Abstract:
A high-frequency bias supply circuit includes a plurality of bias supply lines each of which is connected in parallel with others between a corresponding one of selected points along the main signal line and a common bias supply terminal. Each of the bias supply lines has a length and a distance from an adjacent bias supply line different from those of others, the length and the distance being in a range of a 1/3 to 3/8 wavelength of the center frequency.

Description:
FIELD OF THE INVENTION 
     This invention relates to a high-frequency bias supply circuit, and more particularly to, a high-frequency bias supply circuit used for a transmitter, etc. in a satellite communication system, a telephone communication system, etc. 
     BACKGROUND OF THE INVENTION 
     Recently, a high-frequency bias supply circuit having a plurality of quarter-wave lines has been used for a transmitter, etc. in a radio communication system, such as a satellite communication system, a telephone communication system, etc. in order to increase a level of current without affecting a RF (radio frequency) signal to be transmitted. 
     According to a conventional high-frequency bias supply circuit, however, there is a disadvantage in that available frequency bandwidth is not expanded sufficiently, because a ripple occurs in a signal-loss characteristic in the frequency bandwidth. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to provide a high-frequency bias supply circuit in which a signal-pass characteristic of a wide bandwidth having less ripple is obtained. 
     According to the invention, a high-frequency bias supply circuit, includes: 
     a main signal line through which a radio frequency signal of a predetermined center frequency is passed; and 
     a plurality of bias supply lines each of which is connected in parallel with others between a corresponding one of selected points along the main signal line and a common bias supply terminal to which a bias current is supplied; 
     wherein each of the bias supply lines has a length and a distance from an adjacent bias supply line different from those of others, the length and the distance being in a range of a 1/3 to 3/8 wavelength of the center frequency. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings; wherein: 
     FIG. 1 is a block diagram showing an amplifier used in a RF transmitter including bias supply circuits; 
     FIG. 2 is a circuit diagram showing a self-bias circuit used in the amplifier of FIG. 1; 
     FIG. 3 is a circuit diagram showing a first conventional high-frequency bias supply circuit; 
     FIG. 4 is a graph showing a signal-pass characteristics of the first conventional high-frequency bias supply circuit; 
     FIG. 5 is a circuit diagram showing a second conventional high-frequency bias supply circuit; 
     FIG. 6 is a graph showing a signal-pass characteristics of the second conventional high-frequency bias supply circuit; 
     FIG. 7 is a circuit diagram showing a high-frequency bias supply circuit of a first preferred embodiment according to the invention; 
     FIG. 8 is a graph showing a signal-pass characteristics of the first preferred embodiment; 
     FIG. 9 is an equivalent circuit of a high-frequency bias supply circuit according to the invention; and 
     FIG. 10 is a circuit diagram showing a high-frequency bias supply circuit of a second preferred embodiment according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For better understanding the background of the present invention, the basic principle of the conventional technology is first described hereinafter with reference to FIGS. 1 to 6. 
     FIG. 1 shows a conventional transistor amplifier 10 which includes a bias supply circuit 12 connected to an input terminal 10A, an input matching circuit 14 connected between the bias supply circuit 12 and a gate of a FET 16 which is connected at a source to ground, an output matching circuit 18 connected to a drain of the FET 16, and a bias supply circuit 20 connected between the output matching circuit and an output terminal 10B. 
     In operation, when a predetermined bias current is supplied to a bias terminal 12a of the bias supply circuits 12, a RF input signal supplied from, for instance, a modulator (not shown) thereto is biased in current. And, a current biased signal is supplied through the input matching circuit 14 to the gate of the FET 16, so that an amplified RF signal is obtained at the drain thereof. Then, the amplified RF signal is supplied through the output matching circuit 18 to the bias supply circuit 20, from which a RF signal to be biased by a predetermined bias current supplied to the bias supply terminal 20a is supplied to an output terminal 10B which is connected, for instance, to a transmitting antenna. 
     In the amplifier 10, a self-bias circuit 22 shown in FIG. 2 may be used therein in stead of the bias supply circuit 12. The self-bias circuit 22 is composed of a FET 24, a resistor 26 having a large resistance connected between a gate of the FET 24 and ground, a resistor 28 having a small resistance connected between a source of the FET 24 and ground, and a capacitor 30 connected between the source of the FET 24 and ground. Further, the high-frequency bias supply circuit may be used for an amplifier of a bipolar transistor (not shown), and for a two electrode device such as a diode, respectively, in place of the FET 16. 
     FIG. 3 shows a first conventional high-frequency bias supply circuit 30 which includes a main signal line 32 connected between RF (radio frequency) terminals 34a and 34b, a quarter-wave line 36 connected at one end to the main signal line 32 and at another end to a bias supply terminal 40, and a capacitor 38 connected between ground and a connection point 39. 
     In the high-frequency bias supply circuit 30, when a predetermined bias current is supplied to the bias supply terminal 40, the FET 16 is biased in current with no effect on a radio signal passing through the main signal line 32, because the quarter-wave line 36 functions as an open circuit for the RF signal. 
     FIG. 4 shows a radio frequency characteristic of the first conventional high-frequency bias supply circuit 30, wherein a radio signal passing through the main signal line 34 has a center frequency of 15 GHz. In this graph, passing loss of a radio signal is shown by a solid line, and return loss is shown by a dashed line. According to the graph, it is found that the frequency characteristic of the first conventional high-frequency bias supply circuit 30 becomes a curve of secondary degree having a peak at 15 GHz. 
     FIG. 5 shows a second conventional high-frequency bias supply circuit 50 which includes a main signal line 52 connected between RF (radio frequency) terminals 54a and 54b, five of quarter-wave lines 56a, 56b, 56c, 56d and 56e each connected between one side of the main signal line 52 and a bias supply terminal 60 in common, and five of capacitors 58a, 58b, 58c, 58d and 58e connected between ground and connection points 59a, 59b, 59c, 59d and 59e, respectively. Each of the quarter-wave lines 56a, 56b, 56c, 56d and 56e has a predetermined length to provide a quarter wave of a radio signal passing through the main signal line 54. 
     FIG. 6 shows a radio frequency characteristic of the second conventional high-frequency bias supply circuit 50, wherein a radio signal having a center frequency of 15 GHz is passed through the main signal line 52. In this graph, passing loss of the radio signal is shown by a solid line, and return loss is shown by a dashed line. According to the graph, it is found that the frequency characteristic of the second conventional high-frequency bias supply circuit 50 has a flat portion around 15 GHz center frequency. 
     According to the second conventional high-frequency bias supply circuit 50, the quarter-wave lines 56a, 56b, 56c, 56d and 56e are used therein, so that a bias current supplied to the main signal line 52 is increased in proportional to the number thereof. 
     Therefore, a frequency bandwidth is expanded as understood from FIG. 6. However, there is a disadvantage in that a ripple occurs in the frequency characteristic as shown in FIG. 6, so that the frequency bandwidth is not so wide as expected. 
     FIG. 7 shows a high-frequency bias supply circuit 70 of a first preferred embodiment according to the invention, which includes a main signal line 72 connected between RF (radio frequency) terminals 74a and 74b, six of bias supply lines 76a, 76b, 76c, 76d, 76e and 76f each connected between one side of the main signal line 72 and a common bias supply terminal 80, and six of capacitors 78a, 78b, 78c, 78d, 78e and 78f connected between ground and connection points 79a, 79b, 79c, 79d, 79e and 79f, respectively. The bias supply lines 76a, 76b, 76c, 76d, 76e and 76f are distributed parameter lines which have lengths of L1, L2, L3, L4, L5 and L6, and are positioned having distances of B1, B2, B3, B4 and B5. 
     The lengths of L1, L2, L3, L4, L5 and L6, and the distances of B1, B2, B3, B4 and B5 are determined by following expressions, respectively, on condition that the center frequency of a radio signal passing through the main signal line 72 is 15 GHz. In these expressions, &#34;Lc&#34; is a quarter wavelength of the center frequency &#34;fc&#34; of the radio frequency signal passing through the main signal line 72. 
     
