Abstract:
In a mixer of a converter, a bipolar transistor is connected between the base of a bipolar transistor and a positive power-supply node within a driving circuit, as a temperature characteristic compensating circuit. When the ambient temperature of the driving circuit rises, collector current of the transistor is increased due to the temperature characteristic of the transistor. This lowers the drain voltage of a high electron mobility transistor. However, collector current of the transistor within the temperature characteristic compensating circuit is also increased, increasing the voltage at a point, so that the increase in the collector current of the transistor is suppressed. This results in a stabilized drain voltage of the transistor. Therefore, the low noise down-converter can supply a stable voltage to a transistor performing frequency conversion, without any effect from the change in the ambient temperature.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a Low Noise Block down-converter (hereinafter referred to as LNB), and more particularly, to an LNB in a satellite broadcast receiving system. 
   2. Description of the Background Art 
   In the satellite broadcast receiving system, an LNB has a function of amplifying, with low noise, a satellite broadcast wave signal in a 12 GHz band received from a broadcast satellite, and also frequency-converting the signal to be in an intermediate frequency (IF) band. The frequency conversion described above is performed in a mixer (hereinafter referred to as MIX) within the LNB. 
     FIG. 9  is a circuit diagram of an MIX. The MIX circuit includes a high electron mobility transistor (hereinafter referred to as HEMT)  50  used as a transistor for actually performing frequency conversion, and a driving circuit  10 . Driving circuit  10  includes a PNP bipolar transistor Tr 1  and a plurality of resistance elements. 
   Driving circuit  10  supplies a constant voltage to the drain of HEMT  50 , and the constant voltage allows the MIX to perform stable frequency conversion. That is, characteristics of the MIX largely depend on the drain voltage of HEMT  50 . 
   However, in the MIX, when an ambient temperature of PNP bipolar transistor Tr 1  within driving circuit  10  is changed, the temperature characteristic of PNP bipolar transistor Tr 1  itself causes a change in the voltage supplied to the drain of HEMT  50 . The change in the drain voltage causes a change in the gain frequency characteristic of HEMT  50 , interfering stable frequency conversion. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a low noise down-converter including a driving circuit that can supply a stable voltage to a transistor performing frequency conversion, without any effect of a change in an ambient temperature. 
   According to an aspect of the present invention, a low noise down-converter for satellite broadcast receiving includes a mixer converting a received high-frequency signal into an intermediate-frequency signal. The mixer includes a transistor performing frequency conversion, a first bipolar transistor having an emitter connected to a drain of the transistor and a collector connected to a gate of the transistor, and a temperature characteristic compensating circuit connected to a base of the first bipolar transistor and canceling a temperature characteristic of the first bipolar transistor. 
   Thus, the temperature characteristic compensating circuit cancels the temperature characteristic of the first bipolar transistor, allowing the first bipolar transistor to supply a stable voltage to the drain of the transistor performing frequency conversion even in a case where the ambient temperature of the first bipolar transistor is changed. 
   Preferably, the temperature characteristic compensating circuit includes a second bipolar transistor having a conductive terminal connected to the base of the first bipolar transistor. 
   Thus, the second bipolar transistor stabilizes collector current of the first bipolar transistor with respect to a change in the ambient temperature, and as a result, the first bipolar transistor can supply a stable voltage to the drain of the transistor performing frequency conversion. 
   More preferably, the first and second bipolar transistors are packaged into a dual transistor. 
   Thus, the first bipolar transistor and the second bipolar transistor are packaged together, so that both of them operate under the same temperature condition. As a result, more stable voltage can be supplied to the drain of the transistor performing frequency conversion. Moreover, the area occupied by the transistors within the circuit can be smaller by collecting two transistors together into a single package. 
   As described above, according to the present invention, in the MIX within the LNB, installation of a temperature characteristic compensating circuit in a driving circuit allows a stable voltage to be supplied to a transistor performing frequency conversion, without any effect of a change in the ambient temperature. 
   The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows the schematic configuration of a satellite broadcast receiving system; 
       FIG. 2  is a block diagram showing the schematic configuration of an LNB in  FIG. 1 ; 
       FIG. 3  is a circuit diagram showing the configuration of an MIX in  FIG. 2  according to an embodiment of the present invention; 
       FIG. 4  is a circuit diagram showing an example of the MIX shown in  FIG. 3 ; 
       FIG. 5  is a circuit diagram showing another example of the MIX shown in  FIG. 3 ; 
       FIG. 6  is a circuit diagram showing the configuration of an MIX according to an example of the present invention used for the measurement of a drain voltage; 
