Patent Publication Number: US-2005140455-A1

Title: Power amplifying module

Description:
BACKGROUND OF INVENTION  
      1. Field of the Invention  
      The present invention relates to a power amplifying module, and more specifically, to an optimal heterojunction bipolar transistor (HBT) power amplifier.  
      2. Description of the Prior Art  
      With the rapid development of the communication industry, power amplifier integrated circuits using HBTs are suitable for a wide range of applications and are particularly well adapted for use as high power microwave amplifiers such as those used in mobile phones.  
      In fact, in order to improve the amplifying effect, it is important to design multi-stage amplifiers. Two factors, maximum power gain and linearity, are used to determine the effect of an amplifier. Please refer to  FIG. 1 , which shows a relationship of a ratio of input power to average input power versus a ratio of output power to average output power, in which curve A indicates an ideal power of the amplifier, curve B indicates an actual power of the amplifier, and input 1 dB compression point IP 1dB  and output 1 dB compression point OP 1dB  indicate the point on curve B in which a difference of 1 dB between curve A and curve B is obtained. From  FIG. 1 , the larger the input 1 dB compression point IP 1dB  is, the better the linearity is.  
      Generally speaking, an amplifier consists of at least a HBT coupled in parallel. A frequent and often serious problem with HBT power amplifiers is excessive heat buildup. Power amplifier integrated circuits operate at high current density, and hence high power density, and thus, heat generated by devices of the HBT elevates junction temperature significantly above ambient temperature. High junction temperature degrades the device reliability and limits the maximum power density of the device. Additionally, operating at higher power density risks thermal runaway of the power amplifier, in which the power amplifier suffers catastrophic device failure. Furthermore, operating at a higherjunction temperature reduces device mean time to failure (MTTF). Typically, it is an important issue to design an amplifier using fewer HBTs and keeping the linearity effect.  
     SUMMARY OF INVENTION  
      It is therefore a primary objective of the claimed invention to provide a power amplifying module, to solve the above-mentioned problem.  
      According to the claimed invention, a power amplifying module includes an input end for inputting power, an output end for outputting amplified power, and two cascaded power amplifiers, each comprising at least a heterojunction bipolar transistor (HBT) coupled in parallel, wherein the number of HBTs of the amplifier closer to the input end is less than that of the amplifier closer to the output end.  
      According to the claimed invention, a method of forming a power amplifying module includes a step of cascading two power amplifiers of which the power amplifier closer to the input end has a smaller number of heterojunction bipolar transistors (HBTs) coupled in parallel than the power amplifier closer to the output end does.  
      These and other objects of the claimed 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 DRAWINGS  
       FIG. 1  shows a relationship of a ratio of input power to average input power versus a ratio of output power to average output power  
       FIG. 2  is a schematic diagram of power amplifying module according to the present invention.  
       FIG. 3  is a circuit diagram of a power amplifier comprising a number of HBTs in parallel.  
       FIG. 4  shows a relationship chart of power gain, an input 1 dB compression point IP 1dB , and an output 1 dB compression point OP 1dB  versus a various number of parallel HBTs.  
       FIG. 5  is a relationship diagram in accordance with  FIG. 4   
    
