AMPLIFIER CIRCUIT

PURPOSE:To improve the distortion of a final stage amplifier with a small mount area at low cost, by generating distortion components from the output of the amplifier for a detector and adding the distortion components as compensation components again to the amplifier signal on a main line via a phase shift or and an attenuater. CONSTITUTION:The signal of two waves in equal amplitude with frequencies f1 and f2 is impressed on a final stage amplifier FET-Q via a variable attenuator 11 and an amplifier 12. An FET amplifies the signal of the two waves and outputs it, and the output signal includes part of an amplifier signal and third inter-modulation components 2f1-f2 and 2f2-f1 because an operating range includes a nonlinear are. The output signal is transmitted via the main line 31 of a directional coupler, and part is impressed on a diode D through a sub-line 32 and a phase shifter and attenuator 4. Then detection voltage is added to a comparator 21 via a variable resistor Rv. The threshold voltage Vreff corresponding to the level of the output signal is impressed on a comparator 21 and an attenuation control signal is transmitted to the variable attenuator 11 to eliminate the difference of threshold voltage and the impressed detection voltage. The attenuation of the attenuator 11 is controlled so that the output signal may be always held to a prescribed level.