       FIG. 7  is a circuit diagram showing the configuration of the conventional MIX used for the measurement of a drain voltage; 
       FIGS. 8A and 8B  show measurement results of the drain voltage; and 
       FIG. 9  is a circuit diagram showing the configuration of the conventional MIX. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments of the present invention will be described below in detail with reference to the drawings. It is noted that the same or corresponding portions are denoted by the same reference characters, and the description thereof will not be repeated. 
   First Embodiment 
     FIG. 1  shows a typical satellite broadcast receiving system. The satellite broadcast receiving system includes an antenna  101 , an LNB  102 , a BS-IF cable  103 , a BS tuner  104 , and a television  105 . 
   An uplink signal of 14 GHz sent out from a ground station is received by a broadcast satellite  100 . Broadcast satellite  100  converts and amplifies the uplink signal to be in the 12 GHz band, and transmits the signal again toward the ground as a satellite broadcast wave signal. The satellite broadcast wave signal is received by antenna  101  and is input into LNB  102 . LNB  102  amplifies, with low noise, the satellite broadcast wave signal which is a weak electric wave having a frequency in the 12 GHz band, and thereafter frequency-converts the signal into a signal in an intermediate frequency band (hereinafter referred to as a BS-IF signal) at an MIX which is an internal circuit. The BS-IF signal is transmitted to BS tuner  104  via BS-IF cable  103 . At BS tuner  104 , after a user selects the channel of a desired problem, the signal is FM-demodulated to reproduce a video signal and an aural signal. The reproduced video and aural signals are transmitted to television  105 . 
   Next, the internal structure of LNB  102  will be described. 
     FIG. 2  is a circuit block diagram of LNB  102 . 
   LNB  102  is includes a circular waveguide  1  receiving a satellite broadcast wave signal; a Low Noise Amplifier (hereinafter referred to as LNA)  2  amplifying the satellite broadcast wave signal with low noise; a BPF (Band Pass Filter)  3  passing a signal in a desired frequency band and rejecting a signal in an image frequency band; an MIX  4 ; a local oscillator  5  supplying an oscillation signal to MIX  4 ; an intermediate frequency amplifier (hereinafter referred to as IF-AMP)  6  amplifying a signal to be transmitted to BS tuner  104 ; a power-supply unit  7 ; and an output terminal  8 . 
   A satellite broadcast wave signal having a frequency between 11.71 and 12.01 GHz is received by an antenna probe within waveguide  1 . Subsequently, the satellite broadcast wave signal is input into LNA  2 , and is amplified with low noise. The amplified signal in the 12 GHz band is input into BPF  3 , and the signal in the image frequency band is rejected therefrom. 
   The satellite broadcast signal passed through BPF  3  is input into MIX  4 . An oscillation signal of 10.678 GHz from local oscillator  5  is supplied to MIX  4 . In MIX  4 , the satellite broadcast signal in the 12 GHz band is frequency-converted into a BS-IF signal between 1035 and 1335 MHz. 
   Subsequently, the BS-IF signal is input into IF-AMP  6 , where the signal is amplified to have appropriate noise characteristic and gain characteristic. The amplified BS-IF signal is output from output terminal  8 , and is transmitted to BS tuner  104  via BS-IF cable  103 . 
   Next, MIX  4  within LNB  102  will be described. 
     FIG. 3  is a circuit diagram showing the configuration of MIX  4  according to an embodiment of the present invention. 
   MIX  4  includes an input terminal  20 , an output terminal  21 , an HEMT  50  frequency-converting an input satellite broadcast wave signal, resistance elements R 1 , R 2 , and a driving circuit  10 . 
   Driving circuit  10  includes a PNP transistor Tr 1  supplying a stable drain voltage to HEMT  50 , a temperature characteristic compensating circuit  11  canceling the temperature characteristic of transistor Tr 1 , and resistance elements R 3 , R 4 , R 5 . 
   Resistance element R 1  is connected between the gate of HEMT  50  and the collector of transistor Tr 1 . Resistance element R 2  is connected between the drain of HEMT  50  and the emitter of transistor Tr 1 . Resistance element R 3  is connected between a negative power-supply node and the collector of transistor Tr 1 . Moreover, resistance element R 4  is connected between a positive power-supply node  22  and the emitter of transistor Tr 1 . Resistance element R 5  is connected between the base of transistor Tr 1  and a ground node  24 . Temperature characteristic compensating circuit  11  is connected between a positive power-supply node  23  and the base of transistor Tr 1 . Furthermore, the source of HEMT  50  is connected to ground node  24 . 
   HEMT  50  frequency-converts a satellite broadcast signal input from input terminal  20  into the gate of HEMT  50 , using a local oscillation signal input from local oscillator  5  into the drain of HEMT  50 . 
     FIG. 4  is a circuit diagram showing the configuration of MIX  4  according to the first embodiment. 
   Here, an NPN bipolar transistor is added as temperature characteristic compensating circuit  11  in  FIG. 3 . Temperature characteristic compensating circuit  11  includes an NPN bipolar transistor Tr 2 , and resistance elements R 6 , R 7  and R 8 . 
   Resistance element R 6  is connected between positive power-supply node  23  and the collector of transistor Tr 2 . Resistance element R 7  is connected between a positive power-supply node  26  and the base of transistor Tr 2 . Resistance element R 8  is connected between the base of transistor Tr 2  and ground node  24 . The emitter of transistor Tr 2  is connected to the base of transistor Tr 1 . 
