A bias voltage is applied via a first resistance to the base of a first transistor, and a radio frequency signal is input via a first capacitor to the base of the first transistor. The bias voltage is applied via a second resistance to the base of a second transistor. The bias voltage is applied via a third resistance to the base of a third transistor, and the radio frequency signal RF is input via a third capacitor to the base of the third transistor. A first band rejection filter is provided between the base of the first transistor and the base of the second transistor. A second band rejection filter is provided between the base of the second transistor and the base of the third transistor. The collectors of the first to third transistors are connected in common and the emitters thereof are all grounded.

A transistor integrated circuit apparatus generating less noise, having superb RF characteristics, and preventing thermal runaway of transistors is provided. Owing to capacitors C11 through C1n having one end commonly connected to an RF signal input terminal RFin and the other end connected to a base electrode of a corresponding transistor, and inductors L11 through L1n having one end commonly connected to a DC power supply input terminal DCin and the other end connected to a base electrode of a corresponding transistor, RF noise generated in a DC power supply circuit is reduced. This can reduce the RF noise output from the transistors Tr11 through Tr1n. The inductors L11 through L1n prevent an RF signal input from the RF input terminal RFin from flowing toward the DC power supply circuit. This can prevent the RF signal from being lost by the flow thereof toward the DC power supply circuit.

In order to supply a bias voltage to the base terminals of heterojunction bipolar transistors (HBTs) Q1 to Q3 connected in parallel to one another, resistors RB1 to RB3 and heterojunction bipolar transistors QB1 to QB3 whose base terminals are connected to the collector terminal thereof are provided. The amplifier transistors Q1 to Q3 have the same temperature characteristics as those of the bias-producing transistors QB1 to QB3. With the bias circuit, it is possible to compensate for the temperature characteristics of the amplifier transistors Q1 to Q3. Since the resistance values of the resistors RB1 to RB3 can be decreased, it is possible to suppress the decrease in the output power and to prevent the occurrence of the collapse phenomenon. Thus, it is possible to obtain a power amplifier capable of preventing the thermal runaway and compensating for the temperature characteristics without deteriorating the output characteristics.

A side fed RF amplifier comprising a plurality of transistors connected in parallel such that the base, emitter, and collector leads of each transistor are electrically connected to the base, emitter, and collector leads, respectively, of all other transistors. A common, physical point interconnects the power amplifier current source and the base leads of every transistor. The transistors are arranged such that the impedance between the common physical point and the base lead of any one transistor is substantially equivalent to the impedance between the common point and the base lead of any other transistor within the power amplifier.

A linear amplifier comprises a first current-mirror circuit including a first transistor whose base and collector are short-circuited for diode connection and whose collector is connected via a first resistance to a power-supply terminal, a second current-mirror circuit including a second transistor whose collector and base are connected to power-supply terminals, and an amplification transistor whose emitter is grounded. The base of the first transistor and the emitter of the second transistor are connected to the base of the amplification transistor.