Power amplifier circuit
A power amplifier circuit includes a transistor, a bias current source, and an adjustment circuit. The transistor amplifies an RF signal when supplied with a variable power supply voltage. The bias current source supplies a bias current to the base of the transistor through a first current path. The adjustment circuit increases a current flowing from the bias current source to an input terminal of a matching circuit through a second current path as the variable power supply voltage decreases, and decreases the bias current flowing from the bias current source to the base of the transistor through the first current path as the current flowing from the bias current source to the input terminal through the second current path increases.
1. A power amplifier circuit comprising:
a matching circuit having an input terminal and an output terminal;
a transistor having a base connected to the output terminal, and a collector supplied with a variable power supply voltage greater than or equal to a first voltage and less than or equal to a second voltage, the transistor being configured to amplify a radio frequency signal input to the base of the transistor via the matching circuit;
a bias current source configured to supply a bias current to the base of the transistor through a first current path; and
an adjustment circuit configured to adjust the bias current supplied to the base of the transistor in accordance with the variable power supply voltage,
wherein when the variable power supply voltage is greater than or equal to the first voltage and less than or equal to a third voltage, the third voltage being greater than the first voltage and less than the second voltage, the adjustment circuit is configured to:
increase a current flowing from the bias current source to the input terminal through a second current path as the variable power supply voltage decreases, and
decrease the bias current flowing from the bias current source to the base of the transistor through the first current path as the current flowing from the bias current source to the input terminal through the second current path increases.
2. The power amplifier circuit according to claim 1 , wherein:
the adjustment circuit includes a heterojunction bipolar transistor, a first resistor element, a second resistor element, and a third resistor element,
the heterojunction bipolar transistor has an emitter connected to the first current path via the first resistor element,
the heterojunction bipolar transistor has a base connected to the bias current source via the second resistor element,
the heterojunction bipolar transistor has a collector connected to the input terminal via the third resistor element, and
the second current path is a path through which current flows from the bias current source to the input terminal via the second resistor element, the base and collector of the heterojunction bipolar transistor, and the third resistor element.
3. The power amplifier circuit according to claim 2 , wherein:
the bias current source includes a first diode, a second diode, and an emitter follower transistor,
the first diode has an anode connected to a power supply terminal,
the first diode has a cathode connected to an anode of the second diode,
the second diode has a cathode connected to ground,
the emitter follower transistor has a base connected to the anode of the first diode and also connected to the second resistor element via the second current path, and
the emitter follower transistor has an emitter connected to the base of the transistor via the first current path.
4. The power amplifier circuit according to claim 2 , further comprising:
a first capacitor element connected between the base and collector of the heterojunction bipolar transistor.
5. The power amplifier circuit according to claim 3 , further comprising:
a first capacitor element connected between the base and collector of the heterojunction bipolar transistor.
6. The power amplifier circuit according to claim 2 , further comprising:
a second capacitor element connected in parallel with the third resistor element.
7. The power amplifier circuit according to claim 3 , further comprising:
a second capacitor element connected in parallel with the third resistor element.
8. The power amplifier circuit according to claim 4 , further comprising:
a second capacitor element connected in parallel with the third resistor element.
9. The power amplifier circuit according to claim 5 , further comprising:
a second capacitor element connected in parallel with the third resistor element.
10. The power amplifier circuit according to claim 2 , wherein:
the third resistor element has a first end connected to the input terminal,
the third resistor element has a second end connected to the collector of the heterojunction bipolar transistor,
the first resistor element has a first end connected to the first current path,
the first resistor element has a second end connected to the emitter of the heterojunction bipolar transistor, and
the power amplifier circuit further comprises a third capacitor element connected between the first end of the third resistor element and the second end of the first resistor element.
11. The power amplifier circuit according to claim 3 , wherein:
the third resistor element has a first end connected to the input terminal,
the third resistor element has a second end connected to the collector of the heterojunction bipolar transistor,
the first resistor element has a first end connected to the first current path,
the first resistor element has a second end connected to the emitter of the heterojunction bipolar transistor, and
the power amplifier circuit further comprises a third capacitor element connected between the first end of the third resistor element and the second end of the first resistor element.
12. The power amplifier circuit according to claim 4 , wherein:
the third resistor element has a first end connected to the input terminal,
the third resistor element has a second end connected to the collector of the heterojunction bipolar transistor,
the first resistor element has a first end connected to the first current path,
the first resistor element has a second end connected to the emitter of the heterojunction bipolar transistor, and
the power amplifier circuit further comprises a third capacitor element connected between the first end of the third resistor element and the second end of the first resistor element.
13. The power amplifier circuit according to claim 5 , wherein:
the third resistor element has a first end connected to the input terminal,
the third resistor element has a second end connected to the collector of the heterojunction bipolar transistor,
the first resistor element has a first end connected to the first current path,
the first resistor element has a second end connected to the emitter of the heterojunction bipolar transistor, and
the power amplifier circuit further comprises a third capacitor element connected between the first end of the third resistor element and the second end of the first resistor element.
14. The power amplifier circuit according to claim 1 , wherein:
the variable power supply voltage is supplied by an envelope tracking power supply.