    
     DETAILED DESCRIPTION  
      Please refer to  FIGS. 2 and 3 .  FIG. 2  is a schematic diagram of a power amplifying module  10  according to the present invention.  FIG. 3  is a circuit diagram of a power amplifier comprises a number of HBTs  22  in parallel. A power amplifying module  10  comprises an input end  12  for inputting power, an output end  14  for outputting amplified power, and two cascade power amplifiers  121 ,  122 , where the power amplifier  121  is connected with the input end  12  and the power amplifier  122  is connected with the output end  14 . As shown in  FIG. 3 , each of power amplifiers  121 ,  122  comprises at least one HBT in parallel, where IN indicates an input of a power amplifier and OUT indicates an output of a power amplifier. A matching circuit  18  is connected between the two power amplifiers  121 ,  122 , and is used for matching power thereof.  
      Please refer to  FIGS. 2, 3  in conjunction with  FIGS. 4, 5 .  FIG. 4  shows a relationship chart of power gain, an input 1 dB compression point IP 1dB , and an output 1 dB compression point OP 1dB  versus a various number of parallel HBTs  22 .  FIG. 5  is a relationship diagram in accordance with  FIG. 4 , where a bias voltage of each HBT is 0.87V and both resistances of an input resistor and output resistor are 50 ohms. From  FIGS. 4 , and  5 , the higher the number of the parallel HBTs is, the less the power gain of the power amplifier is and the larger both the input 1 dB compression point IP 1dB  and an output 1 dB compression point OP 1dB  are. In other words, the higher the parallel number of the HBTs  22  is, the worse the amplifying effect is, but the better a linearity (i.e. an improvement of distortion) is. Please notice that if more than 8 HBTs are parallel connected, an output 1 dB compression point OP 1dB  tends towards a constant value.  
      Additionally, the data shown in  FIGS. 4 and 5  is measured under both the input impendence and the output impendence of 50 ohms. The higher the number of HBTs formed in an amplifier is, the smaller the measured power gain of the amplifier is, as well as the larger both the input 1 dB compression point IP 1dB  and output 1 dB compression point OP 1dB  are. But the value of the output 1 dB compression point OP 1dB  will tend towards a constant when the number of the HBTs  22  of an amplifier is over a threshold. For convenience, the associated data as follows is based on  FIGS. 4 and 5 .  
      Please refer to  FIGS. 2, 4  and  5 . Suppose that the input power of −5 dBm is to be amplified to 20 dBm, an amplifier module  10  having 25 dBm power gain is required, of which the amplifier  121  closer to the input end  12  has 4 parallelly-coupled HBTs  22 , and the amplifier  122  closer to the output end  11  has  11  parallelly-coupled HBTs  22 , respectively. The reason the differentnumbers of the HBTs of the amplifiers  121 ,  122  are chosen results from the fact that the input power of −5 dBm is smaller than the 1 dB compression point IP 1dB-121  of the amplifier  121  (i.e. 5.7 dBm), and the 10.235 dBm output power of the amplifier  121  available is smaller than both the output 1 dB compression point OP 1dB-121  of the amplifier  121  (i.e. 18.239 dBm) and the input 1 dB compression point IP 1dB-122  of the amplifier  122  (i.e. 11.6 dBm). In doing so, the output power via the amplifier  121  is not distorted at all and does not exceed a limitation of the input power of the next amplifier  122 . Finally, the output power via the amplifier  122  is 19.802 dBm, which is lower than the output 1 dB compression pointOP 1dB-122  of the amplifier  122  (i.e. 20.168 dBm), meaning that the output power is still not distorted. Generally speaking, a matching circuit  18  is arranged between the two amplifiers  121 ,  122  for matching the output power of the amplifier  121  with the input power of the amplifier  122  for preventing power loss.  
      The input power of −5 dBm is smaller than the 1 dB compression point IP 1dB-121  of the amplifier  121  (i.e. 5.7 dBm), and the 10.235 dBm available output power of the amplifier  121  is smaller than both the output 1 dB compression point OP 1dB-121  of the amplifier  121  (i.e. 18.239 dBm) and the input 1 dB compression point IP 1dB-122  of the amplifier  122  (i.e. 11.6 dBm). In doing so, the output power via the amplifier  121  is not distorted at all and does not exceed a limitation of the input power of the next amplifier  122 . Finally, the output power via the amplifier  122  is 19.802 dBm, which is lower than the output 1 dB compression pointOP 1dB-122  of the amplifier  122  (i.e. 20.168 dBm), meaning that the output power is still not distorted.  
      Suppose that the input power of −5 dBm remains, but amplifiers  121 ,  122  are swapped. Input power of −5 dBm is smaller than the input 1 dB compression point IP 1dB-121  of the amplifier  122  (i.e. 11.6 dBm) and 4.567 dBm output power of the amplifier  122  available is smaller than both the output 1 dB compression point OP 1dB-122  of the amplifier  122  (i.e. 20.168 dBm) and the input 1 dB compression point IP 1dB-121  of the amplifier  121  (i.e. 5.7 dBm). In doing so, the output power via the amplifier  122  is not distorted at all and does not exceed a limitation of the input power of the next amplifier  121 . Afterwards, the output power via the amplifier  121  is 19.802 dBm, which is larger than the output 1 dB compression point OP 1dB-121  of the amplifier  121  (i.e. 18.239 dBm), meaning that the output power is distorted. Consequently, if the power amplifier comprising a higher number of HBTs is set close to the input end, distortion is more likely.  
      Certainly, the number of the HBTs contained in the amplifiers  121 ,  122  is not limited to the above combination. For example, the amplifier  121  can be formed by 5 HBTs coupled in parallel, and the amplifier  122  can be formed by 10 HBTs  22  coupled in parallel. But the amplifier  121  close to the input end should have a smaller number of coupled-in-parallel HBTs than the amplifier  122  close to the output end does.  
      From  FIG. 4 , the power amplifier having only one HBT  22  has a power gain of 17.245 and a minimum output 1 dB compression point of OP 1dB . If the designer requires the output power of up to 10.6 dBm and the gain of over 17.245, the concept of the present invention facilitates design efficiency.  
      Notice that although the data of the output 1 dB compression point OP 1dB  shown in  FIGS. 4 and 5  is obtained under the condition of both the input impendence and output impendence being 50 ohms, a similar tend, in which the output 1 dB compression point OP 1dB  tends towards to a constant (about 20.5 dBm) as output 1 dB compression pointOP 1dB  increases, remains. If necessary, a designer can adjust both input impendence and output impendence to change the constant of the output 1 dB compression pointOP 1dB.  To sum up, the concept of controlling the amplifier close to the input end to have a lower number of HBTs belongs to the scope of the present invention.  
      Generally speaking, for design of the power amplifying module the better anti-distortion effect is required for the amplifier closer to the output end. However, if the power amplifier having a smaller number of HBTs, which has smaller input 1 dB compression point IP 1dB  and output 1 dB compression point OP 1dB , is set closer to the output end, this leads to worse distortion and bad output effect. Consequently, the present power amplifying module is best suited for use in designing high power microwave integrated circuits. This is because that the power amplifier having a higher number of HBTs, which has a lower power gain, larger input 1 dB compression point IP 1dB  and output 1 dB compression point OP 1dB ,is set closer to the output end, thereby preserving the power gain of the complete power amplifying module and also reducing the distortion risk.  
      Those skilled in the art will readily observe that numerous modifications and alterations of the device 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.