not can be detected on the basis of the magnitude of the output signal voltage vs since the feedback resistor 201 is omitted in the circuit of fig6 the voltage across the capacitance cg can follow a variation of the ac voltage vac better than in the circuit of fig4 circuit example 5 of detection circuit section 20 will be explained as follows circuit example 5 of the detection circuit section 20 will be described below in a specific manner with reference to a circuit diagram of fig7 this circuit is characterized in that the circuit of fig3 is modified in such a manner that the feedback resistor 201 is omitted to make the differential amplifier circuit section a voltage follower circuit and that the output end of the differential amplifier circuit section is connected to the top electrode 11 by a capacitor 205 having a capacitance value c 1 the operation of this circuit will be described below the two input ends of the operational amplifier 200 are imaginarily short circuited and hence the potentials of the top electrode 11 and the bottom electrode 12 can be regarded as identical the output voltage of the voltage follower circuit i e the differential amplifier circuit section without the feedback resistor 201.