Compressing and expanding operational circuit with no use of clock signal

An operational circuit to be used for a compressing or expanding circuit comprising:

• an input terminals pair;
• a polarity judging circuit which judges the polarity of an AC signal voltage to be applied to said input terminals pair thereby to generate and output a first DC signal and a second DC signal whose voltage difference is predetermined and different in polarity depending on the judgment of being positive and negative;
• a first differential amplifier circuit which differentially amplifies said AC signal and said first DC signal thereby to output a first differential output signal;
• a second differential amplifier circuit which is equal in circuit structure to said first differential amplifier circuit and differentially amplifies said AC signal and said second DC signal thereby to output a second differential output signal;
• an averaging circuit which obtains a rectified signal of said AC in response to said first and second differential output signals and averages said rectified signal thus obtained to generate an averaged rectified signal thereby to generate and output a first signal and a second signal in accordance with the averaged rectified signal, the difference of said first and second signals being dependent only on said averaged rectified signal and the sum of said first and second signals being independent of said averaged rectified signal;
• a third differential amplifier circuit which differentially amplifies said AC signal and said first signal thereby to output a third differential output signal;
• a fourth differential amplifier circuit which is equal in circuit structure to said third differential amplifier circuit and differentially amplifies said AC signal and said second signal thereby to output a fourth differential output signal;
• and
• a differential output terminals pair which are applied with said third and fourth differential output signals so as to be opposite in polarity to each other thereby to output the product of said AC signal and said averaged rectified signal as its output signal.