High efficiency rf power amplifier
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to high efficiency RF (Radio Frequency) power amplifiers, and more particularly to a high efficiency RF power amplifier having an improved circuit configuration which creates a short circuit impedance at the second harmonic of the fundamental operating frequency. 
     2. Description of the Prior Art 
     Recently, portable telephone sets or mobile radio communication devices have been practically used. These devices use a battery as a power source. A transmission power amplifier of the devices consumes approximately 75% of power supplied by the battery. Thus, it is necessary to improve the efficiency of the transmission power amplifier. 
     A conventional high efficiency power amplifier used in the microwave range uses a GaAs field effect transistor, as disclosed in U.S. Pat. No. 4,717,884, the disclosure of which is hereby incorporated by reference. 
     FIG. 1 is a circuit diagram of the high efficiency RF power amplifier disclosed in the above patent. The amplifier shown in FIG. 1 has a GaAs field effect transistor 10, which has a grounded source, a gate receiving an RF input signal, and a drain. The amplifier functions as a class F amplifier capable of efficiently amplifying input power. 
     At the third harmonic of the fundamental operating frequency, a parallel resonance circuit which comprises a lead inductance L1 of a bonding wire, a parasitic capacitance C1 of the GaAs FET 10, and an external inductor 11 is created, so that the drain of the GaAs FET 10 is opened (in other words, a high-impedance circuit is connected to the drain of the GaAs FET 10). In this case, a stub 12 formed of, for example, a micro-strip line, has a length of λ 3  /4 where λ 3  is the wavelength of the third harmonic. At the second harmonic of the fundamental operating frequency, a series resonance circuit composed of the capacitance of the stub 12, the lead inductance L1 and the external inductor 11 is created, so that the drain of the GaAs FET 10 is substantially shortcircuited (in other words, a low-impedance circuit is connected to the drain of the GaAs FET 10). It will be noted that the amplifier shown in FIG. 1 is designed taking into account the lead inductance L1 and the parasitic capacitance C1. 
     A choke coil 13 is coupled to the drain of the GaAs FET 10 via the bonding wire having the lead inductance L1. A drain bias voltage is applied to the drain of the GaAs FET 10 via the choke coil 13 and the lead inductance L1. It is necessary for the choke coil 13 to have a high impedance in the frequency range between the fundamental operating frequency and the third harmonic thereof. If this requirement is not satisfied, the above-mentioned short and open circuits will not be created, so that the second and third harmonics cannot be removed effectively. In order to meet the above requirement, the choke coil 13 must be formed of a lumped-constant circuit element, such as a coil obtained by turning a wire several times. In other words, the choke coil 13 cannot be formed of a distributed-constant circuit, such as a strip line. When the lumped-constant circuit element is used for forming the drain bias circuit, it is difficult to provide a compact, less expensive RF power amplifier. 
     SUMMARY OF THE INVENTION 
     It is a general object of the present invention to provide an improved high efficiency RF power amplifier in which the above-mentioned disadvantages are eliminated. 
     A more specific object of the present invention is to provide a compact, less expensive high efficiency RF power amplifier without a drain bias circuit element formed of a lumped-constant circuit element. 
     The above-mentioned objects of the present invention are achieved by a high efficiency RF power amplifier which includes a field effect transistor having a grounded source, a gate receiving an input signal, and a drain. A first inductor has a first end coupled to the drain via a lead inductance, and a second end via which an amplified output signal is output. A first capacitor has a first end grounded and a second end coupled to the second end of the first inductor. A second inductor has a first end receiving a drain bias voltage and a second end coupled to the second end of the first inductor. The second inductor is formed of a distributed-constant element. The first capacitor and the second inductor form a parallel resonance circuit coupled to the drain at a fundamental operating frequency of the high efficiency RF power amplifier, so that the drain is set to a high-impedance state. The first inductor, the first capacitor and the lead inductance form a serial resonance circuit coupled to the drain at a second harmonic of the fundamental operating frequency, so that the drain is set to a low-impedance state. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a circuit diagram of a conventional high efficiency RF power amplifier; 
     FIG. 2 is a circuit diagram of a high efficiency RF power amplifier according to a first preferred embodiment of the present invention; 
     FIG. 3 is a circuit diagram of a variation of the circuit configuration shown in FIG. 2; 