   Next, the operation of MIX  4  shown in  FIG. 4  will be described. 
   In  FIG. 4 , collector current of transistor Tr 1  is determined in accordance with a voltage at a B 1  point, which is a connecting point of the base of transistor Tr 1  and resistance element R 5 . Further, a drain voltage of HEMT  50  is determined by the collector current of transistor Tr 1 . 
   Whereas, the voltage at the B 1  point is determined in accordance with the collector current of transistor Tr 2  determined by a voltage at a B 2  point, which is a connecting point of transistor Tr 2  and resistance element R 7 . 
   A case where a temperature change is applied to driving circuit  10  is now considered. When the ambient temperature of transistor Tr 1  is raised, the voltage between the base and emitter of transistor Tr 1  indicates a negative temperature dependency, and thus the collector current of transistor Tr 1  is increased in accordance with the temperature characteristic. Therefore, the increase in the collector current of transistor Tr 1  affects the drain voltage of HEMT  50  such that the drain voltage is lowered. 
   Then, as in the case with transistor Tr 1 , the ambient temperature is also raised in transistor Tr 2 . The voltage between the base and emitter of transistor Tr 2  indicates a negative temperature dependency as in the case with transistor Tr 1 . Thus, the collector current of transistor Tr 2  is also increased. The increase of the collector current of transistor Tr 2  raises the voltage at the B 1  point. The raise of the voltage at the B 1  point means lowering of the voltage between the base and emitter of transistor Tr 1 , and affects the collector current of Tr 1  such that the collector current is reduced. Therefore, the raise of the collector current of transistor Tr 1  due to the temperature raise is suppressed, and, as a result, the drain voltage of HEMT  50  is stabilized even though the ambient temperature is raised. 
   As described above, by connecting transistor Tr 2 , as a compensating circuit for the temperature characteristic, having an opposite polarity structure to that of transistor Tr 1  to transistor Tr 1  as a driving circuit supplying a stable voltage to HEMT  1 , variation in the drain voltage of HEMT  50  due to temperature can be lessened. 
   Second Embodiment 
   Though an embodiment of the present invention has been described, the invention is not limited to the embodiment described above, and can be implemented in other forms. 
     FIG. 5  is a circuit diagram showing another example of MIX  4  within LNB  102 . 
   This MIX  4  uses a dual transistor  12  in which transistor Tr 1  and transistor Tr 2  are packaged together, in place of transistor Tr 1  and transistor Tr 2  being installed as independent elements as shown in  FIG. 4 . 
   The operation of MIX  4  with respect to a change in the ambient temperature is similar to that in the first embodiment, and dual transistor  12  stabilizes the drain voltage of HEMT  50  with respect to the change of the ambient temperature. 
   By packaging transistor Tr 1  and transistor Tr 2  together to form dual transistor  12 , the temperature condition comes to be the same for each of transistors Tr 1 , Tr 2 , and therefore variation with respect to the temperature change is suppressed and more stable voltage can be supplied to HEMT  50 . 
   Furthermore, the area occupied by the transistor within the circuit can be reduced by the packaging technique, and thus weight reduction of devices can be achieved due to the saving in space. 
   EXAMPLE 
     FIG. 6  is a circuit diagram showing the configuration of the MIX of the present invention when a change in the drain voltage of HEMT  50  with respect to a change in the temperature is measured. 
   For the driving circuit, dual transistor  12  in which transistor Tr 1  and transistor Tr 2  are packaged together is used. 
   Resistance element R 1  in  FIG. 5  is, in  FIG. 6 , constituted by resistance elements R 11 , R 12 , and a bypass capacitor C 1  connected in series. Similarly, in  FIG. 6 , resistance element R 2  in  FIG. 5  is constituted by resistance elements R 21 , R 22 , and a bypass capacitor C 2  connected in series. 
   Furthermore, in  FIG. 6 , a bypass capacitor C 3  is connected between the base of transistor Tr 1  and ground node  24 , and a bypass capacitor C 4  is connected between the base of transistor Tr 2  and ground node  24 . 
   As a comparative example,  FIG. 7  shows a circuit configuration of the conventional MIX when a change in the drain voltage of HEMT  50  is measured with respect to a change in the temperature. 
   A positive power-supply is set as 7 volts and a negative power-supply is set as −2 volts, and the changes in the drain voltage of HEMT  50  were measured in the conventional MIX and in the MIX of the present invention at the ambient temperatures of −30° C., 25° C. and 50° C. 
   As for the MIX to be measured, two circuits having an identical structure were fabricated for the conventional MIX and named as the first conventional circuit and the second conventional circuit, and similarly, two circuits having an identical structure were fabricated also for the MIX of the present invention and named as the first invention circuit and the second invention circuit. 
   The measurement result of drain voltage Vd value (V) of HEMT  50  at each ambient temperature is indicated in Table 1, and the chart illustrating the result is shown in  FIG. 8A . Moreover, the rate of change (%) in the drain voltage of HEMT  50  at each ambient temperature is indicated in Table 2 using the value of the drain voltage of HEMT  50  at the ambient temperature of 25° C. as a reference, and the chart illustrating thereof is shown in  FIG. 8B . 
   
