An amplifier circuit includes a first amplifier that amplifies a high frequency signal, and a load circuit that changes a load impedance of the first amplifier without being controlled by an external circuit so that a saturation power at a first temperature is higher than a saturation power at a second temperature lower than the first temperature, and an efficiency at the first temperature is lower than an efficiency at the second temperature.

Amplifiers, amplification circuits, and phase shifters, for example, for flexibly adjusting an output phase to thereby meet a requirement of a constant phase on a link in a communications field, are provided. In one aspect, an amplifier includes first, second, and third MOS transistors. The first MOS transistor includes a gate separately coupled to a signal input end and a bias voltage input end, a source coupled to a power supply, and a drain separately coupled to sources of the second and third MOS transistors. A drain of the third MOS transistor is coupled to a ground, and a drain of the second MOS transistor is coupled to a signal output end. The bias voltage input end is configured to receive a bias voltage to adjust a phase difference between an input signal at the signal input end and an output signal at the signal output end.

A dual-band MMIC power amplifier and method of operation to amplify frequencies in different RF bands while only requiring input drive signals at frequencies f.sub.1 and f.sub.2 in a narrow RF input band. This allows for the use of a conventional narrowband RF IC to drive the MMIC and does not require additional circuitry (e.g., a LO) on the MMIC power amplifier. The matching network of the last amplification stage is modified to pass f.sub.1 (or a harmonic thereof), reflect f.sub.2, pass a P.sup.th harmonic of f.sub.2 where P is 2 or 3 and to reflect any unused 1.sup.st, 2.sup.nd or 3.sup.rd order harmonics of f.sub.1 or f.sub.2 back into the MMIC. In response to an input signal at f.sub.1, the MMIC power amplifier amplifies and outputs a signal at f.sub.1 (or a harmonic thereof). In response to an input signal at f.sub.2 at sufficient RF power, the last amplification stage operates in compression such that the MMIC power amplifier generates the harmonics, selects the P.sup.th harmonic and outputs an amplified RF signal at P*f.sub.2.

A compound semiconductor device comprises a heterojunction bipolar transistor including a plurality of unit transistors, a capacitor electrically connected between a RF input wire and a base wire for each unit transistor of the unit transistors, and a bump electrically connected to emitters of the unit transistors. The unit transistors are arranged in a first direction. The bump is disposed above the emitters of the unit transistors while extending in the first direction. The transistors include first and second unit transistors, the respective emitters of the first and second unit transistors being disposed on first and second sides, respectively, of a second direction, perpendicular to the first direction, with respect to a center line of the bump extending in the first direction. The capacitor is not covered by the bump, and respective lengths of the respective base wires connected respectively to the first and second unit transistors are different.

A current flowing through a transistor of a final-stage amplifier is suppressed. A power amplification circuit includes a driving-stage amplifier, a final-stage amplifier, a power supply terminal, a first voltage control circuit, and a second voltage control circuit. The driving-stage amplifier includes a first transistor having a first input terminal, a first output terminal, and a first ground terminal. The final-stage amplifier includes a second transistor having a second input terminal, a second output terminal, and a second ground terminal. The first voltage control circuit is connected between the power supply terminal and the first output terminal, and controls a first power supply voltage applied to the first transistor. The second voltage control circuit is connected between the power supply terminal and the second output terminal, and controls a second power supply voltage applied to the second transistor.

A circuit comprises an amplifier network including a first amplifier and a second amplifier and a first transistor having a first base. The first transistor is thermally isolated from the second amplifier. The circuit further comprises a second transistor having a second base. The second transistor is thermally linked to the second amplifier. The circuit further comprises coupling circuitry configured to couple the first base to the second base.

A matching circuit structure for effectively suppressing the low-frequency clutter of a power amplifier of a mobile phone, falling within the technical field of radio frequency Pas is provided. The circuit structure includes an input end, a blocking capacitor, a power amplifier (PA), an output matching network and an output end connected in series; and the matching circuit structure further includes a negative feedback network connected in parallel to a transmission end of the PA; the negative feedback network includes a resonant capacitor, a resonant inductor and a matching inductor; the resonant capacitor and the resonant inductor are connected in parallel to form a frequency selecting network, and the frequency selecting network is connected in series with the matching inductor and to the ground. The matching circuit structure above can be used to effectively suppress the low-frequency clutter of a power amplifier.

A power amplifier includes an over-current protection loop and/or an over-voltage protection loop to assist in preventing operation outside a safe operation zone. In a further exemplary aspect, triggering of the over-current protection loop adjusts a threshold voltage for the over-voltage protection loop. In further exemplary aspects, the over-current protection loop may adjust not only a bias regulator, but also provide an auxiliary control signal that further limits signals reaching the power amplifier. In still further exemplary aspects, the over-voltage protection loop may operate independently of the over-current protection current loop or the over-voltage protection loop contribute to an over-current protection signal.

A power amplifier includes an over-current protection loop and/or an over-voltage protection loop to assist in preventing operation outside a safe operation zone. In a further exemplary aspect, triggering of the over-current protection loop adjusts a threshold voltage for the over-voltage protection loop. In further exemplary aspects, the over-current protection loop may adjust not only a bias regulator, but also provide an auxiliary control signal that further limits signals reaching the power amplifier. In still further exemplary aspects, the over-voltage protection loop may operate independently of the over-current protection current loop or the over-voltage protection loop contribute to an over-current protection signal.

A power amplifying circuit includes multi-stage power amplifiers, bias circuits, and a control circuit. The bias circuits output corresponding bias currents based on corresponding control currents. The control circuit outputs the control currents to the bias circuits based on a control voltage. The power amplifiers include a first stage of first and second power amplifiers connected in parallel electrically. The bias circuits include first and second bias circuits. The control circuit includes first and second current output units. The first current output unit outputs, to the first bias circuit, a first control current which has a first current value when the control voltage is a first threshold voltage, and which increases linearly with the control voltage, and the second current output unit outputs, to the second bias circuit, a second control current, having a second constant current value, when the control voltage is the first threshold voltage or greater.