Patent Publication Number: US-2005128003-A1

Title: Transistor assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application is a continuation of copending International Application No. PCT/EP03/00670, filed Jan. 23, 2003, which designated the United States and was not published in English, and is incorporated herein by reference in its entirety.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a transistor assembly and, in particular, to such a transistor assembly comprising a plurality of transistors connected in parallel which together represent a power amplifier.  
      2. Description of the Related Art  
      In order to measure the collector current of a bipolar power transistor, a resistor is usually connected into the collector-emitter path. In analogy, in field-effect transistors a resistor is usually connected into the drain-source path in order to measure the drain current.  
      The resulting circuit structure of a bipolar power transistor  10  is illustrated in  FIG. 4 . The power transistor  10  includes a plurality of individual transistors of which three transistors  10   a ,  10   b  and  10   n  are shown in  FIG. 4 . The individual transistors  10   a ,  10   b  and  10   n  are connected in parallel in that the control terminals thereof are connected to an HF input  12  of the transistor assembly, the collector terminals thereof are connected to a voltage source  18  via a resistor  20  and, additionally, connected to an HF output  16  of the transistor assembly, and the emitter terminals thereof are connected to ground. The voltage source represents means for providing the operating current of the power transistor and thus of the individual transistors  10   a ,  10   b  and  10   n . A resistor  20  is connected between the voltage source  18  and a branching point  24  after which the current supply line branches off to the individual transistors  10 ,  10   b , . . .  10   n . Thus, each of the individual transistors  10   a ,  10   b  and  10   n  is effectively connected to this resistor  20 . The operating current of the power transistor formed by the sum of the collector-emitter currents of the individual transistors can be detected via the voltage drop at this resistor  20 , as is schematically illustrated in  FIG. 4  by a voltage-measuring device  22 .  
      In the circuit assembly shown in  FIG. 4 , the collector resistor  20  results in a reduction of the collector-emitter voltage of each of the individual transistors and the controllability thereof is impeded. Additionally, loss power is generated in the resistor  20 . In particular in applications operating at a small supply voltage, such as, for example, in mobile radio or in power amplifiers, this can result in a significant impairment of the output power of the transistor assembly and the efficiency thereof. Consequently, the transmitting time of a battery-operated mobile phone will be reduced.  
     SUMMARY OF THE INVENTION  
      It is the object of the present invention to provide a transistor assembly offering a simple low-loss possibility to provide a measure of the operating current of one or several transistors of the transistor assembly.  
      In accordance with a first aspect, the present invention provides a transistor assembly having: a plurality of transistors connected in parallel, the control terminals of which are connected to a common HF input; a current path for feeding an operating current for the transistor assembly; a plurality of sub-current paths branching off from the current path for feeding a respective operating current for the transistors; a resistor connected into one of the sub-current paths, wherein a voltage across the resistor is a measure of the operating current of the transistor assembly; and means for regulating the operating current of the transistor assembly based on the voltage across the resistor.  
      The present invention is based on the finding that it is possible to effectively connect only one of the transistors to a resistor for detecting the operating current of a transistor assembly formed of a plurality of transistors and to detect the voltage across this resistor to act as a measure of the operating current of the entire transistor assembly. This is possible since the operating current through the transistor effectively connected to the resistor and the operating current of the other transistors have a determined relation to one another.  
      The voltage across the resistor is a measure of the operating current of the entire transistor assembly and, additionally, a measure of the operating currents of the individual transistors since the operating current of the transistor assembly is formed by the sum of the collector-emitter currents (operating currents) of the individual transistors.  
      According to the invention, only the performance of the one transistor which is effectively connected to the resistor and can thus be considered as an auxiliary transistor is impeded by the wiring to the resistor. In addition, the loss power in the resistor is considerably smaller than in wiring the entire transistor assembly to the resistor, i.e. when-providing the resistor in the current path conducting the operating current of the entire transistor assembly.  
      The present invention can be applied with particular advantage in power transistors which basically include several, sometimes up to several hundred, individual transistors connected in parallel. In these power transistors, all the transistor terminals are usually merged on busses, wherein all the individual transistors which are also referred to as transistor fingers are lined up in a spatially close relation on a semiconductor chip and thus comprise almost identical electrical features. Thus, an individual transistor, i.e. a transistor finger, represents the features of the entire power transistor. If the detection of a measure of the operating current of this individual transistor is made possible, conclusions can be drawn to the operating current of the remaining transistors of the power transistor.  
