1. A membership function generator comprising: a maximum value selector having a plurality of first input terminals and an output terminal, wherein said maximum value selector selects and outputs a maximum voltage from a plurality of first input voltages provided to said first input terminals; a minimum value selector having a plurality of second input terminals and an output terminal, wherein said minimum value selector selects and outputs a minimum voltage from a plurality of second input voltages provided to said second input terminals; and a differential amplifier circuit having input terminals connected to the output terminals of the maximum value selector and the minimum value selector, wherein said differential amplifier circuit amplifies a difference of the voltages output from the maximum value selector and the minimum value selector to generate an amplified result as a membership function, a connected one of the plurality of first input terminals is connected to a connected one of the plurality of second input terminals, the voltage applied to the connected first and second input terminals is an inference-input voltage to form the basis of an inference and remaining ones of the plurality of first input terminals are set at a first reference voltage, and remaining ones of the plurality of second input terminals are set at a second reference voltage.