Radio frequency power amplifier having a tertiary harmonic wave feedback circuit

An RF power amplifier includes an input-side tertiary harmonic wave control circuit and an output-side tertiary harmonic wave control circuit, respectively connected to a gate and a drain of a signal amplification FET. The RF power amplifier also includes a tertiary harmonic wave feedback circuit connected in parallel with the signal amplification FET. The tertiary harmonic wave feedback circuit includes an input-side tertiary harmonic wave bandpass filter, a tertiary harmonic wave amplification FET, a phase shifter and an output-side tertiary harmonic wave bandpass filter. These circuit elements are connected in series. Due to such a configuration, a voltage waveform and a current waveform each having a nearly rectangular shape can be easily generated at an output terminal (a drain) of the signal amplification FET, not only in the region around the efficiency saturation point whore the FET operates in a non-linear mode, but also in other linear-operation regions. Thus, the RF power amplifier can operate with improved efficiency.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a radio frequency (hereinafter, referred 
to as simply "RF") power amplifier for obtaining RF power using a 
semiconductor device, which is mainly used for a transmitting circuit 
section of a communication apparatus and the like which utilizes a 
microwave band. 
2. Description of the Related Art 
Since various types of communication apparatuses, such as a portable 
telephone, have become widely used in recent years, there is an increasing 
demand for an RF power amplifier in a microwave band. The length of "talk 
time", i.e., time period during which a user of such a communication 
apparatus is allowed to talk, considerably depends on the performance of 
an RF power amplifier used for a transmitting circuit section of the 
communication apparatus. Therefore, an RF power amplifier is required to 
operate with a higher efficiency. 
Concerning the circuit design of an RF power amplifier using a 
semiconductor active device such as transistors, the following has been 
reported. When the output circuit is designed in view not only of a 
frequency of a fundamental wave (hereinafter, also referred to as 
"fundamental wave frequency") but also of frequencies of harmonic waves 
(hereinafter, also referred to as "harmonic wave frequencies"), the RF 
power amplifier operates with a higher efficiency as compared with the 
case where the output circuit is designed in view of the fundamental wave 
frequency alone. For example, "A Theoretical Analysis and Experimental 
Confirmation of the Optimally Loaded and Overdriven RF Power Amplifier" 
written by David M. Snider, IEEE Trans. on Electron Devices, Vol. ED-14, 
No.12, pp. 851-857 (Dec. 1967), describes the optimum efficiency 
conditions for realizing 100% efficiency in converting DC power into RF 
power. The conditions include making impedances zero for harmonic wave 
components having a frequency obtained by multiplying the fundamental wave 
frequency by an even number and making impedances infinite for harmonic 
wave components having a frequency obtained by multiplying the fundamental 
wave frequency by an odd number, in addition to obtaining the impedance 
matching at the fundamental wave frequency at an output end of a 
semiconductor active element (a transistor) included in an RF power 
amplifier. Furthermore, Japanese Laid-Open Patent Publication No. 
58-159002 discloses a specific circuit configuration for satisfying the 
above optimum efficiency conditions. 
FIG. 1 is a circuit diagram of a basic RF power amplifier 100. 
The RF power amplifier 100 includes a signal amplification field-effect 
transistor (hereinafter referred to as "FET") 130, an input-side impedance 
matching circuit 140 connected to the input side of the signal 
amplification FET 130, and an output-side impedance matching circuit 150 
connected to the output side of the signal amplification FET 130. These 
circuit elements are provided between an input RF terminal 121 and an 
output RF terminal 124. The input-side impedance matching circuit 140 
includes an input-side impedance matching line 141 and an input-side 
impedance matching capacitor 142. The input-side impedance matching 
circuit 140 matches the impedance for the fundamental wave component of an 
external circuit connected to the input RF terminal 121 to the internal 
impedance of the signal amplification FET 130. Similarly, the output-side 
impedance matching circuit 150 includes an output-side impedance matching 
line 151 and an output-side impedance matching capacitor 152. The 
output-side impedance matching circuit 150 matches the impedance for the 
fundamental wave component of the external circuit connected to the output 
RF terminal 124 to the internal impedance of the signal amplification FET 
130. 
Specifically, a gate 131 of the signal amplification FET 130 is connected 
to the input RF terminal 121 via a DC blocking capacitor 101 and the 
input-side impedance matching line 141 of the input-side impedance 
matching circuit 140. The input-side impedance matching line 141 is 
connected to a gate bias voltage supplying terminal 122 via a resistance 
102 and is grounded via the input-side impedance matching capacitor 142. 
Similarly, a drain 132 of the signal amplification FET 130 is connected to 
the output RF terminal 124 via a DC blocking capacitor 104 and the 
output-side impedance matching line 151 of the output-side impedance 
matching circuit 150. The input-side impedance matching line 151 is 
connected to a drain bias voltage supplying terminal 123 via a choke coil 
103 and is grounded via the output-side impedance matching capacitor 152. 
