A semiconductor device microwave integrated circuit includes at least a transistor stage which stage includes a microwave matching circuit and a d.c. bias circuit interconnected at a link node (A). The transistor stage further includes a stabilizing circuit which includes an open stub line connected to the link node. The open stub line connected to the link node is a .lambda./4 line which at the operating frequency imposes a short circuit on the link node. A matching circuit made up of a low-value resistor connected to ground in the microwave mode through a d.c. isolating capacitor is connected to the link node. A .lambda./4 line of the radial type (.lambda./4 radial stub) provides broadband operation.

In a radio frequency amplifier, a resistor and an inductor are connected to a gate terminal of transistor such as an FET. Another resistor for preventing oscillation is connected to the inductor. A capacitor and a third resistor connected to each other in parallel are connected to the first resistor while the other ends thereof are grounded. A resistance value of the resistor for preventing oscillation is in the range of about 30 .OMEGA. to about 70 .OMEGA.. An inductance value of the inductor is such that an impedance value for a low frequency becomes negligibly small.

There is provided an electrical power amplifier device in which an output voltage amplitude is increased without generating any problems such as a reduction of gain, an increasing of loss and providing a large-sized device. The electrical amplifier device of the present invention is operated such that a bias point and a gradient of a load line are set as follows. That is, in the electrical power amplifier device of the present invention, a bias point of a source-grounded field-effect transistor is set to a point where a drain bias current is higher than that of the class A bias point. In addition, in the electrical power amplifier device of the present invention, a gradient of a load line is set to have a relation of a first voltage V.sub.1 >a second voltage V.sub.2. In this case, the first voltage V.sub.1 is a voltage obtained by subtracting a power supply voltage V.sub.dd from a voltage attained at a crossing point between the load line and the voltage axis. In addition, the second voltage V.sub.2 is a voltage obtained by subtracting a knee voltage V.sub.knee from the power supply voltage V.sub.dd. Further, an input voltage where an output voltage wave is saturated at a low voltage side is applied to the electrical power amplifier device of the present invention to operate it.

A capacitor is connected between the gate of an FET and an input node, and a resistor is connected between the input node and a ground terminal, thereby preventing the FET from oscillating in a low-frequency domain. A capacitor is connected between the drain of the FET and a ground terminal, or a line and a capacitor are connected in series between the drain of the FET and a ground terminal, thereby preventing the FET from oscillating in a high-frequency domain or at a specific frequency in the high-frequency domain.

A high efficiency dual-band RF power amplifier has an output and/or input of a high frequency transistor well terminated at the second harmonic frequency for dual-band operation. Diode switches or voltage controlled varactors implement dual-band resonators such that they can offer low impedance at the second harmonic of each frequency band, while, in some cases, presenting high impedance at the fundamental frequency as an RF choke circuit.