Variable gain amplifier circuit and filter circuit

A variable gain amplifier circuit whose gain can vary, comprising:

• an operational amplifier whose non-inversion input terminal is connected to a reference potential;
• a feedback resistor that is connected between an output terminal and an inversion input terminal of the operational amplifier;
• an input resistor that is inserted between the inversion input terminal of the operational amplifier and an input terminal of the variable gain amplifier circuit;
• and
• an adjustment resistor that is connected between the inversion input terminal of the operational amplifier and the reference potential and whose resistance value is controlled in such a way as to maintain constant against the resistance value change a ratio between a combined resistance value in a parallel connection between the input resistor and the adjustment resistor and the resistance value of the feedback resistor when changing one resistance value of the input resistor and the feedback resistor, wherein
• a resistance value of the feedback resistor is fixed;
• a resistance value of the input resistor is variable;
• and
• a resistance value of the adjustment resistor is controlled in such a way as to maintain a constant combined resistance value in the parallel connection between the input resistor and the adjustment resistor against the resistance value change when changing the resistance value of the input resistor.