A source 133 of the signal amplification FET 130 is grounded. 
An RF signal input to the RF power amplifier 100 having the above-described 
configuration through the input RF terminal 121 is transmitted to the 
signal amplification FET 130 via the input-side impedance matching circuit 
140 and amplified by the FET 130. The amplified signal is then output 
through the output RF terminal 124 via the output-side impedance matching 
circuit 150. 
FIG. 2 is a circuit diagram showing an RF power semiconductor amplifier 200 
disclosed in Japanese Laid-Open Patent Publication No. 58-159002. 
The RF power amplifier 200 of FIG. 2 includes a secondary harmonic wave 
control circuit 260 connected between the ground level and point A of a 
line connected to a drain 232 of a signal amplification FET 230. Other 
portions of the configuration of the RF power amplifier 200 are the same 
as that of the RF power amplifier 100 shown in FIG. 1 and therefore will 
not further be described. In FIG. 2, each circuit element is numbered with 
a reference numeral obtained by adding 100 to the reference numeral of the 
corresponding circuit element in FIG. 1. For example, a DC blocking 
capacitor, whose reference numeral in FIG. 1 is "101", is numbered with a 
reference numeral "201" in FIG. 2. 
The secondary harmonic wave control circuit 260 includes a secondary 
harmonic wave control line 261 and a secondary harmonic wave control 
capacitor 262. The drain 232 of the signal amplification FET 230 is 
grounded via the line 261 and the capacitor 262. The line 261 and the 
capacitor 262 of the secondary harmonic wave control circuit 260 are so 
adjusted that the impedance becomes zero for the secondary harmonic wave 
component when the secondary harmonic wave control circuit 260 is viewed 
from point A. 
The operation of the RF power amplifier 200 is basically the same as that 
of the RF power amplifier 100. However, in the RF power amplifier 200, 
generation of secondary harmonic wave component at point A is depressed 
since the impedance is zero for the secondary harmonic wave component when 
the secondary harmonic wave control circuit 260 is viewed from point A. 
On the other hand, by using a rectangular wave as an input signal, the RF 
power amplifier can operate with a high efficiency. For example, Japanese 
Laid-Open Patent Publication No. 7-231231 discloses an RF power amplifier 
having a circuit configuration where an input signal having a nearly 
rectangular waveform can be supplied. FIG. 3 schematically shows a circuit 
configuration of an RF power amplifier 300 disclosed in this publication. 
The RF power amplifier 300 includes a rectangular wave generation circuit 
310 for generating a rectangular wave using an input signal 301 supplied 
through an RF input terminal 311 and a power amplification circuit 350 for 
amplifying the thus generated rectangular wave and supplying the amplified 
signal to an RF output terminal 312. The circuits 310 and 350 are 
connected to each other in series. 
The rectangular wave generation circuit 310 includes a tertiary harmonic 
wave adjustment circuit 320 and a fundamental wave adjustment circuit 330 
which are connected to each other in parallel. The circuits 320 and 330 
are each connected to a signal synthesis circuit 340. The tertiary 
harmonic wave adjustment circuit 320 includes a tertiary harmonic wave 
bandpass filter 321, a tertiary harmonic wave amplification element 322, a 
phase shifter 323 and a capacitor 324. On the other hand, the fundamental 
wave adjustment circuit 330 includes a fundamental wave bandpass filter 
331 and a capacitor 332. In the signal synthesis circuit 340, a gate bias 
resistance 341, a drain bias choke coil 345 and a dual gate FET 342 having 
two gates 343 and 344 are connected between a gate bias terminal 314 and a 
drain bias terminal 313. 
The power amplification circuit 350 includes, as well as a capacitor 351, a 
gate bias resistance 352, a power amplification element 353 and a drain 
bias choke coil 354 connected between a drain bias terminal 315 and a gate 
bias terminal 316. 
The signal 301 input to the RF power amplifier 300 through the RF input 
terminal 311 is first transmitted to the rectangular wave generation 
circuit 310, where a fundamental wave component 303 and a tertiary 
harmonic wave component 302 are taken out of the input signal 301. Only 
the tertiary harmonic wave component 302 of the input signal 301 is 
selectively transmitted through the tertiary harmonic wave adjustment 
circuit 320 due to the tertiary harmonic wave bandpass filter 321 included 
therein. The tertiary harmonic wave component 302 is then amplified in the 
tertiary harmonic wave amplification element 322, and the phase thereof is 
adjusted by the phase shifter 323. Then, the tertiary harmonic wave 
component 302 is input to the dual gate FET 342 through the gate 343. 
On the other hand, only the fundamental wave component 303 of the input 
signal 301 is selectively transmitted through the fundamental wave 
adjustment circuit 330 due to the fundamental wave bandpass filter 331 
included therein. The fundamental wave component 303 is input to the dual 
gate FET 342 through the other gate 344. 
The tertiary harmonic wave component 302 and the fundamental wave component 
303 thus input to the dual gate FET 342 are synthesized with each other 
into a synthesized waveform signal 304 having a nearly rectangular 
waveform and is output to the power amplification circuit 350. The 
synthesized waveform signal 304 is input to and amplified in the power 
amplification element 353 of the power amplification circuit 350 and is 
output through the RF output terminal 312. 
The above-explained conventional RF power amplifiers have the following 
disadvantages. 
In the RF power amplifier 200 disclosed in Japanese Laid-open Patent 
Publication No. 58-159002, the operational efficiency is improved 
especially in the region around the efficiency saturation point where 
harmonic wave components of a high amplitude are likely to be generated 
because of the non-linear operation. However, in other regions, the 
operational efficiency is not improved to a satisfactory level, although a 
good linearity of the operations is obtainable in those regions. 
In accordance with the RF power amplifier 300 disclosed in Japanese 
Laid-Open Patent Publication No. 7-231231, the tertiary harmonic wave 
component 302 in taken out of the input signal 301 provided to the RF 
power amplifier 300 and is then amplified in the tertiary harmonic wave 
amplification element 322. Thereafter, the amplified tertiary harmonic 
wave component 302 is added to the fundamental wave component 303 
contained in the input signal 301. Thus, the rectangular wave (synthesized 
waveform signal) 304 is obtained, which is to be supplied to the power 
amplification element 353 of the power amplification circuit 350. 
However, in general, an input signal provided to an RF power amplifier used 
in an ordinary communication apparatus practically contains only a vary 
small amount of a tertiary harmonic wave component. Therefore, in order to 
obtain a sufficient amplitude level for the amplified tertiary harmonic 
wave component which is required to realize a desired operation by 
amplifying the tertiary harmonic wave component of such a small amount, a 
number of the tertiary harmonic wave amplification elements 322 must be 
connected in multiple stages in the tertiary harmonic wave adjustment 
circuit 320 of the RF power amplifier 300. This not only leads to an 
undesirable increase in the area occupied by the entire circuit, but also 
deteriorates the overall efficiency of the RF power amplifier 300 due to 
the increased amount of power consumed by the tertiary harmonic wave 
amplification elements 322. 
Moreover, In accordance with the RF power amplifier 300, the fundamental 
wave component 303 and the tertiary harmonic wave component 302 must he 
taken out of the input signal 301 and synthesized back together to obtain 
the rectangular wave 304 to be supplied to the power amplification element 
353. The circuit elements required for these processes also increase the 
area occupied by the entire circuit. 
SUMMARY OF THE INVENTION 
A radio frequency amplifier of the present invention includes: a power 
amplification circuit including a power amplification element; and a 
tertiary harmonic wave feedback circuit connected in parallel with the 
power amplification circuit. The tertiary harmonic wave feedback circuit 
feeds back a portion of a tertiary harmonic wave component contained in an 
amplified signal, which is output from an output aide of the power 
amplification circuit, to an input side of the power amplification 
circuit. 
According to another aspect of the present invention, a radio frequency 
amplifier includes: a power amplification circuit including a plurality of 
power amplification elements connected in series in multiple stages; and a 
tertiary harmonic wave feedback circuit connecting an output side of the 
power amplification circuit to an input side of one of the power 
amplification elements in a selected stage. The tertiary harmonic wave 
feedback circuit feeds back a portion of a tertiary harmonic wave 
component contained in an amplified signal, which is output from the 
output side of the power amplification circuit, to the input side of the 
power amplification element in the selected stage. 
Preferably, the tertiary harmonic wave feedback circuit comprises a 
tertiary harmonic wave bandpass filter and a phase shifter. 
Preferably, the tertiary harmonic wave feedback circuit comprises at least 
one tertiary harmonic wave amplification active element. 
Preferably, the radio frequency amplifier further includes a circuit, 
connected to at least one of the input side and the output side of the 
power amplification circuit, for controlling an impedance for a secondary 
harmonic wave component. 
Preferably, the radio frequency amplifier further includes a circuit, 
connected to at least one of the input side and the output side of the 
power amplification circuit, for controlling an impedance for a tertiary 
harmonic wave component. 
Thus, the invention described herein makes possible the advantage of 
providing an RF power amplifier which easily exhibits an improved 
efficiency even in regions other than the region around the efficiency 
saturation point without undesirably increasing the area occupied by the 
entire circuit or the amount of power consumed. 
This and other advantages of the present invention will become apparent to 
those skilled in the art upon reading and understanding the following 
detailed description with reference to the accompanying figures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereinafter, the present invention will be described by way of illustrative 
examples with reference to the accompanying figures. 
FIG. 4 is a circuit diagram of an RF power amplifier 400 according to an 
example of the present invention. 
The RF power amplifier 400 includes a signal amplification FET 430, an 
input-side impedance matching circuit 440 connected to the input side of 
the signal amplification FET 430 and an output-side impedance matching 
circuit 450 connected to the output side of the signal amplification FET 
430. These circuit elements are provided between an input RF terminal 421 
and an output RF terminal 424. Moreover, a tertiary harmonic wave feedback 
circuit 480 is connected between the input RF terminal 421 and the output 
RF terminal 424 in parallel with the signal amplification FET 430. 
The input-side impedance matching circuit 440 includes an input-side 
impedance matching line 441 and an input-side impedance matching capacitor 
442. The input-side impedance matching circuit 440 matches the impedance 
for the fundamental wave component of an external circuit connected to the 
input RF terminal 421 to the internal impedance of the signal 
amplification FET 430. Similarly, the output-side impedance matching 
circuit 450 includes an output-side impedance matching line 451 and an 
output-side impedance matching capacitor 452. The output-side impedance 
matching circuit 450 matches the impedance for the fundamental wave 
component of the external circuit connected to the output RF terminal 424 
to the internal impedance of the signal amplification FET 430. 
Specifically, a gate 431 of the signal amplification FET 430 is connected 
to the input RF terminal 421 via a DC blocking capacitor 401 and the 
input-side impedance matching line 441 of the input-side impedance 
matching circuit 440. The input-side impedance matching line 441 is 
connected to a gate bias voltage supplying terminal 422 via a resistance 
402 and is grounded via the input-side impedance matching capacitor 442. 
Similarly, a drain 432 of the signal amplification FET 430 is connected to 
the output RF terminal 424 via another DC blocking capacitor 404 and the 
output-side impedance matching line 451 of the output-side impedance 
matching circuit 450. The output-side impedance matching line 451 is 
connected to a drain bias voltage supplying terminal 423 via a choke coil 
403 and is grounded via the output-side impedance matching capacitor 452. 
A source 433 of the signal amplification FET 430 is grounded. 
An input-side tertiary harmonic wave control circuit 460 is connected to 
point C of a line connected to the gate 431 of the signal amplification 
FET 430. On the other hand, an output-side tertiary harmonic wave control 
circuit 470 is connected to point D of a line connected to the drain 432 
of the signal amplification FET 430. 
The input-side tertiary harmonic wave control circuit 460 includes an 
input-side tertiary harmonic wave control line 461 and an input-side 
tertiary harmonic wave control capacitor 462. The gate 431 of the signal 
amplification FET 430 is grounded at point C via the line 461 and the 
capacitor 462. The line 461 and the capacitor 462 of the input-side 
tertiary harmonic wave control circuit 460 are so adjusted that the 
impedance becomes very high for the tertiary harmonic wave component when 
the input-side tertiary harmonic wave control circuit 460 is viewed from 
point C. 
Similarly, the output-side tertiary harmonic wave control circuit 470 
includes an output-side tertiary harmonic wave control line 471 and an 
output-side tertiary harmonic wave control capacitor 472. The drain 432 of 
the signal amplification FET 430 is grounded at point D via the line 471 
and the capacitor 472. The line 471 and the capacitor 472 of the 
output-side tertiary harmonic wave control circuit 470 are so adjusted 
that the impedance becomes very high for the tertiary harmonic wave 
component when the output-side tertiary harmonic wave control circuit 470 
is viewed from point D. 
The tertiary harmonic wave feedback circuit 480 is connected in parallel 
with the signal amplification FET 430 between point E near the output RF 
terminal 424 and point B near the input RF terminal 421. The tertiary 
harmonic wave feedback circuit 480 included an input-side tertiary 
harmonic wave bandpass filter 487, a DC blocking capacitor 486, an 
input-side tertiary harmonic wave impedance matching circuit 445, a 
tertiary harmonic wave amplification FET 435, an output-side tertiary 
harmonic wave impedance matching circuit 455, another DC blocking 
capacitor 483, a phase shifter 482 and an output-side tertiary harmonic 
wave bandpass filter 481. The circuit elements included in the tertiary 
harmonic wave feedback circuit 480 are connected in series. 
The input-side tertiary harmonic wave impedance matching circuit 445 
includes an input-side tertiary harmonic wave impedance matching line 446 
and an input-side tertiary harmonic wave impedance matching capacitor 447. 
The input-side tertiary harmonic wave impedance matching circuit 445 
matches the impedance for the tertiary harmonic wave component of an 
external circuit connected to the output RF terminal 424 to the internal 
impedance of the tertiary harmonic wave amplification FET 435. 
Specifically, the input-side tertiary harmonic wave impedance matching 
line 446 is connected to an gate bias voltage supplying terminal 426 via a 
resistance 485 and is grounded via the input-side tertiary harmonic wave 
impedance matching capacitor 447. 
Similarly, the output-side tertiary harmonic wave impedance matching 
circuit 455 includes an output-side tertiary harmonic wave impedance 
matching line 456 and an output-side tertiary harmonic wave impedance 
matching capacitor 457. The output-side tertiary harmonic wave impedance 
matching circuit 455 matches the impedance for the tertiary harmonic wave 
component of an external circuit connected to the input RF terminal 421 to 
the internal impedance of the tertiary harmonic wave amplification FET 
435. Specifically, the output-side tertiary harmonic wave impedance 
matching line 456 is connected to a drain bias voltage supplying terminal 
425 via a choke coil 484 and is grounded via the output-side tertiary 
harmonic wave impedance matching capacitor 457. 
A gate 436 of the tertiary harmonic wave amplification FET 435 is connected 
to point E being the input section of the tertiary harmonic wave feedback 
circuit 480 via the input-side tertiary harmonic wave impedance matching 
line 446, the DC blocking capacitor 486 and the input-side tertiary 
harmonic wave bandpass filter 487. On the other hand, a drain 438 of the 
tertiary harmonic wave amplification FET 435 is connected to point B being 
the output section of the tertiary harmonic wave feedback circuit 480 via 
the output-side tertiary harmonic wave impedance matching line 456, the DC 
blocking capacitor 483, the phase shifter 482 and the output-side tertiary 
harmonic wave bandpass filter 481. 
A source 437 of the tertiary harmonic wave amplification FET 435 is 
grounded. 
Hereinafter, the operation of the RF power amplifier 400 having the 
above-described configuration will be described. 
An RF signal input to the RF power amplifier 400 through the input RF 
terminal 421 is transmitted to the signal amplification FET 430 via the 
input-side impedance matching circuit 440 and amplified therein. A 
harmonic wave component is generated in this amplification process while 
the signal is amplified. Herein, the impedance is very high for the 
tertiary harmonic wave component when the input-side tertiary harmonic 
wave control circuit 460 is viewed from point C. The impedance is also 
very high for the tertiary harmonic wave component when the output-side 
tertiary harmonic wave control circuit 470 is viewed from point D. 
Therefore, a tertiary harmonic wave component having a large amplitude is 
generated at each of points C and D. 
A portion of the tertiary harmonic wave component thus generated by the 
signal amplification FET 430 is input to the tertiary harmonic wave 
feedback circuit 480 via point E. The tertiary harmonic wave component is 
then input to the tertiary harmonic wave amplification FET 435 via the 
input-side tertiary harmonic wave band pass filter 487, the DC blocking 
capacitor 486 and the input-side tertiary harmonic wave impedance matching 
circuit 445. Herein, the input-side tertiary harmonic wave bandpass filter 
487 prevents the fundamental wave component, which would lower the gain of 
the RF power amplifier 400, and unnecessary tertiary harmonic wave 
components from entering the tertiary harmonic wave feedback circuit 480. 
The amplified tertiary harmonic wave component is output from the tertiary 
harmonic wave amplification FET 435 and fed back to point B being the 
input section of the signal amplification FET 430 via the output-side 
tertiary harmonic wave impedance matching circuit 455, the DC blocking 
capacitor 483, the phase shifter 482 and the output-side tertiary harmonic 
wave bandpass filter 481. The output-side tertiary harmonic wave bandpass 
filter 481 prevents the fundamental wave component from entering the 
tertiary harmonic wave feedback circuit 480 via point B to lower the gain 
of the RF power amplifier 400. 
The tertiary harmonic wave component fed back to point B is output from the 
signal amplification FET 430 through the drain 432. FIG. 5 schematically 
shows voltage waveforms of the respective components at the drain 432. 
Specifically, it shows each of the waveforms of the fundamental wave 
component (a), the tertiary harmonic wave component (b), and a synthesized 
wave (c) obtained by synthesizing the two components (a) and (b). In FIG. 
5, the horizontal axis represents the lapse of time, and the vertical axis 
represents the voltage intensity. 
The amount by which the phase of the tertiary harmonic wave component is 
rotated by the phase shifter 482 can be adjusted so that, as shown in FIG. 
5, the phase of the tertiary harmonic wave component (b) is substantially 
inverted, i.e., rotated by about 180.degree., with respect to the phase of 
the fundamental wave component (a) at .theta.-0. Moreover, the rate at 
which the tertiary harmonic wave component is amplified by the tertiary 
harmonic wave amplification FET 435 can be adjusted so that the amplitude 
of the tertiary harmonic wave component (b) is set to an appropriate level 
with respect to the amplitude of the fundamental wave component (a), 
whereby the waveform of the synthesized wave (c) becomes more ideally 
rectangular. In the example of the FIG. 5, the amplitude of the tertiary 
harmonic wave component (b) is adjusted to be about 1/6 to about 1/7 of 
the amplitude of the fundamental wave component (a). 
FIG. 6 shows an output current waveform 602 and an output voltage waveform 
603 at the drain 432 of the signal amplification FET 430 of the RF power 
amplifier 400 (i.e., waveforms of a current I.sub.DS and voltage V.sub.DC 
between the drain 432 and the source 433), in the case where an input 
power is of a relatively small signal. In the description below, for 
simplicity, it is assumed that an input voltage waveform 601 to the signal 
amplification FET 430 is rectangular. It is also assumed that the signal 
amplification FET 430 is subject to an AB class operation. 
In a graph showing static characteristics 600 of the signal amplification 
FET 430 in FIG. 6, the vertical axis represents the drain-source current 
(I.sub.DS) , and the horizontal axis represents the drain-source voltage 
(V.sub.DS). In a graph of the input voltage waveform 601 provided between 
the gate and the source, the vertical axis represents a gate-source input 
voltage (V.sub.DS), and the horizontal axis represents the lapse of time. 
When the input voltage 601 having a rectangular waveform is input between 
the gate 431 and the source 433, the output voltage waveform 603 and the 
output current waveform 602, which are observed between the drain 432 and 
the source 433, both become rectangular. In this case, the phase of the 
output voltage waveform 603 and the phase of the output current waveform 
602 are shifted from each other by about 180.degree.. 
In practice, the amplification rate of the signal amplification FET 430 for 
the fundamental wave component differs from that for the tertiary harmonic 
wave component. Therefore, even when the input voltage 601 having a 
rectangular waveform is input to the signal amplification FET 430 through 
the gate 431, the output current waveform 602 and the output voltage 
waveform 603 obtained at the drain 432 do not have a completely 
rectangular shape. In order to obtain the output current waveform 602 and 
the output voltage waveform 603 having more ideally rectangular shapes at 
the drain 432, it is desirable to optimize the amplification rate of the 
tertiary harmonic wave amplification FET 435 and the amount of phase 
rotation of the phase shifter 482 for the tertiary harmonic wave 
component, with the current waveform 602 and the voltage waveform 603 at 
the drain 432 of the signal amplification FET 430 being observed. 
FIGS. 7A and 7B show, in a manner different from that of FIG. 6, the output 
current waveform I.sub.DS and the output voltage waveform V.sub.DS at the 
drain 432 of the signal amplification FET 430 in the case where a small 
power input signal is provided. Specifically, FIG. 7A shows the output 
voltage waveform (V.sub.DS) 603 and the output current waveform (I.sub.DS) 
602 obtained by the RF power amplifier 400 of the present invention, in 
which the waveforms 603 and 602 are arranged along the same time axis. For 
comparison with the present invention, FIG. 7B shows an output current 
waveform I.sub.DS and an output voltage waveform V.sub.DS at the drain 232 
of the signal amplification FET 230 of the conventional RF power amplifier 
200 described above with reference to FIG. 2. 
As shown in FIG. 7A, in the RF power amplifier 400 of the present 
invention, there is no flow of the output current I.sub.DS during a period 
Z. Accordingly, the amount of power consumed during the period Z is zero. 
To the contrary, as shown in FIG. 7B, in the case of the conventional RF 
power amplifier 200, there is a flow of the output current I.sub.DS during 
a period Z' corresponding to the period Z in FIG. 7A. Accordingly, the 
amount of power consumed during the period Z' is not zero. 
Thus, in the RF power amplifier 400 of the present invention, even when it 
is not possible to obtain a voltage and a current having a completely 
rectangular shape, it is possible to obtain an output voltage and an 
output current each having a nearly rectangular waveform even in the case 
where a small signal power is input. Therefore, it is possible to minimize 
the period during which both of a non-zero voltage and a non-zero current 
are output. As a result, only a reduced amount of DC power supplied from 
the drain is wasted, i.e., converted into heat and the like. Thus, the 
operational efficiency of the amplifier is improved. 
FIG. 8 shows input power dependence of the operational efficiency of each 
of the RF power amplifier 400 of the present invention, the conventional 
RF power amplifier 100 of FIG. 1 and the conventional RF power amplifier 
200 of FIG. 2. The horizontal axis represents the input power, and the 
vertical axis represents the operational efficiency. 
In the region around the efficiency saturation point, a relatively large 
power input (large signal input) Y is provided, and therefore a signal 
amplification FET operates in a non-linear mode. It is observed in FIG. 8 
that, in this region, the conventional RF power amplifier 200 having the 
characteristic represented by line (b) operates with a higher efficiency 
as compared with the conventional RF power amplifier 100 having the 
characteristic represented by line (a). The reason for this is as follows. 
When the relatively large power input Y is supplied to the RF power 
amplifier 200, and the signal amplification FET 230 thereof thereby 
operates in the non-linear mode in the region around the efficiency 
saturation point, only generation of the secondary harmonic wave component 
is depressed due to the secondary harmonic wave control circuit 260. 
In this region around the efficiency saturation point, the RF power 
amplifier 400 of the present invention having the characteristic 
represented by line (c) operates with an efficiency of the same level as 
that of the conventional RF power amplifier 200. 
On the other hand, in regions where a relatively small power input (small 
signal input) X is provided, the RF power amplifier 400 of the present 
invention operates with a higher efficiency as compared with the 
conventional RF power amplifier 100 and 200, for reasons previously 
described. 
As shown in FIG. 8, the RF power amplifier 400 of the present invention 
operates with a high efficiency in the region around the efficiency 
saturation point where the large signal input Y is provided. Moreover, 
even when operating in a linear region where the small signal input X is 
provided, the RF power amplifier 400 of the present invention is capable 
of easily generating a voltage and a current each having a nearly 
rectangular waveform at an output terminal of a power amplification 
element (e.g., a drain of an FET). Thus, the operational efficiency can be 
improved. 
In the circuit configuration shown in FIG. 4, the tertiary harmonic wave 
feedback circuit 480 is connected between points B and E. Points B and E 
are respectively located at positions beyond the input-side tertiary 
harmonic wave control circuit 460 and beyond the output-side tertiary 
harmonic wave control circuit 470, as viewed from the signal amplification 
FET 430. Alternatively, a similar effect can be realized with points B and 
E being respectively located between the signal amplification FET 430 and 
the input-side tertiary harmonic wave control circuit 460; and between the 
signal amplification FET 430 and the output-side tertiary harmonic wave 
control circuit 470. 
Furthermore, the above-described example of the present invention can be 
partially modified as shown in FIG. 9 and FIG. 10. 
FIG. 9 shows a configuration obtained by modifying the section between 
points C and D, which includes the signal amplification FET 430, in the 
circuit configuration of the RF power amplifier 400 described above with 
reference to FIG. 4. 
Specifically, another signal amplification FET 434 is connected to the 
signal amplification FET 430 in series in order to correspond to the case 
where an RF power amplifier is required to provide a higher gain. 
In such a configuration, a tertiary harmonic wave component is fed back to 
the input side of the signal amplification FET 434 in the former stage but 
not to point H which corresponds to the input terminal of the signal 
amplification FET 430 in the latter stage. As a result, a rectangular wave 
is input to each of the signal amplification FETs 430 and 434. Thus, both 
of the signal amplification FETs 430 and 434 operate with high efficiency. 
Moreover, even in the case where the tertiary harmonic wave component fed 
back by the tertiary harmonic wave feedback circuit 480 shown in FIG. 4 
does not have a sufficiently large amplitude, the amplitude of the 
tertiary harmonic wave component can be amplified by the signal 
amplification FET 434 in the former stage to an amplitude level required 
to shape the signal input to the signal amplification FET 430 in the 
letter stage into a sufficiently rectangular waveform. 
It is also applicable to connect further signal amplification FETs in 
series in multiple stages. 
FIG. 10 shows another configuration obtained by modifying the section 
between points C and D, which includes the signal amplification FET 430, 
in the circuit configuration of the RF power amplifier 400 described above 
with reference to FIG. 4. 
Specifically, in the configuration shown in FIG. 10, an input-side 
secondary harmonic wave control circuit 490 is connected to point F of a 
line connected between the gate 431 of the signal amplification FET 430 
and point C. Similarly, an output-side secondary harmonic wave control 
circuit 495 is connected to point G of a line connected between the drain 
432 of the signal amplification FET 430 and point D. 
The input-side secondary harmonic wave control circuit 490 includes a 
secondary harmonic wave control line 491 and a secondary harmonic wave 
control capacitor 492. The gate 431 to the signal amplification FET 430 is 
grounded via the line 491 and the capacitor 492. The line 491 and the 
capacitor 492 of the input-side secondary harmonic wave control circuit 
490 are so adjusted that the impedance becomes very small or zero for the 
secondary harmonic wave component when the input-side secondary harmonic 
wave control circuit 490 is viewed from point F. 
Similarly, the output-side secondary harmonic wave control circuit 495 
includes a secondary harmonic wave control line 496 and a secondary 
harmonic wave control capacitor 497. The drain 432 of the signal 
amplification FET 430 is grounded via the line 496 and the capacitor 497. 
The line 496 and the capacitor 497 of the output-side secondary harmonic 
wave control circuit 495 are so adjusted that the impedance becomes very 
small or zero for the secondary harmonic wave component when the 
output-side secondary harmonic wave control circuit 495 is viewed from 
point G. 
By providing the secondary harmonic wave control circuits 490 and 495 as 
described above, generation of the secondary harmonic wave component at 
points F and G is depressed, whereby it is possible to obtain a more 
ideally rectangular voltage waveform. 
In the description above, the secondary harmonic wave control circuits 490 
and 495 are respectively connected to points F and G corresponding to the 
input side and the output side, respectively, of the signal amplification 
FET 430. However, the generation of the secondary harmonic wave component 
can be depressed also in a configuration where a secondary harmonic wave 
control circuit is connected to either one of points F or G. 
In the above description, the phrase "signal amplification" is used, such 
as "signal amplification FET". This phrase can be referred to as "power 
amplification" because a power of a signal is amplified in an actual 
situation. 
As described above, in an RF power amplifier of the present invention, an 
amplified power is generated by a power amplification circuit including 
one power amplification element in a single stage or a plurality of power 
amplification elements connected in series in multiple stages. A portion 
of the tertiary harmonic wave component contained in the amplified power 
is fed back to the input section of the power amplification circuit by a 
tertiary harmonic wave feedback circuit and is synthesized with a 
fundamental wave component contained in an input signal. 
An RF input signal supplied to the power amplification circuit is amplified 
by the power amplification element such as an FET. The amplified power 
output from the power amplification circuit contains a tertiary harmonic 
wave component generated in the amplification process. The tertiary 
harmonic wave component is taken out of the output signal from the power 
amplification circuit and is fed back to the input side of the power 
amplification circuit by the tertiary harmonic wave feedback circuit. 
The tertiary harmonic wave component fed back to the input section of the 
power amplification circuit is synthesized with the fundamental wave 
component contained in the input signal. Due to this synthesis process, 
the voltage waveform of the input signal is shaped into a nearly 
rectangular waveform. According to the configuration of the present 
invention, the tertiary harmonic wave component generated by the power 
amplification element of the power amplification circuit is utilized for 
the above-described waveform shaping process for the waveform of the 
signal input to the power amplification circuit. Therefore, it is possible 
to very easily obtain a tertiary harmonic wave component of a large 
amplitude. 
The power amplification circuit may include a plurality of the power 
amplification elements (e.g., the FETs) connected in series in multiple 
stages. In this case, the tertiary harmonic wave component can be fed back 
by the tertiary harmonic wave feedback circuit beyond a plurality of the 
power amplification elements over the multiple stages. This makes it 
possible to obtain a tertiary harmonic wave component of an even larger 
amplitude. Moreover, it becomes possible to improve the efficiency of the 
plurality of power amplification elements being in parallel with the 
tertiary harmonic wave feedback circuit. 
Upon shaping the input signal waveform into a nearly rectangular waveform 
by synthesizing the fed back tertiary harmonic wave component with the 
fundamental wave component contained in the input signal, in order to 
obtain a more ideally rectangular waveform, it is preferable to adjust the 
phase and amplitude of the tertiary harmonic wave component to be fed back 
in accordance with the phase and amplitude of the fundamental wave 
component. By such an adjustment, it is also possible to improve the 
efficiency of the waveform shaping process. This can be realized by so 
configuring the tertiary harmonic wave feedback circuit that it includes a 
tertiary harmonic wave bandpass filter and a phase shifter. The tertiary 
harmonic wave bandpass filter selectively feeds back only the tertiary 
harmonic wave component and prevents unnecessary harmonic wave components 
and the fundamental wave component, which would lower the gain of the RF 
power amplifier, from entering the tertiary harmonic wave feedback 
circuit. The phase shifter, on the other hand, adjusts the phase of the 
tertiary harmonic wave component. 
Moreover, in the case where at least one tertiary harmonic wave 
amplification element (e.g., an FET for amplifying tertiary harmonic wave 
component) is provided in the tertiary harmonic wave feedback circuit, it 
is possible to adjust the amplitude of the tertiary harmonic wave 
component to be fed back. 
When the power amplification device operates in a non-linear mode, the 
amount of the tertiary harmonic wave component to be generated is small. 
In this case, a circuit for controlling an impedance for the tertiary 
harmonic wave component may be connected to at least one of the input side 
and the output side of the power amplification circuit. Thus, the 
impedance becomes high for the tertiary harmonic wave component, and the 
amplitude of the tertiary harmonic wave component can thereby be made 
large. 
Moreover, a circuit for controlling an impedance for the secondary harmonic 
waves may be connected to at least one of the input side and the output 
side of the power amplification circuit. In this case, the impedance can 
be made low for the secondary harmonic wave components. Thus, it becomes 
possible to shape the waveform of the signal input to the power 
amplification circuit into a more ideally rectangular waveform. 
Various other modifications will be apparent to and can be readily made by 
those skilled in the art without departing from the scope and spirit of 
this invention. Accordingly, it is not intended that the scope of the 
claims appended hereto be limited to the description as set forth herein, 
but rather that the claims be broadly construed.