A voltage control variable gain circuit constructed as a semiconductor integrated circuit comprising:
- (a) a first operational amplifier circuit having an inverting input terminal, a non-inverting input terminal and an output terminal, said inverting input terminal leading out of said semiconductor integrated circuit to act as a first input terminal of said semiconductor integrated circuit, said non-inverting input terminal leading out of said semiconductor integrated circuit and connected to a reference potential;
- (b) a second operational amplifier circuit having an inverting input terminal, a non-inverting input terminal and an output terminal, said inverting input terminal leading out of said semiconductor integrated circuit to act as a second input terminal of said semiconductor integrated circuit, said non-inverting input terminal leading to the outside of said semiconductor integrated circuit and connected to said reference potential;
- (c) a first gain control section connected to said output terminal of said first operational amplifier circuit within said semi-integrated circuit and having an input terminal, a control input terminal and an output terminal, said input terminal being electrically connected to said inverting input terminal of said first operational amplifier circuit within said semiconductor integrated circuit, said control input terminal leading to the outside of said semiconductor integrated circuit to act as a first control input terminal of said semiconductor integrated circuit;
- (d) a second gain control section connected to said output terminal of said second operational amplifier circuit within said semiductor integrated circuit and having an input terminal, a control input terminal and an output terminal, said input terminal being electrically connected to said inverting input terminal of said second operational amplifier circuit within said semiconductor integrated circuit, said control input terminal leading to the outside of said semiconductor integrated circuit to act as a second control input terminal of said semiconductor integrated circuit;
- (e) a third operational amplifier circuit having an inverting input terminal, a non-inverting input terminal and an output terminal, said inverting input terminal being electrically connected to said output terminal of said first gain control section within said semiconductor integrated circuit and leading to the outside of said semiconductor integrated circuit, said non-inverting input terminal leading to the outside of said semiconductor integrated circuit and connected to said reference potential, and said output terminal leading to the outside of said semiconductor integrated circuit to act as a first output terminal of said semiconductor integrated circuit;
- (f) a fourth operational amplifier circuit having an inverting terminal, a non-inverting terminal and an output terminal, said inverting input terminal being electrically connected to said output terminal of said second gain control section within said semiconductor integrated circuit and lading to the outside of said semiconductor integrated circuit, said non-inverting input terminal leading to the outside of said semiconductor integrated circuit and connected to said reference potential, and said output terminal leading to the outside of said semiconductor integrated circuit to act as a second output terminal of said semiconductor integrated circuit;
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- (g) fifth operational amplifier circuit having an inverting input terminal, a non-inverting input terminal and an output terminal, said inverting input terminal leading to the outside of said semiconductor integrated circuit to serve as an optional input terminal for the semiconductor integrated circuit, said non-inverting input terminal leading to the outside of said semiconductor integrated circuit and connected to said reference potential, and said output terminal leading to the outside of said semiconductor integrated circuit to provide an external terminal to permit either the connection or disconnection of said fifth operational amplifier circuit to the inverting input terminal of the second operational amplifier circuit.