Amplifiers – With semiconductor amplifying device – Including particular biasing arrangement
Reexamination Certificate
2000-03-28
2001-10-09
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including particular biasing arrangement
C330S267000, C330S273000
Reexamination Certificate
active
06300837
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention is in the field of transistor amplifier circuits, and relates more particularly to a power amplifier circuit having a bias boosting circuit for increasing maximum power output and reducing power dissipation at low power levels.
Amplifiers of this general type are frequently used in high-frequency RF amplifiers, as well as in audio amplifiers and other applications. In order to obtain a linear input-output relationship and high operating efficiency, such amplifiers are typically operated with a conduction angle of about 180° (Class B) or slightly greater (Class AB) to avoid crossover distortion.
Typically, amplifiers of this type require a dc bias circuit to establish the quiescent bias current in the amplifier circuit to ensure operation in the Class B or Class AB mode. In the prior art, bias is typically provided by a fixed current source, as shown in U.S. Pat. No. 5,844,443, or else by an external supply, which can be set to a desired constant value to secure the quiescent current necessary to operate in the desired mode, as shown in U.S. Pat. No. 5,548,248.
However, in amplifiers of the type described above the average current drawn from the supply depends upon the input signal level. As the output power increases so does the average current in both the emitter and the base of the power transistor. This increased average current causes an increased voltage drop in the biasing circuitry and in ballast resistors (which are used to avoid hot-spotting and thermal runaway in transistors using an interdigitated design). This in turn reduces the conduction angle (i.e. the number of degrees out of 360° that the amplifier is conducting), and forces the amplifier deep into Class B or even Class C operation, thereby reducing the maximum power output by about 25%. To avoid this power reduction, the amplifier must have a larger quiescent bias. In prior-art circuitry this inevitably leads to a higher power dissipation at low power output levels and therefore an undesirable tradeoff in operating characteristics.
Accordingly, it would be desirable to have a power amplifier circuit which offers the advantages of optimum maximum output power and reduced power dissipation at low power levels. Additionally, it would be desirable for such a circuit to be simple and compact in design, and economical to manufacture.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a power amplifier circuit which provides improved maximum output power and less power dissipation at low power levels. It is a further object of the invention to provide a power amplifier circuit which is both simple and compact in design and which is economical to manufacture.
In accordance with the invention, these objects are achieved by a new power amplifier circuit for amplifying an input signal and having a conduction angle of at least about 180°, the amplifier circuit including an amplifying transistor and a dc bias circuit for biasing the amplifier transistor to obtain the desired conduction angle. The dc bias circuit includes a dynamic bias boosting circuit for increasing the dc bias current provided to the amplifying transistor by the dc bias circuit in direct proportion to an increase in the input signal provided to the power amplifier.
In a preferred embodiment of the invention, the amplifier circuit is either a Class B or a Class AB amplifier circuit.
In a further preferred embodiment of the invention, the dynamic bias boosting circuit includes a voltage-controlled current source controlled by a dc voltage proportional to the amplitude of the input signal provided to the power amplifier circuit.
In a further preferred embodiment of the invention, the power amplifier circuit also includes a driver transistor, and the dynamic bias boosting circuit senses a voltage proportional to a current in the driver transistor and increases the dc bias current provided to the amplifying transistor in direct proportion to an increase in the voltage to and current in the driver transistor.
A power amplifier circuit in accordance with the present invention offers a significant improvement in that a particularly advantageous combination of features, including increased maximum output power and reduced power dissipation at low power levels can be obtained in a simple, compact and economical configuration.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
REFERENCES:
patent: 3855540 (1974-12-01), Leidich
patent: 4638260 (1987-01-01), Hamley
patent: 4663599 (1987-05-01), Patch
patent: 5548248 (1996-08-01), Wang
patent: 5844443 (1998-12-01), Wong
patent: 6147543 (2000-11-01), Gailus et al.
Luo Sifen
Sowlati Tirdad
Biren Steven R.
Nguyen Patricia T.
Pascal Robert
Philips Electronics North America Corporation
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