Patent ID: 8842855

Claim:
A sound volume control circuit that controls sound volume by digitally changing an output voltage comprising: a first operational amplifier including: an inverting input; a non-inverting input connected to a voltage source of a reference voltage; and an output that outputs the output voltage; a variable resistor circuit that is connected between the output and the inverting input, and has a plurality of resistance values; an R-2R ladder circuit that is connected between a voltage source of an input voltage and the inverting input, and changes the magnitude of electric current flowing to the variable resistor circuit, the R-2R ladder circuit including: a resistor chain that has a plurality of resistors connected in series, the resistor chain having: a plurality of first resistors having a third resistance value, respectively; and one second resistor that is provided closest the inverting input and has a fourth resistance value that is twice as large as the third resistance value; a plurality of third resistors that has the fourth resistance value, respectively, and is branched from the resistor chain, respectively; a plurality of first switches by which the branched plurality of the third resistors is connected to either the inverting input or the voltage source of the reference voltage, respectively; and a second switch that is provided between the resistor chain and the inverting input; and a control circuit that controls the variable resistor circuit and the R-2R ladder circuit, wherein when changing a resistance value of the variable resistor circuit from a first resistance value to a second resistance value, so as to change the output voltage in a step size where a difference between the output voltage corresponding to the first resistance value and the output voltage corresponding the second resistance value is further segmented, the control circuit changes the magnitude of the electric current flowing to the variable resistor circuit by use of the R-2R ladder circuit, and wherein each of the plurality of the first switches further has a state of being connected to neither the inverting input nor the voltage source of the reference voltage, and in a case of muting, the control circuit changes the resistance value of the variable resistor circuit to a minimum value, switches each of the plurality of the first switches to the state of being connected to neither the inverting input nor the voltage source of the reference voltage, and turns oil the second switch.