Abstract:
A power amplifier (PA) has been disclosed for linearity improvement. The PA comprises at least an amplifying transistor and at least an auxiliary transistor. Each amplifying transistor of the at least an amplifying transistor includes a first terminal for receiving an input signal of the PA, a second terminal for delivering an output signal of the PA, and a third terminal. Each auxiliary transistor of the at least an auxiliary transistor includes a first terminal, a second terminal coupled to the second terminal of the at least an amplifying transistor, and a third terminal electrically connected to the first terminal of the at least an amplifying transistor.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a power amplifier, and more particularly, to a power amplifier with low circuit complexity, high stability against temperature and bias variations, and high signal linearity. 
     2. Description of the Prior Art 
     As technology evolves, wireless communication is an important part of human life. Various electronic devices, such as smart phones, smart wearable devices, tablets, etc., utilize wireless radio frequency (RF) system to transmit and receive wireless signals. A power amplifier (PA) is a necessary amplifier in a transmitter composing a wireless RF system. Small- and large-signal performances of the PA usually determine the performance of the overall wireless RF system. Moreover, signal linearity and stability against temperature and bias variations are key performance metrics of the PA. 
     In the prior art, PA circuits within using the techniques of harmonic signal detection and compensation, high-order harmonic adjustment, tunable output matching network, parallel type transconductance compensation, and envelope tracking are provided to improve the linearity thereof. The harmonic signal detection and compensation offers an adjustable function of automatic bias control by detecting and filtering an output signal through a feedback network when the output amplitude becomes a large signal. However, the circuit complexity thereof grows for practical implementation. In addition, using high-order harmonic adjustment and tunable output matching network controls a reflection condition of high-order harmonic waves and adjusts the waveform of an output signal by slightly tuning the harmonic impedances of an output impedance network in the PA circuit, such that the signal linearity of the PA circuit can be improved. Since a complicated output impedance network results in a worse performance such as an output power loss or the sensitivity to frequency response, more concerns are need to be considered in PA circuit design. 
     The parallel type transconductance compensation may adjust bias and generate a mixed output signal through two transistors combined in parallel, so as to achieve a compensationon third-order intermodulation (IMD3), which improves an overall linearity to the LNA circuits. Generally, high capacitance at both input and output ports presents in PA circuits for high output power consideration, which influences high frequency performance, i.e., high frequency impedance matching and high frequency signal gain. In addition, the envelope tracking architecture provides a dynamic bias control under different operating powers via feedback sensing network and digital controller, which improves the DC and RF power efficiencies. However, using envelope tracking requires more active and inactive components such as analog and digital control circuits, which could increase the circuit complexity and production cost of the PA circuit. 
     Therefore, how to provide a power amplifier with low circuit complexity, high stability in temperature and bias variations, and superior signal linearity is a significant objective in the field. 
     SUMMARY OF THE INVENTION 
     It is therefore a primary objective of the present invention to provide a power amplifier with low circuit complexity, high stability against temperature and bias variations, and high signal linearity. 
     The present invention discloses a power amplifier (PA). The PA comprises at least an amplifying transistor and at least an auxiliary transistor. Each amplifying transistor comprises a first terminal configured to receive an input signal of the PA, a second terminal configured to deliver an output signal of the PA, and a third terminal. Each auxiliary transistor comprises a first terminal, a second terminal coupled with the second terminal of the at least an amplifying transistor, and a third terminal electrically connected to the first terminal of the at least an amplifying transistor. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a power amplifier according to an embodiment of the present invention. 
         FIG. 2  is a signal spectrum diagram of adjacent channel power ratio (ACPR), generating by the power amplifiers with an auxiliary transistor and without the auxiliary transistor. 
         FIG. 3  is IMD3 performance comparison of the power amplifiers with an auxiliary transistor and without the auxiliary transistor. 
         FIG. 4  is a schematic diagram of a power amplifier according to an embodiment of the present invention. 
