Transconductance amplifier

• US 7,847,635 B2
• Filed: 08/27/2007
• Issued: 12/07/2010
• Est. Priority Date: 08/28/2006
• Status: Active Grant

• Current Assignee: Asahi Kasei EMD Corp (Asahi Kasei Corporation)
• Original Assignee: Asahi Kasei EMD Corp (Asahi Kasei Corporation)
• Inventors: Aiba, Yusuke
• Primary Examiner(s): Mottola; Steven J
• Application Number:
  US12/439,093
• Publication Number:
  US 20100013556A1
• Time in Patent Office: 1,198 Days
• Field of Search: 330/254, 330/260, 330/305, 327/359
• US Class Current: 330/254
• CPC Class Codes: H03F 1/3211 in differential amplifiers
• CPC Class Codes: H03F 2203/45052 the cascode stage of the cascode differential amplifier being controlled by a controlling signal, which controlling signal can also be the input signal
• CPC Class Codes: H03F 2203/45066 the resulting deducted common mode signal being added at the one or more inputs of the differential amplifier
• CPC Class Codes: H03F 2203/45101 Control of the DC level being present
• CPC Class Codes: H03F 2203/45292 the AAC comprising biasing means controlled by the signal
• CPC Class Codes: H03F 2203/45356 the AAC comprising one or more op-amps, e.g. IC-blocks
• CPC Class Codes: H03F 2203/45394 the AAC of the dif amp comprising FETs whose sources are not coupled, i.e. the AAC being a pseudo-differential amplifier
• CPC Class Codes: H03F 2203/45481 the CSC comprising only a direct connection to the supply voltage, no other components being present
• CPC Class Codes: H03F 2203/45586 the IC comprising offset generating means
• CPC Class Codes: H03F 3/45179 using MOSFET transistors as the active amplifying circuit H03F3/45278 takes precedence
• CPC Class Codes: H03F 3/45183 Long tailed pairs H03F3/4521, H03F3/45237 take precedence
• CPC Class Codes: H03F 3/45188 Non-folded cascode stages
• CPC Class Codes: H03G 1/0023 in emitter-coupled or cascode amplifiers H03G1/0029 takes precedence
• CPC Class Codes: H03G 1/0029 using FETs
• CPC Class Codes: H03G 1/04 Modifications of control circuit to reduce distortion caused by control modifications to reduce influence of variations of internal impedance of amplifying elements caused by control H03F1/08

Abstract

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.

Claims

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.