      In such an application of the present invention to a power transistor including many individual transistors connected in parallel, the current determination takes place more or less without power. Since additionally only the electrical features of the effectively connected individual transistor but not the electrical features of all the other transistors of the power transistor are influenced, the electrical characteristics of the entire power transistor are hardly influenced.  
      Preferably, the individual transistor effectively connected to the resistor is completely HF (high-frequency) coupled to the entire transistor, i.e. base, collector and emitter conduct almost exactly the same HF signals as corresponding transistor terminals of the transistors not connected effectively, which is the case when connecting in parallel a plurality of transistors in power transistors. In such a design, changes in the surroundings of the entire transistor, such as, for example, mismatching at the HF output, are also reflected by a changed current consumption in the transistor effectively connected to the resistor.  
      In preferred embodiments of the present invention, means is further provided to adjust the operating current of the power transistor and thus the respective operating currents of the individual transistors thereof based on the voltage across the resistor. In particular, the operating current of the entire transistor can be controlled to a constant value based on the voltage across the resistor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Preferred embodiments of the present invention will be detailed subsequently referring to the appendage drawings, in which:  
       FIG. 1  shows a circuit diagram of an inventive transistor assembly;  
       FIG. 2  shows a circuit diagram of an inventive transistor assembly having operating current regulation;  
       FIG. 3  shows a circuit diagram of an alternative inventive transistor assembly having operating current regulation; and  
       FIG. 4  shows a circuit diagram of a conventional transistor assembly. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
      Preferred embodiments of the present invention will be detailed subsequently referring to FIGS.  1  to  3  with the example of bipolar power transistors. It is to be pointed out here that the present invention can be applied in the same manner to field-effect transistors having gate, drain and source instead of base, collector and emitter.  
      As is shown in  FIG. 1 , the embodiment of the inventive transistor assembly illustrated includes a plurality of bipolar transistors  32 ,  34  and  36 . The bipolar transistors  32 ,  34  and  36  are connected in parallel and together form a power transistor which, as the interrupted lines in  FIG. 1  show, is usually formed by a considerably greater number, such as, for example, several hundred, of individual transistors.  
      The bipolar transistors  32 ,  34  and  36  are connected in parallel in that the base terminals, i.e. the control terminals, thereof are connected to an HF input  38 , the emitter terminals thereof are connected to ground and the collector terminals thereof are coupled to an HF output  40 . An operating current of the transistor assembly is provided by a voltage source  42  which is connected to a current path  48  conducting the entire operating current for the transistor assembly via a coil  44  serving for HF decoupling. The current path  48  branches off into individual sub-lines  50 ,  52  and  54  or sub-current paths, respectively, which are connected to the collector terminals of the transistors  32 ,  34  and  36 .  
      As can be seen in  FIG. 1 , the collector terminals of the transistors  34  and  36  are directly connected to the current path  48  via the sub-lines  52  and  54 , while a resistor  60  is connected into the sub-line  50 . In addition, for purposes of illustration, a voltage-measuring device  62  enabling detection of the voltage across the resistor  60  is shown, since the coil  44  represents a short circuit for the direct current considered. As an alternative to the coupling illustrated, the current-measuring device  62  could be directly connected in parallel to the resistor  60 .  
      In the transistor assembly shown in  FIG. 1 , the voltage source  42  provides an operating current for the entire transistor assembly via the coil  44  on the current path  48 . This total current branches off into individual partial currents fed to the individual transistors on the sub-lines  50 ,  52  and  54  at the branching point of the current path  48 . The partial current fed to the transistor  32  can be detected by the resistor  60  connected into the sub-line  50  by detecting a voltage across it. According to the invention, the resistor  60  is not connected into the current path conducting the entire operating current of the transistor assembly but into the current path carrying the operating current of just one individual transistor.  