         FIG. 5  is a schematic diagram of a power amplifier according to an embodiment of the present invention. 
         FIG. 6  is a schematic diagram of a power amplifier according to an embodiment of the present invention. 
         FIG. 7  is a schematic diagram of a power amplifier according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic diagram of a power amplifier  10  according to an embodiment of the present invention. The power amplifier  10  is a single-stage power amplifier, which comprises an amplifying transistor  100  and an auxiliary transistor  102 . The amplifying transistor  100  may be a bipolar junction transistor (BJT) or a heterojunction bipolar transistor (HBT). The auxiliary transistor  102  may be a metal-oxide-semiconductor field effect transistor (MOSFET) or a high electron mobility transistor (HEMT). The amplifying transistor  100  comprises a base B (first terminal), a collector C (second terminal) and an emitter E (third terminal). The base B may be coupled from a radio frequency processing circuit  105  through an input network  104 . The input network  104  may comprise an impedance matching network, a gain amplifier, or a band pass filter. The base B is utilized to receive an input signal of the power amplifier  10 . The collector C is coupled to an antenna ANT through an output network  103 . The output network  103  may comprise an impedance matching network, a RF switch circuit, or a band pass filter (not illustrated in  FIG. 1 ). The collector C is utilized to deliver an output signal of the power amplifier  10  to the antenna ANT. The emitter E may be connected to a system ground. The auxiliary transistor  102  comprises a gate G (first terminal), a drain D (second terminal) and a source S (third terminal). The drain D is coupled to the collector C of the amplifying transistor  100 . The source S is electrically connected to the base B of the amplifying transistor  100 . The gate G may be connected to a bias driving circuit  106 , wherein the bias driving circuit  106  may be a fixed biasing circuit or a digital controllable and adjustable biasing circuit. 
     Note that, the auxiliary transistor  102  is combined between the input terminal (the base B) and the output terminal (the collector C) of the amplifying transistor  100 . The bias condition of the power amplifier  10  may be adjusted by auxiliary transistor  102  to the input terminal of the amplifying transistor  100  according to an output amplitude level at the output terminal of the amplifying transistor  100 . The signal linearity therefore is improved due to the bias adjustable function. Meanwhile, a temperature variation and a bias variation of the power amplifier  10  may be counteracted by a feedback mechanism provided by the auxiliary transistor  102 , so as to cancel a self-heating effect and a bias shifting effect caused by the amplifying transistor  100  under a large signal operation and to stabilize the variation of the amplifying transistor  100  in high frequency, forming the stable power amplifier  10  which would not be significantly affected by the variations of temperature and bias. In other words, the power amplifier  10  only utilizes the single auxiliary transistor  102  to adjust the bias at the input terminal of the amplifying transistor  100  and counteract the temperature and bias variations of the amplifying transistor  100 , so as to improve the signal linearity of the power amplifier  10 . In addition, the power amplifier  10  can be achieved with low circuit complexity. 
       FIGS. 2 and 3  are the comparisons of adjacent channel power ratio (ACPR) and linearity performance (i.e., third-order intermodulation (IMD3)) of power amplifiers with and without the auxiliary transistor, respectively, wherein the solid line represents the ACPR spectrum and the IMD3 performance of the power amplifier comprising the auxiliary transistor, and the dashed line represents the ACPR spectrum and the IMD3 performance of the power amplifier without the auxiliary transistor. As can be seen from  FIGS. 2 and 3 , the power amplifier comprising the auxiliary transistor may effectively suppress sideband transmitted power of and has a better linearity performance. 
