Patent Publication Number: US-6701134-B1

Title: Increased dynamic range for power amplifiers used with polar modulation

Description:
FIELD OF THE INVENTION 
     The present invention relates to an improvement for an amplifier, and particularly to an amplifier circuit that provides increased dynamic range in polar modulation applications. 
     BACKGROUND OF THE INVENTION 
     Competitive forces in the wireless communication industry are continuously forcing prices down and requiring higher data rates, especially for non-voice applications. To achieve lower costs, wireless communication device designers must reduce parts count as well as take advantage of economical components. Higher data rate requirements force higher performance from wireless communication devices. In addition to these pressures, wireless communication devices must operate very efficiently to reduce power consumption in an effort to maximize battery life. 
     Most of the power consumption in a wireless communication device takes place in the power amplifier circuitry. In an effort to maximize efficiency, power amplifiers must operate near peak efficiency over varying power ranges, which are required in most wireless communication architectures. In addition to controlling output power, certain communication architectures incorporate polar modulation, wherein part of the modulation scheme requires modulation of the supply voltages provided to the power amplifiers. Unfortunately, the dynamic range required for advanced wireless communication architectures, such as the Enhanced Data Rates for GSM Evolution (EDGE), requires the power amplifiers to provide output power over a significant dynamic range, such as 45 dB. These large dynamic range requirements have forced additional complexity and parts counts into these wireless communication devices. Further, the additional parts and complexity have led to less efficient operation. 
     Accordingly, there is a need for power amplifier circuitry in polar modulation applications that provides efficient operation in a cost-effective and easy to implement manner. 
     SUMMARY OF THE INVENTION 
     The present invention provides power amplification circuitry having multiple amplifier stages in series and adapted to facilitate polar modulation. As such, the amplifier circuitry receives a frequency signal representing the frequency and phase components of the signal to be transmitted, as well as an amplitude signal presenting the amplitude component of the signal to be transmitted. The frequency component is amplified by the amplification circuitry. The amplitude component controls the supply voltage provided to the amplifier stages in the amplification circuitry. To increase the dynamic range of the amplification circuitry, the final stage or stages of the amplification circuitry are selectively disabled when lower output powers are necessary, and enabled when higher output powers are required. When disabled, the output stage is biased in a manner such that it effectively functions as a series capacitor, coupling the amplified signal from the prior amplifier stages through the final output stage to an antenna for transmission. 
     Preferably, the transistors forming the output stage or stages are disabled by allowing the collectors or drains, as the case may be, to float, while the base or gate remains biased. Application of a constant bias to the base or gate of the output stage transistors provides a constant load for the previous stages and allows the devices to effectively operate as a capacitor. As such, when the output stage is disabled, the prior amplifier stages can operate at more efficient levels while providing lower output powers. 
    
    
     Those skilled in the art will appreciate the scope of the present invention and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures. 
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the invention, and together with the description serve to explain the principles of the invention. 
     FIG. 1 is a schematic drawing of a user element constructed according to the present invention; 
     FIG. 2 is a schematic drawing of a power amplifier chain and control logic used in a user element according to one embodiment of the present invention; 
     FIG. 3 is a more detailed schematic of the amplifier chain illustrated in FIG. 2; and 
     FIG. 4 is a more detailed schematic of the amplifier chain illustrated in FIG. 2 illustrating the effective circuit configuration when a third output stage is disabled. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the invention and illustrate the best mode of practicing the invention. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the invention and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims. 
     The present invention is preferably incorporated in a user element  20 , such as a mobile telephone, personal digital assistant, wireless modem, wireless network interface, or the like, which implements polar modulation and requires substantially linear operation over a significant amount of dynamic range. The basic architecture of the user element  20  is represented in FIG.  1  and may include a receiver front end  22 , a radio frequency transmitter section  24 , an antenna  26 , a duplexer or switch  28 , a baseband processor  30 , a control system  32 , and interface circuitry  36 . The receiver front end  22  receives information bearing radio frequency signals from one or more remote transmitters provided by a base station (not shown). A low noise amplifier  38  amplifies the signal. A filter circuit (not shown) minimizes broadband interference in the received signal, while a downconverter  40  downconverts the filtered, received signal to an intermediate or baseband frequency signal, which is then digitized into one or more digital streams. 