     FIG. 4 is a circuit diagram of a high efficiency RF power amplifier according to a second preferred embodiment of the present invention; 
     FIG. 5 is a circuit diagram of a variation of the circuit configuration shown in FIG. 4; 
     FIGS. 6A and 6B are circuit diagrams of variations of a gate bias circuit; and 
     FIG. 7 is a cross-sectional view of a strip line. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2 shows a high efficiency RF power amplifier according to the first preferred embodiment of the present invention. An RF input signal applied to a terminal 20 passes through a coupling capacitor 21, and is applied to an input matching circuit 22. Then, the input signal is applied to the gate of a GaAs FET 23. A D.C. gate bias voltage V G  applied to a terminal 26 is input to the gate of the GaAs transistor 23 via a gate bias supply line 27 having a lead inductance, and an RF choke coil 28. The RF choke coil 28 is formed of a strip line, and has a length λ 1  /4 where λ 1  is the wavelength of the fundamental wave having a frequency equal to, for example, about 900 MHz. A node where the gate bias supply line 27 is connected to the choke coil 28 is grounded via a capacitor 29, which allows high frequency components to pass. 
     The source of the GaAs FET 23 is grounded. The GaAs FET 23 has a parasitic capacitance 24 formed between the drain and source of the GaAs FET 23, and a lead inductance 25 of a bonding wire connected to the drain thereof. The drain of the GaAs FET 23 is coupled, via the lead inductance 25, to one end of an inductor 30 formed of a strip line as shown in FIG. 7, which is composed of a dielectric element 51 and a conductor 52. The other end of the inductor 30 is connected to a lowpass type output matching circuit 31, and grounded via a capacitor 32 formed of a lumped-constant circuit element. Further, the other end of the inductor 30 is connected to one end of an inductor 33 which is an element of a drain bias circuit. The output matching circuit 31 has a high efficiency at the fundamental operating frequency and a high impedance at the second harmonic thereof. 
     The inductor 33 is formed of a strip line having a length equal to or smaller than λ 2  /4 where λ 2  is the wavelength of the second harmonic of the fundamental operating frequency. A D.C. drain bias voltage V D  is applied to the other terminal of the inductor 33 via a terminal 34 and a drain bias supply line 35 having a lead inductance. A node where the inductor 33 is connected to the drain bias supply line 35 is grounded via a capacitor 36, which allows high frequency components to pass. 
     An amplified RF signal output by the GaAs FET 23 passes through the lead inductance 25, the inductor 30 and the output matching circuit 31, and is then output to an output terminal 38 via a coupling capacitor 37. The inductors 30 and 33 and the capacitor 32 are designed so that the amplifier can operate in the following way. At the fundamental operating frequency, a parallel resonance circuit comprising the capacitor 32 and the inductor 33 is created, so that a high-impedance circuit (open impedance circuit) is coupled to the drain of the GaAs FET 23. At the second harmonic of the fundamental operating frequency, a series resonance circuit comprising the lead inductance 25, the inductor 30 and the capacitor 32 is created, so that a low-impedance (short circuit impedance) is coupled to the drain of the GaAs FET 23 (in other words, the drain is substantially grounded). 
     It should be noted that the RF power amplifier shown in FIG. 2 can execute class F amplification when the drain of the GaAs FET 23 is shortcircuited at the second harmonic of the fundamental operating frequency even if the drain is not in the open state at the third harmonic thereof. The drain bias voltage V D  is supplied to the drain of the GaAs FET 23 via the inductor 33. The inductor 33 forms the parallel resonance circuit together with the capacitor 32 at the fundamental operating frequency. The inductor 33 can be formed of a distributed-constant circuit, such as a strip line as shown in FIG. 7. With this arrangement, it is possible to provide the compact, less expensive drain bias circuit without any lumped-constant circuit element, such as a choke coil. 
     It is possible to design the inductors 30 and 33 and the capacitor 32 so that at the third harmonic a parallel resonant circuit comprised of the capacitor 24, the inductors 25, 30 and 33, and the capacitor 32 is created and thus a high-impedance circuit is coupled to the drain of the GaAs FET 23. 
     It should be noted that the capacitor 32 is designed so that the parallel resonant circuit is formed together with the inductor 33 at the fundamental operating frequency, and the inductor 33 is designed so that it has a length λ 2  /4 where λ 2  is the wavelength of the second harmonic. Thus, the capacitor 32 and the inductor 33 have a small degree of freedom in design. Meanwhile, the inductor 30 has a large degree of freedom in design. 
     The capacitance of the capacitor 32, which forms the parallel resonant circuit together with the inductor 33 at the fundamental operating frequency, is determined as follows. The impedance of the drain bias circuit, Z l , is described as follows: 
     
