Abstract: A feed-forward amplifier is provided, which can implement satisfactory linearity immediately after its activation. The amplifier has a high productivity and exhibits less long term fluctuations of its characteristics during the operation. In the feed-forward amplifier, the transfer function of a path from an input terminal to a directional coupler via a main amplifier and the transfer function of a path from the input terminal to the directional coupler via a first delay means are automatically controlled so as to be equal in amplitude but anti-phase relative to each other. The transfer function of a path from the main amplifier to an output terminal via a second delay means and the transfer function of a path from the main amplifier to the output terminal via an auxiliary amplifier are automatically controlled so as to be equal in amplitude but anti-phase relative to each other.

Abstract: In an automatic predistortion adjusting circuit, a splitter splits an input signal into first and second signals. A predistortion circuit applies first non-linear characteristics to the first signal, a variable attenuator adjusts an amplitude of an output signal of the predistortion circuit, a variable phase shifter adjusts a phase of an output signal of the variable attenuator, and a main amplifier amplifies an output signal of the variable phase shifter, so that second non-linear characteristics opposite to the first non-linear characteristics are applied to an output signal of the main amplifier. A directional coupler takes out a part of an output signal of the main amplifier as a branch signal, and a delay line delays the second signal. An adder adds a delayed signal passed through the dely line to the branch signal.

Abstract: The compensation range of feed-forward amplifiers is expanded. The peak value of the input or output of an auxiliary amplifier 15 is detected with a peak power measuring instrument 24, and the characteristic of a distortion detecting loop 100 is controlled with a vector adjuster 3 and a control circuit 9 so as to minimize this peak value. By controlling the characteristic of the distortion detecting loop by detecting the peak value, the range of linearity compensation is expanded compared with conventional control based on the average level.

Abstract: A first pilot signal is superimposed onto an input signal thereto to distribute the signal as a first signal and a second signal to two destinations. The first signal is fed via an equalizer to a main amplifier for amplification thereof such that a second pilot signal is mixed with the amplified signal to attain a main amplification signal. Subtraction is conducted between the second signal and the main amplification signal to extract a distortion signal to be fed via an equalizer to an auxiliary amplifier so as to produce an auxiliary amplification signal. Between the main and auxiliary amplification signals, there is effected subtraction to attain a differential signal. The pilot signals are detected according to the differential and output signals. The equalizers are controlled to set the pilot signals to a minimum value. Consequently, there is achieved a linear amplifier obtaining a stable amplification signal with reduced distortion in a wide band.

Abstract: In a first step, a control circuit 61 turns a switch 63 to detect a frequency of an input signal input to an input terminal 1 and distortion generated by amplification of this input signal or mixed in and sets a frequency of a pilot signal output from a variable frequency oscillator 53 at a value other than the input signal frequency and a frequency of the distortion and close to an average value of the input signal. Thereafter, the control circuit 61 turns the switch 63 off and controls a distortion detecting loop 7 by using the pilot signal in a second step and controls a distortion removing loop 4 in a third step. In a fourth step, it succeeds the control of the distortion removing loop 4 in a normal state and controls the distortion detecting loop 7 again only when environment is changed largely.

Abstract: An FSK modulating apparatus includes a first phase locked loop arrangement having a first voltage controlled oscillator for generating a space frequency signal of a modulation signal and a second phase locked loop arrangement having a second voltage controlled oscillator for generating a mark frequency signal of the modulation signal. The apparatus also includes a third voltage controlled oscillator used to produce an FSK modulated output signal. After input data changes frequency to a space frequency or a mark frequency, a control voltage for the phase locked loop arrangement that is associated with the resulting frequency is routed through a low pass filter having a flat group delay characteristic to the third voltage controlled oscillator. After a set period of time has elapsed, the third voltage controlled oscillator replaces the voltage controlled oscillator of the particular phase locked loop arrangement associated with the current state of the modulator signal.