     Notably, the embodiments stated in the above are utilized for illustrating concepts of the present invention. Those skilled in the art may make modifications and alternations accordingly, and not limited herein. For example,  FIG. 4  is a schematic diagram of a power amplifier  40  according to an embodiment of the present invention. The power amplifier  40  is similar to the power amplifier  10 , and thus, the same symbols are denoted by the same notations. Different from the power amplifier  10 , the power amplifier  40  is combined with a feedback circuit  406 , the feedback circuit  406  is coupled between the gate and the drain of the auxiliary transistor  102 , which satisfies the requirements of the present invention, and not limited herein. In addition, the amplifying transistor is not limited to be a BJT or an HBT. The amplifying transistor may also be a field effect transistor (FET) such as a MOSFET or an HEMT. In such a situation, the first terminal of the amplifying transistor is a gate of the FET, and the second terminal and the third terminal of the amplifying transistor are a drain or a source of the FET. In addition, the auxiliary transistor is not limited to be a MOSFET or a HEMT. The auxiliary transistor may also be a BJT or an HBT. In such a situation, the first terminal of the auxiliary transistor is a base of the BJT/HEMT, and the second terminal and the third terminal of the auxiliary transistor are a collector and an emitter of the BJT/HEMT. 
     In addition, the power amplifier of the present invention is not limited to comprise only one amplifying transistor. The power amplifier may comprise a plurality of amplifying transistors. For example,  FIG. 5  is a schematic diagram of a power amplifier  50  according to an embodiment of the present invention. The power amplifier  50  is similar to the power amplifier  10 , and thus, the same symbols are denoted by the same notations. Different from the power amplifier  10 , the power amplifier  50  comprises a plurality of amplifying transistors  500 _ 1 - 500 _N. The drain D of the auxiliary transistor  102  is coupled to collectors C_ 1 -C_N of the amplifying transistors  500 _ 1 - 500 _N, and the source S is electrically connected to bases B_ 1 -B_N of the amplifying transistors  500 _ 1 - 500 _N, which also meet requirements of the present invention. In addition, the power amplifier of the present invention may be an integrated-stage power amplifier by connecting multiple single-stage power amplifiers. For example,  FIG. 6  is a schematic diagram of a power amplifier  60  according to an embodiment of the present invention. The power amplifier  60  is an integrated-stage power amplifier, comprising a plurality of single-stage power amplifiers  62 _ 1 - 62 _N connected in parallel. Each single-stage power amplifier  62 _ x  is the same as the power amplifier  10 , i.e., each single-stage power amplifier  62 _ x  comprises an amplifying transistor  600 _ x  and an auxiliary transistor  602 _ x . A drain of the auxiliary transistor  602 _ x  is coupled to a collector of the amplifying transistor  600 _ x , and a source of the auxiliary transistor  602 _ x  is electrically connected to a base of the amplifying transistor  600 _ x . Note that, in the power amplifier  60 , a number of the auxiliary transistors  602 _ 1 - 602 _N is the same as a number of the amplifying transistors  600 _ 1 - 600 _N, and gates of the auxiliary transistors  602 _ 1 - 602 _N are combined together. Nevertheless, it is not limited thereto. For example,  FIG. 7  is a schematic diagram of a power amplifier  70  according to an embodiment of the present invention. The power amplifier  70  comprises amplifying transistors  700 _ 1 - 700 _N and auxiliary  702 _ 1 - 702 _ 3 . Gates of the auxiliary transistors  702 _ 1 - 702 _ 3  respectively receive bit-controlled signals bit_ 1 -bit_ 3  generated by a control circuit  706  to control on-off statuses of the auxiliary transistors  702 _ 1 - 702 _ 3 . As long as drains of the auxiliary transistors  702 _ 1 - 702 _ 3  are coupled to collectors of the amplifying transistors  700 _ 1 - 700 _N, and sources of the auxiliary transistors  702 _ 1 - 702 _ 3  are electrically connected to bases of the amplifying transistors  700 _ 1 - 700 _N, requirements of the present invention are satisfied. 
     In summary, the power amplifier of the present invention only utilizes the single auxiliary transistor to adjust the bias at the input terminal of the amplifying transistor and counteract the temperature characteristic and bias variation of the amplifying transistor, so as to improve the signal linearity and the stabilities of temperature and bias. In addition, the power amplifier of the present invention has low circuit complexity. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.