The present invention relates to a technique for amplifying a transmit signal, which is employed in mobile communication equipment, and particularly to a technique effective in improving a distortion characteristic of a power amplifier for amplifying a transmit signal, which is based on a third-generation mobile communication system.
A cellular phone has recently been widely prevalent as one for one mobile communication. There has been a demand for diversity even with respect to its function. A power amplifier for power amplification in an IF (high-frequency) module used in the cellular phone requires high efficiency, high linearity, etc. to transmit more information in a short period of time as in an extension of a communication time, a transfer of image information, etc.
For example, a power amplifier typified by a communication system of GSM (Global System for Mobile Communication) comprises a power FET (Field Effect Transistor) nearest to an antenna, an FET for a driver, impedance matching circuits, gate bias circuits and drain bias circuits, etc.
The impedance matching circuits are provided on the input and output sides. Further, the impedance matching circuit is provided between the power FET and the FET for the driver. The gate bias circuits respectively apply bias voltages to the gates of the power FET and the FET for the driver. The drain bias circuits respectively apply bias voltages to the drains of the power FET and the FET for the driver.
In the power amplifier, the positions and capacitance values of electrostatic capacitive elements constituting the impedance matching circuits, and the inductance values of inductors, etc. are adjusted to optimize impedance matching. Then, a propagated harmonic signal is trapped to set input-output characteristics for the purpose of making satisfaction thereof, whereby an improvement in distortion characteristic is made.