11a connected in series with the collector of the transistor 18 on the contrary when the input signal from the input terminal i1 has a level 34 h 34 and the input signal from the inverted input terminal i2 has a level of 34 l 34 the transistor 12 is switched on and the transistor 13 is turned off so that the electric current of the constant electric current source 15 flows in the transistor 16 the transistor 16 forms an electric current mirror circuit together with a transistor 19 thus conduction of the transistor 16 causes the transistor 19 to be switched on so that the electric current flows into the coil 11b connected in series with the collector of the transistor 19 the constant electric current source 20 a variable power source 21 and transistors 22 and 23 form the aforementioned compensation circuit which is connected to the collectors of the transistors 12 and 13 assume in the present example that the variable voltage source 21 disposed between the base of transistor 22 and the base of transistor 23 has an output voltage of v 21 the transistors 22 and 23 have collector electric currents of i c22.