     The baseband processor  30  processes the digitized received signal to extract the information or data bits conveyed in the received signal. This processing typically comprises demodulation, decoding, and error correction operations. As such, the baseband processor  30  is generally implemented in one or more digital signal processors (DSPs). 
     On the transmit side, the baseband processor  30  receives digitized data from the interface circuitry  36  directly or indirectly via the control system  32  to encode for transmission. The encoded data is output to the transmitter section  24 , where it is modulated for transmission using polar modulation techniques. For polar modulation, a polar modulator  42  will process the encoded data and generate an amplitude signal (A) and a phase (or frequency) signal (φ) to drive the power amplifier  44 . The amplitude signal (A) controls the power supply voltage of the power amplifier  44 , while the phase signal (φ) represents the frequency and phase content of the modulated signal. 
     In one embodiment of the present invention, the polar modulator  42  may comprise several components, including, but not limited too, a mapping module  46 , a polar converter  48 , a phase-locked loop (PLL)  50 , a digital-to-analog (D/A) converter  52 , and power control circuitry  54 . Although shown separately, the power amplifier  44  and the power control circuitry  54  may reside in the same module. In this embodiment, the modulation scheme for the polar modulator  42  is an Enhanced Data Rates for GSM Evolution (EDGE) modulation scheme and thus, the bit rate is 812.5 kbps. This data is passed to the mapping module  46 , where every three consecutive data bits are mapped into a corresponding symbol. The resulting symbols are Grey coded and rotated by 3π/8 on each symbol as per European Telecommunications Standards Institute (ETSI) specifications. For EDGE modulation, the output of the mapping module  46  provides a corresponding one of 16 rectangular coordinates, generally referred to an in-phase (I) and quadrature (Q) coordinates, for each symbol. The in-phase and quadrature coordinates (I, Q) corresponding to each symbol are filtered using finite impulse response (FIR) filters (not shown) as dictated by the ETSI specifications and then converted from rectangular to polar coordinates by the polar converter  48 . 
     The in-phase and quadrature coordinates (I, Q) are preferably converted into phase (φ C ) and amplitude (A C ) control signals by the polar converter  48  using a classical CORDIC (coordinate rotation digital computer). Processing by the polar converter  48  also includes a conversion from a true phase signal to a frequency signal. This conversion is well understood in the art and for the purposes of the present invention, this conversion is treated as part of the CORDIC conversion. Further information about CORDIC algorithms may be found in  Proceedings of the  1998  ACM/SIGDA Sixth International Symposium On Field Programmable Gate Arrays  by Ray Andraka, Feb. 22-24, pp. 191-200 and “The CORDIC Trigonometric Computing Technique” by Jack E. Volder,  IRE Trans on Elect. Computers , p.330, 1959, both of which are hereby incorporated by reference in their entireties. 
     At this point, the phase control signal (φ C ), referred to as a frequency control signal (φ C ), and amplitude control signal (A C ) separate and proceed by different paths, a frequency signal processing path and an amplitude signal processing path, respectively, to the power amplifier  44 . With respect to the amplitude signal processing path, the digital amplitude control signal (A C ) is converted to an analog signal by the D/A converter  52 . The output of the D/A converter  52  is an analog version of the amplitude control signal (A C ) and is used by the power control circuitry  54  to set the supply voltage on the power amplifier  44 . Notably, the amplitude control signal (A C ) has a DC component reflecting the desired output power and an AC component reflecting the desired amplitude modulation characteristic. The power control circuitry  54  will process the amplitude control signal (A C ) and provide a proportionate amplitude signal (A) to drive the power amplifier  44 . As the amplitude signal (A) changes, the voltage at the power amplifier  44  changes and the output power will vary as V 2 /2R out , where R out  is effectively the load on the power amplifier  44 . Accordingly, the amplitude signal (A) represents both a modulation and power level component at any given time. The power control circuitry  54  may also receive a transmit (TX) enable signal  56 , which enables the power amplifier  44 , and an output (O/P) stage enable signal  58 , which selectively enables a final stage of the power amplifier  44  as will be described below in greater detail. 
