High frequency power amplifier, transmitter and mobile communication terminal using the power amplifier
A high frequency power amplifier maintains an excellent linearity regardless of a fluctuation of a load impedance and is downsized. The high frequency power amplifier detects an AC voltage amplitude at an output terminal of a final amplification stage transistor, and suppresses an input signal amplitude of a power amplifier when the voltage amplitude exceeds a predetermined threshold value.
1. A mobile communication terminal, comprising:
a base band circuit having a limiter that determines a maximum transmit power of the mobile communication terminal;
a transmit variable gain amplifier that receives the control signal from the base band circuit;
a power amplifier that amplifies a transmit signal from the transmit variable gain amplifier;
a front end module coupled to the power amplifier; and
an antenna coupled to the front end module,
wherein the power amplifier includes two amplification stages for amplifying an input signal from a gain variable amplifier,
wherein the power amplifier includes:
a first amplification stage;
a second amplification stage comprising a transistor, and electrically coupled to the first amplification stage;
an output matching circuit that is coupled to an output side of the second amplification stage; and
a first detection unit for detecting a voltage amplitude at a node between the transistor of the second amplification stage and the output matching circuit, and outputting the voltage amplitude as a first signal of gain control information of the gain variable amplifier, detecting a supply voltage being supplied to the second amplification stage, and outputting the supply voltage as a second signal of the gain control information of the gain variable amplifier, and
wherein the gain variable amplifier is controlled based on the gain control information so as to limit a gain of the gain variable amplifier, upon the voltage amplitude exceeding a predetermined threshold voltage.
2. The mobile communication terminal according to claim 1 , wherein the first detection unit includes a voltage detection diode that is connected to a collector of the transistor of the second amplification stage, and a voltage detection resistor and a capacitor that are coupled to the voltage detection diode.
3. The mobile communication terminal according to claim 2 , further comprising a second detection unit having a directional coupler that is disposed within the output matching circuit or between the output matching circuit and the output terminal, and wherein the second detection unit outputs an output signal of the directional coupler as a third signal of the gain control information of the gain variable amplifier.
4. The mobile communication terminal according to claim 3 , wherein an input matching capacitor and an interstage matching capacitor that are coupled to the transistor of the second amplification stage, the transistor of the second amplification stage, and the first detection unit are integrated on one chip.
5. The mobile communication terminal according to claim 2 , wherein an input matching capacitor and an interstage matching capacitor that are coupled to the transistor of the second amplification stage, the transistor of the second amplification stage, and the first detection unit are integrated on one chip.
6. The mobile communication terminal according to claim 2 , wherein the transistor of the second amplification stage is of a self bias system in which a bias current with respect to the power amplifier changes according to an RF input voltage.