         ______________________________________L1 = 1.1 Lc   B1 = 0.84 LcL2 = 1.3 Lc   B2 = 0.44 LcL3 = 0.7 Lc   B3 = 1.40 LcL4 = 0.8 Lc   B4 = 0.52 LcL5 = 1.2 Lc   B5 = 1.00 LcL6 = 1.0 Lc______________________________________ 
    
     FIG. 8 shows a radio frequency characteristic of the first preferred embodiment. In this graph, passing loss of the radio signal is shown by a solid line, and return loss thereof is shown by a dashed line. According to the graph, it is found that the frequency characteristic having a flat portion around 10 to 20 GHz is obtained. 
     According to the fist preferred embodiment, no ripple occurs in the frequency characteristic as shown in FIG. 8, so that a frequency bandwidth is expanded sufficiently. 
     FIG. 9 shows an equivalent circuit of a high-frequency bias supply circuit having bias supply lines of the number of &#34;n&#34;. In this figure, an impedance of a main signal line is &#34;Z o  &#34;, and an impedance of each of bias supply lines is &#34;Z f  &#34;. 
     In the high-frequency bias supply circuit, lengths L1 to Ln of the bias supply lines, and distances B1 to B n-1  between the two adjacent bias supply lines are determined to meet the following dimensions (1) and (2). 
     
         L1 to Ln=Lc±50%                                         (1) 
    
     
         B1 to B.sub.n-1 =Lc±50%                                 (2) 
    
     In FIG. 9, the following expressions are met, where Y n  is equal to 1/Z n  (Y n  =1/Z n ) ##EQU1## 
     where &#34;β&#34; is a phase constant which is expressed by &#34;β=2π/λg&#34;. 
     In the above expressions, the length L1 to L(n-1) and the distances B1 to B(n-1) are designated to meet the below expression in a desired bandwidth. 
     
         Yn≈Y.sub.o (=1/Z.sub.o) 
    
     This means that a reactance component of each bias supply line &#34;Y f  cot βLn&#34; is mutually cancelled in a wide bandwidth. 
     FIG. 10 shows a high-frequency bias supply circuit 90 of a second preferred embodiment according to the invention. The high-frequency bias supply circuit 90 uses six open stubs 92a, 92b, 92c, 92d, 92e and 92f instead of the capacitors 78a, 78b, 78c, 78d, 78e and 78f of the first preferred embodiment. The open stubs 92a, 92b, 92c, 92d, 92e and 92f operate as same as the capacitors 78a, 78b, 78c, 78d, 78e and 78f, so that the same effect as the first preferred embodiment can be obtained by the second preferred embodiment. 
     Although the invention has been described with respect to specific embodiment for complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modification and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.