     
       
             
           
             
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Change in Drain Voltage Vd of MIX-HEMT [V] 
             
           
        
         
             
               Ambient 
               1st 
               2nd 
               1st 
               2nd 
             
             
               Temperature 
               Conventional 
               Conventional 
               Invention 
               Invention 
             
             
               [° C.] 
               Circuit 
               Circuit 
               Circuit 
               Circuit 
             
             
                 
             
             
               −30   
               0.4  
               0.503 
               0.185 
               0.299 
             
             
               25 
               0.261 
               0.361 
               0.213 
               0.326 
             
             
               50 
               0.183 
               0.287 
               0.226 
               0.34  
             
             
                 
             
           
        
       
     
   
   
     
       
             
           
             
             
             
             
             
           
             
             
             
             
             
           
         
             
               TABLE 2 
             
           
           
             
                 
             
             
               Rate of Change in Vd of MIX-HEMT [%] (Reference: 25° C.) 
             
           
        
         
             
               Am- 
                 
                 
                 
                 
             
             
               bient 
               1st 
               2nd 
               1st 
               2nd 
             
             
               Tem- 
               Conventional 
               Conventional 
               Invention 
               Invention 
             
             
               perature 
               Circuit 
               Circuit 
               Circuit 
               Circuit 
             
             
                 
             
           
        
         
             
               −30   
               53.25670498 
               39.33518006 
               −13.1455399 
               −8.28220859 
             
             
               25 
               0 
               0 
               0 
               0 
             
             
               50 
               −29.8850575 
               −20.498615 
               6.103286385 
               4.294478528 
             
             
                 
             
           
        
       
     
   
   As a result of the measurements, the change in the drain voltage of HEMT  50  with respect to the change in the ambient temperature is smaller in the MIX of the present invention. 
   Therefore, provision of transistor Tr 2  as a temperature compensating circuit allows variation in the collector current of transistor Tr 1  to be suppressed, and therefore, variation in the drain voltage of HEMT  50  can be suppressed. 
   Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.