     The frequency control signal (φ C ) is used to set the frequency of the PLL  50 , or like synthesizer, to generate the frequency signal (φ) having the appropriate frequency and phase component to send to the power amplifier  44  for amplification. Accordingly, the power amplifier  44  will amplify the frequency signal (φ) using the corresponding amplitude signal (A) to provide a modulated and amplified signal for transmission via the antenna  26 . Further information pertaining to polar modulation may be found in commonly assigned U.S. patent application Ser. No. 10/139,560 filed May 6, 2002, 10/147,569 filed May 16, 2002, and 10/147,579 filed May 16, 2002, which are incorporated herein by reference. 
     While a detailed discussion of amplifier chains may be found in commonly owned U.S. patent application Ser. No. 09/878,461 filed Jun. 11, 2001 and 10/080,187 filed Feb. 21, 2002, the disclosures of which are hereby incorporated by reference, the following outlines the basic operation of a multistage amplifier. Turning now to FIG. 2, the power amplifier circuitry  44  is associated with power control circuitry  54 . The power amplifier circuitry  44  may comprise any number of amplifier stages. In this embodiment, the power amplifier circuitry includes three amplifier stages, a first amplifier stage  60 , a second amplifier stage  62 , and a third amplifier stage  64 , as well as a bias network  66  providing bias (V BIAS ) for each of the three amplifier stages  60 ,  62 , and  64 . The first and second amplifier stages  60 ,  62  are driver stages, while the third amplifier stage  64  is a final, output stage. In alternate embodiments, the bias network  66  may comprise multiple bias networks, one for each amplifier stage. Preferably, the bias provided to each stage  60 ,  62 , and  64  is relatively stable regardless of power levels and the enablement of the third amplifier stage  64 . Other bias arrangements are also possible, as those skilled in the art will recognize. 
     The power control circuitry  54  will preferably include two control sections. The first control section will control the final stage  64 , while the second will preferably control the first and second stages  60 ,  62 . The first section includes an error amplifier  68 ′, a voltage regulator  70 ′, and a feedback network  72 ′ having a transfer function H(S)′. As noted, the power amplifier circuitry  44  provides gain for the frequency signal (φ) to be transmitted under control of the power control circuitry  54  based on the amplitude control signal (A C ). 
     In the exemplary embodiment, the amplitude control signal (A C ) may be received by an inverting input  76 ′ of an operational amplifier forming error amplifier  68 ′. The regulated output  78 ′ of the voltage regulator  70 ′ is fed back through a feedback network  72 ′ and received by a non-inverting input  80 ′ of error amplifier  68 ′. An output signal  82 ′ from error amplifier  68 ′ is provided to a control input  84 ′ of voltage regulator  70 ′ to control the regulated output  78 ′ of voltage regulator  70 ′. The regulated output  78 ′ represents the actual amplitude signal (A′), which drives the third output stage  64 . Voltage regulator  70 ′ regulates the voltage and the amplitude signal (A′) supplied to the rail  86  of the third amplifier stage  64  to control the gain according to the amplitude control signal (A C ). 
     The second section of the power control circuitry  54  is similar to the first section and may include an error amplifier  68 ″, a voltage regulator  70 ″, and a feedback network  72 ″ having a transfer function H(S)″. The amplitude control signal (A C ) is also received by an inverting input  76 ″ of an operational amplifier forming error amplifier  68 ″. The regulated output  78 ″ of voltage regulator  70 ″ is fed back through feedback network  72 ″ and received by a non-inverting input  80 ″ of error amplifier  68 ″. An output signal  82 ″ from error amplifier  68 ″ is provided to a control input  84 ″ of voltage regulator  70 ″ to control the regulated output  78 ″ of voltage regulator  70 ″. Voltage regulator  70 ″ regulates the voltage supplied to the rails  88 ,  90  of the first amplifier stage  60  and the second amplifier stage  62 , respectively, to control the gain according to the amplitude control signal (A C ). Accordingly, the rails  88 ,  90 ,  86  of the first, second, and third amplifier stages  60 ,  62 ,  64  may be controlled in unison as a function of the amplitude control signal (A C ) to amplify the frequency signal (φ). The rails  88 ,  90 ,  86  will typically be coupled to the collectors of bipolar transistors or drains of field effect transistors forming the respective amplifier stages  60 ,  62 ,  64 , as will be appreciated by those skilled in the art. 
