PURPOSE:To suppress an influence of fluctuations of an electric power source voltage as to a self-excited D-class amplifier, by making oscillation conditions unchangeable even when load states change by separating a feedback system path, determining an oscillation frequency, from the determining the whole frequency characteristics. CONSTITUTION:By way of the integrating circuit consisting of operational amplifier 9 and capacitor 10, an input signal from input terminal 1 is sent to comparator 2, the output of which is supplied to LPF consisting of coil 4 and capacitor 5 through switching circuit 3. Then, its output is fed back to the other-side input of comparato 2 by way of the 1st feedback circuits 6 and 7 composed of resistor 6 and capacitor 7 and also to integrators 9 and 10 by way of the 2nd feedback circuit composed of resistors 11 and 12. Consequently, frequency response characteristics are flat in the low-frequency range and rise suddenly nearly at the oscillation frequency to make oscillation conditions unchangeable according to the load state, so that an influence of fluctuations of the electric power source voltage can be suppressed.
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