      The currents through the lines  50 ,  52  and  54  are in a determined relation to one another due to the current divider resulting by the branching of the current path  48  into the sub-current paths  50 ,  52  and  54  and due to the wiring of the respective sub-lines. Thus, conclusions can be drawn to the currents through the conductors  52  and  54  from the voltage across the resistor  60 . In addition, the entire operating current or the operating currents of the individual transistors, respectively, can be additionally controlled by controlling the voltage across the resistor  60 .  
      In the circuit assembly shown in  FIG. 1 , the auxiliary transistor finger, i.e. the transistor  32  having the resistor  60 , is completely HF-coupled to the entire transistor, i.e. base, collector and emitter conduct nearly exactly the same high-frequency signals as the corresponding transistor terminals of the remaining transistors. The only difference here is a small amplification provided by the auxiliary transistor  32 , due to the change of the operating point thereof by the wiring to the resistor  60 . This effect can, however, be neglected, wherein nevertheless changes in the surroundings of the entire transistor, such as, e.g., a mismatch of the output  40  of it, are reflected by a changed current consumption in the auxiliary transistor.  
       FIG. 2  shows an embodiment of the present invention comprising means for regulating the operating current of the power transistor based on the voltage across the resistor  60 . Thus, in  FIG. 2  elements corresponding to those of  FIG. 1  are designated by the same reference numerals.  
      According to  FIG. 2 , the voltage-measuring device  62  schematically illustrated in  FIG. 1  is substituted by a regulating element  70  by which the voltage drop at the resistor  60  can be kept constant so that the operating current of the entire transistor, too, is kept constant.  
      The regulating element  70  includes a transistor  72 , resistors  74  and  75 , a loop low-pass filter  76  and a constant current source  78 . In the embodiment illustrated, the transistor  72  is a pnp transistor while the transistors  32 ,  34  and  36  are npn transistors.  
      The emitter terminal of the transistor  72  is connected to the collector terminal of the transistor  32  via the loop low-pass filter  76 . The collector terminal of the transistor  72  is connected to the base terminal of the transistors  32 ,  34  and  36  via the resistor  75 . The base terminal of the transistor is connected to ground via the constant current source  78  and connected to the voltage source  42  via the resistor  74 . The constant current source  78  provides a defined voltage drop at the resistor  74  with an offset by the voltage source  42 .  
      For explaining the mode of functioning of the regulating element  70 , it is assumed that the operating current has a predetermined value resulting in a certain voltage drop at the resistor  60 . If the operating current changes, the voltage across the resistor  60  and thus the potential at the emitter terminal of the transistor  72  will change, wherein high-frequency changes are blocked by the loop low-pass filter  76 .  
      More precisely, the potential at the emitter terminal of the transistor  72  will decrease if the voltage drop at the resistor  60  increases, while the potential at the emitter terminal of the transistor  72  will increase if the voltage drop at the resistor  60  decreases.  
      The base-emitter voltage and thus the collector-emitter current of the transistor  72  decrease by a reduced potential at the emitter terminal of the transistor  72 . Thus, the base current available for the transistors  32 ,  34  and  36  is reduced so that the operating current thereof, i.e. the collector-emitter current thereof, will decrease. Thus, the current through the resistor  60  and consequently the voltage drop over it will be reduced.  
      The base emitter voltage and thus the collector-emitter current, however, will increase by an increased potential at the emitter terminal of the transistor  72 , as is caused by a reduced voltage drop at the resistor  60 . Thus, the base current available for the transistors  32 ,  34  and  36  will increase so that the operating current thereof, i.e. the collector-emitter current thereof, will increase. Consequently, the current through the resistor  60  and the voltage drop across it will increase.  
      According to the above explanations, the operating current through the entire transistor is regulated to a constant value by the regulating element  70  shown in  FIG. 2 .  
      Another embodiment for regulating the operating current of the transistor assembly is shown in  FIG. 3 , which again illustrates a regulating element by which the voltage drop at the resistor  60  can be kept constant so that the operating current of the entire transistor is kept constant, too.  
