Semiconductor integrated circuit and radio communication terminal including the same

1. A semiconductor integrated circuit comprising:

an operational amplifier that amplifies a voltage difference between an input voltage supplied to an inverting input terminal and a reference voltage supplied to a non-inverting input terminal and outputs an amplified signal;

a feedback resistor that performs negative feedback of the amplified signal to the inverting input terminal of the operational amplifier; and

a variable resistor unit that sets a current path with a first resistance value in accordance with a control signal between an external input terminal and the inverting input terminal of the operational amplifier, and sets a first alternative path with a second resistance value in accordance with the control signal between a node on the current path and a reference voltage terminal to which the reference voltage is supplied,

wherein the variable resistor unit includes:

a plurality of first resistor elements placed in parallel between the external input terminal and the inverting input terminal of the operational amplifier,

a plurality of first switching elements respectively placed between one ends of the plurality of first resistor elements and the inverting input terminal of the operational amplifier and having a conducting state controlled based on the control signal,

a plurality of resistance attenuators respectively placed between other ends of the plurality of first resistor elements, and

a plurality of second switching elements respectively placed between common terminals of the plurality of resistance attenuators and the reference voltage terminal and having a conducting state controlled based on the control signal.

2. The semiconductor integrated circuit according to claim 1 , wherein the variable resistor unit sets the first alternative path between the inverting input terminal of the operational amplifier and the reference voltage terminal.

3. The semiconductor integrated circuit according to claim 2 , wherein the variable resistor unit sets the current path and the first alternative path in such a way that impedance of the variable resistor unit as seen from the operational amplifier is constant regardless of a state of the control signal.

4. The semiconductor integrated circuit according to claim 2 , wherein the variable resistor unit further sets a second alternative path with the second resistance value in accordance with the control signal between the external input terminal and the reference voltage terminal.

5. The semiconductor integrated circuit according to claim 4 , wherein the variable resistor unit sets the current path, the first alternative path and the second alternative path in such a way that impedance of the variable resistor unit as seen from the operational amplifier is constant regardless of a state of the control signal and that impedance of the variable resistor unit as seen from the external input terminal is constant regardless of a state of the control signal.

6. The semiconductor integrated circuit according to claim 1 , wherein the variable resistor unit sets the first alternative path between the external input terminal and the reference voltage terminal.

7. A semiconductor integrated circuit comprising:

an operational amplifier;

a first feedback resistor that performs negative feedback of one of differential output signals of the operational amplifier;

a second feedback resistor that performs negative feedback of another one of differential output signals of the operational amplifier; and

a variable resistor unit that sets first and second current paths with a first resistance value in accordance with a control signal between a first external input terminal and an inverting input terminal of the operational amplifier and between a second external input terminal and a non-inverting input terminal of the operational amplifier, respectively, and sets a first alternative path with a second resistance value in accordance with the control signal between nodes on the first and second current paths,

wherein the variable resistor unit includes:

a plurality of first resistor elements placed in parallel between the first external input terminal and the inverting input terminal of the operational amplifier,

a plurality of second resistor elements placed in parallel between the second external input terminal and the non-inverting input terminal of the operational amplifier,

a plurality of first switching elements respectively placed between one ends of the plurality of first resistor elements and the inverting input terminal of the operational amplifier and having a conducting state controlled based on the control signal,

a plurality of second switching elements respectively placed between one ends of the plurality of second resistor elements and the non-inverting input terminal of the operational amplifier and having a conducting state controlled based on the control signal,

a plurality of first resistance attenuators respectively placed between other ends of the plurality of first resistor elements,

a plurality of second resistance attenuators respectively placed between other ends of the plurality of second resistor elements,

a plurality of third switching elements respectively placed between common terminals of the plurality of first resistance attenuators and a common node and having a conducting state controlled based on the control signal, and

a plurality of fourth switching elements respectively placed between common terminals of the plurality of second resistance attenuators and the common node and having a conducting state controlled based on the control signal.

8. The semiconductor integrated circuit according to claim 7 , wherein the variable resistor unit sets the first alternative path between the inverting input terminal and the non-inverting input terminal of the operational amplifier.

9. The semiconductor integrated circuit according to claim 8 , wherein the variable resistor unit sets the first current path, the second current path and the first alternative path in such a way that impedance of the variable resistor unit as seen from the operational amplifier is constant regardless of a state of the control signal.

10. The semiconductor integrated circuit according to claim 8 , wherein the variable resistor unit further sets a second alternative path with the second resistance value in accordance with the control signal between the first and second external input terminals.

11. The semiconductor integrated circuit according to claim 10 , wherein the variable resistor unit sets the first current path, the second current path, the first alternative path and the second alternative path in such a way that impedance of the variable resistor unit as seen from the operational amplifier is constant regardless of a state of the control signal and that impedance of the variable resistor unit as seen from the first and second external input terminals is constant regardless of a state of the control signal.

12. The semiconductor integrated circuit according to claim 7 , wherein the variable resistor unit sets the first alternative path between the first and second external input terminals.

13. A radio communication terminal comprising a semiconductor integrated circuit according to claim 1 .

14. A radio communication terminal comprising a semiconductor integrated circuit according to claim 7 .

15. A semiconductor integrated circuit comprising:

an operational amplifier that amplifies a voltage difference between an input voltage supplied to an inverting input terminal and a reference voltage supplied to a non-inverting input terminal and outputs an amplified signal;

a feedback resistor that performs negative feedback of the amplified signal to the inverting input terminal of the operational amplifier; and

a variable resistor unit that sets a current path with a first resistance value in accordance with a control signal between an external input terminal and the inverting input terminal of the operational amplifier, and sets a first alternative path with a second resistance value in accordance with the control signal between a node on the current path and a reference voltage terminal to which the reference voltage is supplied,

wherein the variable resistor unit includes:

a plurality of first resistor elements placed in parallel between the external input terminal and the inverting input terminal of the operational amplifier,

a plurality of first switching elements respectively placed between the plurality of first resistor elements and the inverting input terminal of the operational amplifier and having a conducting state controlled based on the control signal,

a plurality of second resistor elements placed in parallel between the inverting input terminal of the operational amplifier and the reference voltage terminal, and

a plurality of second switching elements respectively connected in series with the plurality of second resistor elements and having a conducting state controlled based on the control signal, and

wherein the plurality of first switching elements connect each of the plurality of first resistor elements to one of the inverting input terminal of the operational amplifier and the reference voltage terminal based on the control signal.

16. The semiconductor integrated circuit according to claim 15 ,

wherein the variable resistor unit sets the first alternative path between the inverting input terminal of the operational amplifier and the reference voltage terminal, and

wherein the variable resistor unit further sets a second alternative path with the second resistance value in accordance with the control signal between the external input terminal and the reference voltage terminal.

17. The semiconductor integrated circuit according to claim 16 , wherein the variable resistor unit sets the current path, the first alternative path and the second alternative path in such a way that impedance of the variable resistor unit as seen from the operational amplifier is constant regardless of a state of the control signal and that impedance of the variable resistor unit as seen from the external input terminal is constant regardless of a state of the control signal.

18. The semiconductor integrated circuit according to claim 15 , wherein the variable resistor unit sets the first alternative path between the external input terminal and the reference voltage terminal.

19. A radio communication terminal comprising a semiconductor integrated circuit according to claim 15 .