     Each of the voltage regulators  70 ′ and  70 ″ is connected directly to a fixed or primary supply voltage, V PRIM , which will preferably be connected to the terminal for the positive potential of a battery. As noted, the bias network  66  supplies a fixed bias to the three power amplifier stages  60 ,  62 ,  64 , regardless of the collector/drain voltage supplied to the first, second and third amplifier stages  60 ,  62 ,  64 . The fixed bias incorporates traditional V APC  signals, which are configured to maintain a constant bias, in the preferred embodiment. 
     The transmit enable signal, TX ENABLE  56 , is a logic signal used to simultaneously enable or disable the error amplifiers  68 ′ and  68 ″ and the amplifier circuitry  44  by supplying or removing the bias to or from each of the three amplifier stages  60 ,  62 ,  64 . A frequency signal (φ) to be amplified is provided at the input of the first amplifier stage  60  and amplified by either the first and second amplifier stages  60 ,  62  or all three amplifier stages  60 ,  62 ,  64  to provide an amplified output signal  94  (RF OUT ). The third amplifier stage  64  is selectively enabled or disabled by enabling or disabling the voltage regulator  70 ′ with the output (O/P) stage enable signal  58 . When enabled, the third amplifier stage  64  operates to amplify the frequency signal (φ), which has been amplified by the first and second amplifier stages  60 ,  62 . When disabled, the third amplifier stage  64  is configured to effectively become a capacitance in series with the amplification path and output load. 
     While three amplifier stages are preferred, other arrangements of two, four, or more are contemplated. The teachings of the present invention apply to such arrangements, and one of ordinary skill in the art can modify the present teachings to such arrangements. Further, certain advantages may be realized by forming two or more of the amplifier stages  60 ,  62 ,  64  from a plurality of transistor cells arranged in parallel. In an exemplary embodiment, the first amplifier stage  60  comprises a single transistor cell; the second amplifier stage  62  comprises approximately six transistor cells; and the final amplifier stage  64  comprises forty-eight transistor cells. 
     For further information pertaining to the transistor arrays, reference is made to U.S. Pat. Nos. 5,608,353, HBT POWER AMPLIFIER, issued Mar. 4, 1997; and U.S. Pat. No. 5,629,648, HBT POWER AMPLIFIER, issued May 13, 1997, which are assigned to RF Micro Devices, Inc. of 7628 Thorndike Road, Greensboro, N.C. 27409, and wherein the disclosures are incorporated herein by reference in their entireties. Still further information may be found in commonly owned U.S. patent application Ser. No. 09/952,524, filed Sep. 14, 2001, the disclosure of which is hereby incorporated by reference. Exemplary bias networks  66  capable of being used in association with the present invention are described in further detail in U.S. patent application Ser. No. 09/467,415 filed Dec. 20,1999, currently pending, the disclosure of which is incorporated herein by reference in its entirety. Upon understanding the present invention, those skilled in the art will be able to construct any number of bias networks that are compatible with the present invention. 
     With reference to FIG. 3, the first amplifier stage  60  is formed by a single transistor cell, and the second and third amplifier stages  62  and  64  are implemented using parallel transistor cells. A transistor cell is essentially a transistor, a resistor and a capacitor, such as Q 1 ″, RB 1 ″, and C 1 ″. The second amplifier stage  62  is comprised of a transistor network formed by transistors Q 1 ″ to QN″. In this configuration, each transistor Q 1 ″ through QN″ receives identical bias from the bias network  66  through resistors RB 1 ″ through RBN″. Output from the first amplifier stage  60  comes into the second amplifier stage  62  and drives the transistor array through capacitors C 1 ″ through CN″. Notably, the collectors of transistors Q 1 ″ through QN″ of the second stage  62  are coupled together to provide a common output signal. 
