Transconductance amplifier
Provided is a transconductance amplifier capable of suppressing variation in the range of a linear relationship between an input voltage and an output current depending on the magnitude of a tuning voltage Vctrl, thereby adjusting transconductance over a wider range of operating input voltages. The transconductance amplifier is configured by a differential pair formed of MOS transistors ( 111, 112 ) having a common source, MOS transistors ( 113, 114 ), amplifiers ( 106, 107 ), a voltage generator circuit ( 100 ), and a differential-pair input voltage generator circuit ( 120 ). An input differential common voltage Vcm of all differential signals inputted to the differential pair is adjusted so that a difference between Vcm and Vctrl is equal to a constant, in accordance with a change in the tuning voltage Vctrl that controls the transconductance. This enables keeping constant the range in which the transconductance amplifier can achieve good linearity.
1. A transconductance amplifier which supplies an output current proportional to an input voltage, characterized by comprising:
a differential pair which is formed of first and second MOS transistors having a common source, and which operates in a triode region;
a third MOS transistor of which a source terminal is connected to a drain terminal of the first MOS transistor, and which operates in a saturation region;
a fourth MOS transistor of which a source terminal is connected to a drain terminal of the second MOS transistor, and which operates in the saturation region;
a first amplifier of which a negative input terminal is connected to the source terminal of the third MOS transistor and of which an output terminal is connected to a gate terminal of the third MOS transistor;
a second amplifier of which negative input terminal is connected to the source terminal of the fourth MOS transistor and of which output terminal is connected to a gate terminal of the fourth MOS transistor;
a voltage generator circuit which generates a tuning voltage inputted to positive input terminal voltages of the first and second amplifiers, and a common voltage of first and second voltages inputted to the differential pair so that a difference between the tuning voltage and the common voltage is equal to a constant; and
a differential-pair input voltage generator circuit which is supplied with a common voltage, and which generates the first voltage outputted to a gate terminal of the first MOS transistor, and the second voltage outputted to a gate terminal of the second MOS transistor, characterized in that
the second voltage is equal to 2×(the common voltage)−(the first voltage),
the input voltage is equal to a difference between the first voltage and the second voltage, and
the output current is equal to a difference between a first current Ip flowing through the drains and sources of the first and third MOS transistors and a second current In flowing through the drains and sources of the second and fourth MOS transistors.
2. The transconductance amplifier according to claim 1 , characterized in that
the voltage generator circuit includes a voltage generator, a fixed current source, and a resistance connected in series between the voltage generator and an output terminal of the fixed current source,
the tuning voltage is outputted from between the voltage generator and the resistance, and the common voltage is outputted from between the resistance and the fixed current source.
3. The transconductance amplifier according to claim 1 , characterized in that
the voltage generator circuit includes a voltage generator, a fixed current source, and a resistance connected in series between the voltage generator and an input terminal of the fixed current source,
the common voltage is at a contact point between the voltage generator and the resistance, and the tuning voltage is at a contact point between the resistance and the fixed current source.
4. The transconductance amplifier according to claim 3 , characterized in that
the voltage generator includes a second current source, fifth and sixth MOS transistors, and a third amplifier, which are connected in series,
a source terminal of the sixth MOS transistor, a drain terminal of the fifth transistor, and a negative input terminal of the third amplifier are connected to one another,
a gate terminal of the fifth transistor having a common source is connected to a drain terminal of the sixth MOS transistor and an output terminal of the second current source,
a gate voltage of the fifth MOS transistor is the common voltage, and a positive input terminal voltage of the third amplifier is the tuning voltage.
5. The transconductance amplifier according to claim 4 , characterized in that
the fifth MOS transistor is configured to have a current-mirror relationship to the first and second MOS transistors, and the sixth MOS transistor is configured to have a current-mirror relationship to the third and fourth MOS transistors.
6. The transconductance amplifier according to any one of claims 4 and 5 , characterized in that the second current source is variable.
7. The transconductance amplifier according to any one of claims 2 to 5 , characterized in that the voltage generator is variable.
8. The transconductance amplifier according to claim 6 , characterized in that the voltage generator is variable.