         Zl=j·Rc·tanβl                       (1) 
    
     where Rc is the impedance of the drain bias supply line 35 (about 90 ohms, for example), β=2π/λ, and l=l2=λ 2  /4 (l2 is the length of the inductor 33). 
     Formula (1) can be rewritten as follows: ##EQU1## Since λ=λ 1  =2λ 2 , formula (2) can be rewritten as follows: ##EQU2## Thus, the following is obtained: ##EQU3## By squaring both sides of the formula (4), the following is obtained: 
     
         f.sub.1 =1/(2π·90·C)                  (5) 
    
     For f 1  =920 MHz, for example, 
     
         C=1.322 pF≈2 pF                                    (6) 
    
     In order to reduce the area of the inductor 33 on an LSI chip, it is possible for the capacitor 32 to have a capacitance greater than 2 pF without degrading the amplification characteristics, such as 3 or 4 pF. 
     It will be noted that it is not necessary to consider the harmonics on the input side of the GaAs FET 23 since these harmonics arise from the operation of the GaAs FET 23. 
     FIG. 3 is a variation of the first embodiment of the present invention shown in FIG. 2. A strip line 40, which is a distributed-constant element, is substituted for the capacitor 32. The strip line 40 has one end connected to the inductor 33 and the other end in the open state. 
     FIG. 4 shows a high efficiency RF power amplifier according to the second preferred embodiment of the present invention. In FIG. 4, those parts which are the same as those shown in FIG. 2 are given the same reference numerals. The end of the inductor 33 which is connected to the drain bias supply line 35 is grounded via a capacitor 41, and connected to one end of a resistor 42. The other end of the resistor 42 is grounded via the aforementioned capacitor 36. In a similar way, the end of the inductor 28 which is on the side of the gate bias supply line 27 is grounded via a capacitor 43, and connected to one end of a resistor 44. The other end of the resistor 44 is grounded via the capacitor 29. 
     As has been described previously, the parallel resonant circuit composed of the inductor 33 and the capacitor 32 is created at the fundamental operating frequency. However, the inductors 33 and 28 do not have a sufficient impedance at frequencies lower than the fundamental operating frequency. According to the circuit configuration shown in FIG. 4, the impedances of the resistors 42 and 44 are at such frequencies lower than the fundamental operating frequency. Thus, it is possible to improve the operation stability of the amplifier so that the occurrence oscillation can be prevented. Each of the resistors 42 and 44 has a resistance equal to, for example, 50 ohms. In this case, the amplifier operates in a stable area in the Smith chart. 
     FIG. 5 illustrates a variation of the second embodiment of the present invention shown in FIG. 4. The capacitor 32 shown in FIG. 4 is replaced by the aforementioned strip line 40. 
     The gate bias circuit is not limited to the structures shown in FIGS. 2 and 4. As shown in FIG. 6A, the gate bias circuit is made up of resistors 42 and 43 connected in series. The gate bias voltage V G  is applied to one end of the resistor 42, the other end thereof is connected to one end of the resistor 43. The other end of the resistor 43 is grounded. A voltage obtained by dividing the gate bias voltage V G  is applied to the gate of the GaAs FET 23. With the 50-ohm system, each of the resistors 42 and 43 is designed to have a resistance much greater than 50 ohms, for example, 3 or 4 kilo-ohms. The RF input signal is not affected by the resistors 42 and 43 having such high resistance values. 
     It is also possible to employ a gate bias circuit as shown in FIG. 6B, which is made up of resistors 45, 44 and 43 connected in series and a capacitor 46. A node where the resistors 44 and 45 are connected in series is grounded via the capacitor 46. As compared with the gate bias circuit shown in FIG. 6A, the gate bias circuit shown in FIG. 6B operates in a much more stable area in the Smith chart. For the 50-ohm system, the resistor 44 has a resistance of, for example, 100-300 ohms, and the capacitor 46 has a capacitance of, for example, 1000 pF. 
     The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention.