1.  Technical Field
The present invention relates to a radio-frequency (RF) power amplifier of mobile communication equipment, and more particularly, to an RF power amplifier of portable mobile communication equipment which improves the quality of a transmission signal and reduces its electric power consumption. 
2.  Related Art
A mobile communication system has been widely utilized for radio-frequency communication between a number of pieces of mobile.  communication equipment and base stations located at a distance from one another in a service area.  Such communication system is constructed in various manners in accordance with the purpose of service or the like.  For example, in a land-based mobile communication system, the mobile communication equipment is realized as a car telephone, a portable telephone or the like which, when used, is connected with a public telephone network through a base station and a radio link control station. 
A typical portable telephone comprises an equipment body including transmission/receiver sections, a telephone set including telephone transmitter/receiver, and an antenna.  The equipment body with which the telephone set and the antenna are formed into one piece accommodates therein a battery serving as a power source.  The transmitter section includes a sonic-electric converter, modulator, frequency converter, and radio-frequency (RF) power amplifier.  Voice applied through a telephone transmitter to the transmission section is subject to sonic-electric conversion, modulation, frequency conversion and power amplification and is radiated in the form of a radio wave. 
Conventionally, an RF power amplifier of a transmission section is usually formed solely by single-ended circuits which have a proven track record in hybrid ICs, and is mounted on a printed circuit board or printed wiring board of mobile communication equipment, with the RF power amplifier connected with an RF grounding pattern formed in the circuit board.  The RF power amplifier constructed as mentioned above is liable to be affected by the RF grounding pattern, so that the amplification gain and the distortion factor are likely to change, resulting in degraded quality of the transmission signal. 
In the case of land-based mobile communication based on a code division multiple access (CDMA) system, the electric field strengths of radio waves transmitted from all the pieces of mobile communication equipment which commonly utilize the same frequency are preferably equalized at the base station.  To this end, for example, each mobile communication equipment is arranged to adjust the transmission output power in accordance with a transmission power control signal (gain control signal) transmitted from the base station. 
A conventional portable telephone whose transmission section includes an RF power amplifier composed of a single-ended circuit is arranged to make a transmission output adjustment, e.g., by applying the gain control signal to an automatic gain control (AGC) circuit disposed between a modulator and a frequency converter which are located upstream of the RF power amplifier.  In other words, generally, the transmission output adjustment is not performed based on a gain adjustment of the single-ended circuit (RF power amplifier) for the reason that the single-ended circuit is likely to operate abnormally to cause undesired oscillation, etc.,  if the bias current applied thereto is changed so as to change the amplification gain of the single-ended circuit. 
Therefore, the RF power amplifier of a conventional portable telephone is arranged to make an amplifying action at a fixed gain.  This causes a significant reduction in the power efficiency of the RF power amplifier, especially, in a range of low transmission output level.  On the other hand, at the time of transmission, the power consumption of the RF power amplifier makes up a large proportion of the entire power consumption of the mobile communication equipment.  Thus, an improvement of power efficiency of an RF power amplifier is intensively demanded, especially in portable mobile communication equipment which is frequently operated in a low output level range and the power source of which is comprised of a battery.