power Amplifier with 29.3% PAE in 40-nm CMOS,” authored by some of the present inventors and attached hereto as Appendix A, is incorporated herein in its entirety.   
 
         [0044]    According to embodiments of the present invention, to reduce the power consumption and extend the battery lifetime, one unit PA  12  of a dual-mode EHF power amplifier  10  is turned off in low-power mode. A switch  26  is employed to short the output of this off-state unit PA  12  and thus improve the back-off efficiency. The PA  10  achieves a saturated output power (P SAT ) of 17.4 dBm with 29.3% PAE in high power mode and a P SAT of  12.6 dBm with 19.6% PAE in low-power mode. 
         [0045]      FIG. 3  depicts a flow diagram of a method  100  of operating a dual-mode PA  10  operative to amplify RF signals in the EHF range. The PA  10  comprises a plurality of separately-enabled, differential PA units  12 ,  14  arranged in parallel, wherein each PA unit  12 ,  14  comprises an input stage  18  comprising a pair of transistors arranged in a common-gate configuration and a plurality of serially-connected output stages  20 ,  22 , each comprising a pair of transistors arranged in a common-source configuration. A transformer-based coupling circuit  16  combines the outputs of the PA units  12 ,  14 . According to the method  100 , the PA  10  operates in one of a high power mode or a low power mode. If the PA  10  is in the high power mode (block  102 ), if a decision is made to switch operation to a low power mode (block  104 ), such as by a controller (now shown in the drawing figures), the outputs of one or more PA units  12 ,  14  are shorted to remove the PA unit  12 ,  14  from the transformer-based coupling circuit  16 . The outputs may be shorted, for example, by biasing a switching transistor  26  to an “on” or conductive state. This removes the PA unit  12  from the output, resulting in efficient operation in a low power mode (block  108 ). Some time later, if a decision is made to switch operation to the high power mode (block  110 ), the outputs of the disabled PA unit(s)  12  are enabled (block  112 ) to couple the PA unit  12  to the transformer-based coupling circuit  16 . The outputs may be enabled, for example, by biasing the switching transistor  26  to an “off” or non-conductive state. This couples the PA unit  12  to the output, resulting in operation in high power mode (block  102 ). 
         [0046]    Embodiments of the present invention enable the commercialization of EHF band consumer electronics—particularly mobile devices—by providing low-power EHF radios and thus extending the battery lifetime of mobile devices. Although particular embodiments are described herein as operating at or near 60 GHz, the present invention is not limited to this frequency, but may be advantageously employed in any mm-wave, or EHF, radio. 
         [0047]    The present invention may, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present embodiments are to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.