     The output signal from the second amplifier stage  62  is directed to the third (final) amplifier stage  64 . The third amplifier stage  64  is comprised of a transistor network formed by transistors Q 1 ′ to QN′. In this configuration, each transistor Q 1 ′ through QN′ receives identical bias from the bias network  66  through the resistors RB 1 ′ through RBN′. The output signal from the second amplifier stage  62  is coupled to the third amplifier stage  64  and drives the transistor array through capacitors C 1 ′ through CN′. Notably, the collectors of transistors Q 1 ′ through QN′ are coupled together to provide the output signal (RF OUT )  94 . When the third amplifier stage  64  is enabled by turning on the voltage regulator  70 ′ with the output enable signal  58 , transistors Q 1 ′ through QN′ will conduct and provide amplification for the frequency signal (φ). When the third amplifier stage  64  is disabled by turning off the voltage regulator  70 ′ with the output enable signal  58 , transistors Q 1 ′ through QN′ are disabled and do not conduct from the collectors to the respective emitters. 
     When biased, the transistors Q 1 ′ through QN′ will conduct the frequency signal (φ) provided by the second amplifier stage  62  between the collectors and bases of the respective transistors Q 1 ′ through QN′, even when the voltage regulator output is disabled. As shown in FIG. 4, the collectors and bases for each of the disabled transistors Q 1 ′ through QN′ are capacitively coupled by capacitances C Q1 , through C QN , given the inherent characteristics of the transistors. Accordingly, when the transistors Q 1 ′ through QN′ are disabled, the third amplifier stage  64  simply capacitively couples the output of the second amplifier stage  62  to the output of the third amplifier stage  64 , wherein the output of the second amplifier stage  62  becomes the output signal RF OUT    94 . The bias provided to the bases of transistors Q 1 ′ through QN′ from the bias network  66  via resistors RB 1 ′ through RBN′ is maintained even when the voltage regulator  70 ′ is disabled, to set an appropriate impedance between the collectors and bases of the transistors Q 1 ′ through QN′. Accordingly, less overall gain is provided by the signal being transmitted, yet the first and second amplifier stages  60  and  62  can operate at a higher gain and thus at a more efficient level with lower distortion, even when lower levels of output power are needed. 
     The transistors Q 1 ″ through QN″ and Q 1 ′ through QN′ in the exemplary embodiment are biased to operate in saturation and are preferably heterojunction bipolar transistors (HBTs) formed on a single semiconductor and equally sized to form a transistor array. However, the inventive concepts defined herein are independent of technology (Si, GaAs, SiGe, etc.) as well as device type (BJT, FET, MESFET, HBT, etc.). Further, the number of transistors in each array need not be the same. Thus, N″ does not necessarily equal N′, although it may if needed or desired. As noted, in an exemplary embodiment, N″=6 and N′=48. 
     By selectively enabling the final amplifier stage or stages, the dynamic range for the over power amplifier  44  is significantly increased in an efficient manner and without excessive increases in parts count or die area. By allowing the collectors (or drains) of the final amplifier stage to float when disabled, a feed-through capacitor is created between the base (or gate) and collector (or drain). The driver amplifier stages, such as amplifier stages  60  and  62 , are still varied in voltage and output power to provide the appropriate output power level and maintain linear operation throughout the power range. The use of constant base (or gate) bias for all amplifier stages, regardless of the enabling of the final amplifier stage, such as the third amplifier stage  64  maintains a constant load for the driver stages. 
     For an EDGE application requiring 45 dBm of linear power control, the present invention can be configured to enable the final stage when between 15 dBm and 35 dBm of output power is needed. The final stage can be disabled when output power is between 15 dBm and −20 dBm. Preferably, hysteresis is applied about the 15 dBm switching point. 
     Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present invention. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow.