The present invention relates generally to a feedforward amplifier for use mainly in the high-frequency band and, more particularly, to a feedforward amplifier of increased power efficiency.
FIG. 1 illustrates in block form a basic configuration of a feedforward amplifier. The feedforward amplifier comprises a distortion detector 13 made up of an amplifying transfer path 11 of a main amplifier (hereinafter referred to as a main amplifier path) and a first linear signal transfer path (hereinafter referred to as a first linear path) 12 and a distortion canceller or suppressor 16 made up of a second linear signal transfer path (hereinafter referred to as a second linear path) 14 and a distortion injection path 15. The main amplifier path 11 is formed by a series connection of a variable attenuator 21, a variable phase shifter 22 and a main amplifier 23. The first linear path 12 is formed by a series connection of a delay line 24 and a phase inverter 25. The second linear path 12 is formed by a delay line 26. The distortion injection path 15 is formed by a series connection of a variable attenuator 27, a variable phase shifter 28 and a auxiliary amplifier 29. The input into the feedforward amplifier is divided by a power divider 31 to the main amplifier path 11 containing the main amplifier 23 and the first linear path 12. A power combiner/divider 32 generates the sum of and the difference between signals from the main amplifier path 11 and the first linear path 12, and provides them to the second linear path 14 and the distortion injection path 15, respectively. The output from the feedforward amplifier is produced by a power combiner 33 which combines outputs from the second linear path 14 and the distortion injection path 15.
Accordingly, the feedforward amplifier detects a distortion component (a difference component) generated by the main amplifier 23 in the distortion detector 15 and, in the distortion canceller 16, regulates the phase and amplitude of the distortion component and injects it into the output signal from the main amplifier 23 provided via the second linear path 14, thereby canceling nonlinear distortion generated by the main amplifier 23. In general, the amount of nonlinear distortion cancelled by the feedforward amplifier depends on the regulation of the variable attenuator 21 and the variable phase shifter 22 of the distortion detector 13 and the variable attenuator 27, the variable phase shifter 28 and the auxiliary amplifier 29 of the distortion canceller 16. The accuracy of such regulation is disclosed in Japanese Patent Laid-Open Gazette No. 1-198809 entitled "Automatic Regulation Circuit for Feedforward Amplifier." For example, phase and amplitude deviations for achieving a distortion compression of 30 dB or more are .+-.2.degree. and .+-.0.3 dB, respectively. It can be said, therefore, that strict conditions are imposed on the balance and completeness of regulation of the transmission characteristics of the distortion detector 13 and the distortion canceller 16.
The feedforward amplifier compensates for the nonlinear distortion generated by the main amplifier 23. Accordingly, because of its circuit configuration the feedforward amplifier cannot theoretically compensate for nonlinear distortion by the auxiliary amplifier 29. Furthermore, because of the stringent condition on the balance of each of the above-mentioned two paths, linearity of input vs. output power performance is required of the auxiliary amplifier 29 of the conventional feedforward amplifier. To increase the linearity of the amplification circuit using a semiconductor amplifying device, it is customary in the prior art to use a Class A bias for the condition of operation to thereby make the saturation output voltage sufficiently larger than the peak voltage of the signal to be amplified.
In recent years there has been a growing demand for small, light and inexpensive radio equipment of lower-power consumption. The same is true of radio equipment using the feedforward amplifier. To reduce the power consumption of the feedforward amplifier, it is essential to increase the power efficiency of both of the main and auxiliary amplifiers. With the increased power efficiency of the both amplifiers, it is possible to reduce their cooling bodies and so on and hence achieve downsizing of radio equipment.
The power efficiency of the main amplifier can be enhanced as by a push-pull circuit which operates under Class B bias condition. The conventional feedforward amplifier is capable of compensating for the nonlinear distortion that occurs in the main amplifier. On the other hand, to increase the power efficiency of the auxiliary amplifier 29 inserted in the distortion injection path 15 of the feedforward amplifier, it is necessary, in general, that the semiconductor amplifying device forming the auxiliary amplifier be caused to operate under a Class B or C bias condition. The nonlinear distortion that results from such bias condition cannot theoretically be compensated for by the feedforward amplifier as referred to above. For these reasons, enhancement of the power efficiency of the auxiliary amplifier 29 gives rise to the problem of impairing the distortion compensating ability of the feedforward amplifier.
The power-supply efficiency of the feedforward amplifier can be expressed by the ratio between the output power of the feedforward amplifier and the power supplied thereto. For example, according to Toshio Nojima and Shouichi Narahashi, "Ultra-Low, Multi-Frequency Common Amplifier for Mobile Communications, --Self-Adjusting Feedforward Amplifier (SAFF-A)" Technical Report of Institute of Electronics, Information and Communication Engineers of Japan, RCS90-4, 1989, in the case where the saturation output power of the main amplifier is 100 W, the saturation output power of the auxiliary amplifier is 1/8 that of the main amplifier, GaAs-MESFETs are used as semiconductor amplifying devices for the main amplifier and the auxiliary amplifier, the drain voltage and current of the MESFET of the main amplifier are 12 V and 20 A, the drain voltage and current of the MESFET of the auxiliary amplifier are 12 V and 5 A and the both amplifiers are operated in a 1.5 GHz band under the Class A bias condition, the power supply to the feedforward amplifier is 300 W. If mean output power back off is set at 8 dB and the loss of the main amplifier output signal in the distortion canceller is ignored, then the feedforward amplifier output is approximately 15 W. Accordingly, the power-supply efficiency of the feedforward amplifier is in the range of between 15/300 and about 5%. Even in the case where a highly efficient amplification circuit such as a Class B push pull amplifier is used as the main amplifier and a Class A amplifier as the auxiliary amplifier, the power-supply efficiency is around 10% or less at the highest.
To achieve high power efficiency of a high-output power amplifier, there is described in W. H. Doherty, "A new high efficiency power amplifier for modulated waves," Pro. IRE, Vol. 24, No. 9, pp.1163-1182, September 1936 a method which employs a plurality of amplifiers of different saturated output voltages. This method is generally known under the name of Doherty method, which has been implemented, for example, in a medium-wave transmitting power amplifier for broadcasting stations. In the Doherty method, a saturation amplifier and a linear amplifier are connected in parallel. The saturation amplifier amplifies signals of mean power or thereabout and the linear amplifier amplifies signals of peak power. The Doherty method achieves high power efficiency amplification by means of the saturation amplifier, but because of the circuit configuration used, signals that ought to be input into the linear amplifier are also applied to the saturation amplifier--this raises the problem of nonlinear distortion. Moreover, the circuit configuration by this method is incapable of compensating for the nonlinear distortion caused by the saturation amplifier.
One method that has been proposed to further increase the powersupply efficiency of the feedforward amplifier is to set the output backoff (a difference between the saturated output power and the output power at the operating point) of the main amplifier at a small value. As is well-known in the art, however, the nonlinear distortion that can be compensated for by the feedforward amplifier is limited only to incompleteness of the linear region of the input/output power characteristic of the main amplifier. That is, in general, the feedforward amplifier cannot compensate for nonlinear distortion as by clipping in the saturation region of the input characteristic.