PURPOSE: To attain the titled preamplifier suitable for large scale circuit integration by constituting an amplifier circuit of a modulation signal with an operational amplifier, an input/output capacitor pair of set the gain of the operational amlifier and a short-circuit switch for initial setting so as to attain high accuracy of signal amplification to the gain setting in response to a wide range of signal level.

CONSTITUTION: A timing (c) is set during time t1∼T2 prior to timings a, b, switches 22∼29 are turned on so as to apply initial setting to capacitors 81∼84 and 87∼90, that is, to discharge an electric charge stored in the capacitors due to leakage, thereby eliminating the effect of a leakage current. A polarity switching circuit 100 sets the timing (a) during time t2∼t3 succeedingly, the timing (b) is set during t3∼t4, the polarity of an input signal is inverted alternately at the two time sections, the input signal is modulated into a high frequency signal near the frequency of the timings a, b and the result is inputted to an amplifier 101. The gain of the amplifier circuit 101 is a product of the gain of the 1st amplifier stage comprising two non-inverting amplifiers and the 2nd amplifier stage comprising a differential amplifier.