      The regulating element shown in  FIG. 3  includes a loop low-pass filter  76 , a pnp transistor  90 , an npn transistor  92 , resistors  94  and  96  and a voltage source  100 . The loop filter  76  is connected between the collector terminal of the transistor  32  and the base terminal of the pnp transistor  90 . The emitter terminal of the pnp transistor  90  is connected to the voltage source  42 . The collector terminal of the pnp transistor  90  is connected to ground via the resistor  94 . In addition, the collector terminal of the pnp transistor  90  is connected to the base terminal of the npn transistor  92 . The emitter terminal of the transistor  92  is connected to ground and the collector terminal thereof is connected to the base terminal of the transistors  32 ,  34  and  36 . Finally, a series connection of the resistor  96  and the voltage source  100  is connected between the base terminal of the transistors  32 ,  34  and  36  and ground.  
      The regulation of the operating current by the above setup will be explained subsequently. If the operating current of the transistor assembly changes departing from a desired operating current, the voltage across the resistor  60  will change. Due to this, the potential at the base terminal of the pnp transistor  90  will change, wherein high-frequency changes will be blocked by the loop low-pass filter  76 . More explicitly, the potential at the base terminal of the transistor  90  will decrease if the voltage drop across the resistor  60  increases, and vice-versa. The collector-emitter current will change due to the changed potential at the base terminal of the transistor  90 . More precisely, the collector-emitter current through the transistor  90  will increase if the potential at the base terminal thereof decreases, and vice-versa.  
      The change of the collector-emitter current of the transistor will change the voltage drop across the resistor  94  and thus the potential at the base of the resistor  92 . Thus, the collector-emitter current of the transistor  92  and consequently the current consumed by this transistor from the base current of the transistors  32 ,  34  and  36  will change. Consequently, the base current of the transistors  32 ,  34  and  36  will be reduced if the voltage drop across the resistor  60  and thus the voltage drop across the resistor  94  increase, while the base current in the transistors  32 ,  34  and  36  will increase if the voltage drop across the resistor  60  and thus the voltage drop across the resistor  94  decrease.  
      Corresponding to the change of the base current of the transistors  32 ,  34  and  36 , the emitter-collector current consumed by them and thus the operating current of the transistor assembly will change. The operating current is also regulated to a constant value in the example shown in  FIG. 3 .  
      It is to be pointed out here that the regulating elements described referring to  FIGS. 2 and 3  only represent examples and that a plurality of modified regulating elements is obvious to those skilled in the art. The intervention in the regulation can thus take place both in the bias of the power transistor, i.e. the individual transistors thereof, or else in an amplifier upstream of the power transistor acting as an output stage transistor. Here, the input line of the output stage transistor can be reduced so that the current consumption thereof will decrease.  
      Apart from a constant current regulation, as has been explained above referring to  FIG. 2 , different current regulations operating on the basis of the voltage across the resistor  60  can be realized. It is, particularly, possible to provide a regulation offering a current limitation and thus overload protection. This can be of advantage in power transistors, in particular in mobile radio where there are often high collector currents. Such high collector currents can, for example, be caused by mismatching at the output, wherein the transistor may be damaged by these high collector currents. The collector current of all the transistors of a power transistor can be limited to a defined maximum value using a suitable regulating element based on the voltage across the resistor which only the auxiliary transistor finger is connected to. A permanent damage of the power transistor can thus be prevented.  
      In addition, the present invention can be employed to perform a high-frequency output power limitation and a high-frequency output power setting. In order to optimize the efficiency, high-frequency power output stages are often operated in an AB, B and C operation, depending on the operating point setting. In these modes of operation, there is a very precise context between output stage current, i.e. operating current of the high-frequency power output stage, and the high-frequency output power. If the transistors of such a power output stage are compressed as a consequence of a high input power, the high-frequency output power can be set through a collector current limitation, which again takes place on the basis of the voltage across the resistor connected according to the invention.  
      Apart from the embodiments mentioned above, any current regulation can be performed by the remaining transistor fingers on the basis of detecting the current through an auxiliary transistor finger, as is performed according to the invention, since the currents through the different fingers have a reproducible relation to one another. The present invention thus provides the possibility to provide an operating current regulation for transistor assemblies having a plurality of transistors, which, on the one hand, does not reduce significantly the collector-emitter voltages of the individual transistors, except for the transistor effectively connected to the resistor, and, on the other hand, only generates small loss power. According to the invention, this is made possible by not wiring the entire power transistor to the resistor, but only an individual transistor, i.e. individual transistor finger, of the plurality of transistors.  
      While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, and equivalents which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and compositions of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention.