Automatic predistortion adjusting circuit having stable non-linear characteristics regardless of input signal frequency

An automatic predistortion adjusting circuit comprising:

• a first splitter for receiving an input signal and splitting said input signal into first and second signals;
• a predistortion circuit, connected to said first splitter, for applying compensating non-linear characteristics to said first signal;
• a first variable attenuator, connected to said predistortion circuit, for adjusting an amplitude of an output signal of said predistortion circuit;
• a first variable phase shifter, connected to said first variable attenuator, for adjusting a phase of an output signal of said first variable attenuator;
• a main amplifier, connected to said first variable phase shifter, for amplifying an output signal of said first variable phase shifter;
• a directional coupler, connected to said main amplifier, for taking out a part of an output signal of said main amplifier as a branch signal;
• a first delay line, connected to said splitter, for delaying said second signal to produce a delayed second signal;
• a first adder, connected to said directional coupler, for adding said delayed second signal to said branch signal;
• a detector, connected to said first adder, for detecting a strength of an output signal of said first adder;
• and
• a control circuit, connected to said detector, said predistortion circuit, said first variable attenuator and said first variable phase shifter, for controlling said compensating non-linear characteristics, an attenuation amount of said first variable attenuator and a phase amount of said first variable phase shifter in accordance with an output signal of said detector.