Transimpedance amplifier
A variable resistance element is connected between a first input terminal of a first amplifier and a second input terminal of a second amplifier, and has a resistance value between the first input terminal and the second input terminal that is varied according to an amplitude value of a first voltage signal or an amplitude value or a differential voltage signal. A variable current source is connected between the first input terminal and a ground, and controls a current value of a current flowing to the ground from the first input terminal according to a value of an offset of the differential voltage signal. A bias voltage having the same value as that of a bias voltage that is applied to the first input terminal is applied to the second input terminal.
1. A transimpedance amplifier that receives an input current signal from a photodetector, and outputs a differential voltage signal, the amplifier comprising:
an input terminal configured to receive the input current signal;
a first amplifier including a first input terminal electrically connected to the input terminal, and generating a first voltage signal in response to a first current signal input into the first input terminal;
a second amplifier including a second input terminal, and generating a second voltage signal in response to a second current signal input into the second input terminal;
a differential amplifier electrically connected to the first amplifier, the differential amplifier being configured to generate the differential voltage signal in response to a difference between the first voltage signal and a reference voltage signal;
a variable resistance element electrically connected between the first input terminal and the second input terminal, the variable resistance element having a resistance value varied according to an amplitude value of the first voltage signal or an amplitude value of the differential voltage signal; and
a variable current source electrically connected between the first input terminal and a ground, the variable current source being configured to vary a current value of a direct current flowing to the ground from the input terminal according to a value of an offset of the differential voltage signal,
wherein the first input terminal is biased to a voltage value equal to a voltage value to which the second input terminal is biased.
2. The transimpedance amplifier according to claim 1 ,
wherein the variable resistance element is an FET including a first current terminal electrically connected to the first input terminal, a second current terminal electrically connected to the second input terminal, and a control terminal receiving a control signal generated according to the amplitude value of the first voltage signal or the amplitude value of the differential voltage signal, the FET varying a resistance value between the first current terminal and the second current terminal in response to the control signal.
3. The transimpedance amplifier according to claim 1 ,
wherein the resistance value of the variable resistance element between the input terminal and the second input terminal is set to a first resistance value when a value of the first current signal is smaller than a predetermined value, and is set to a second resistance value less than the first resistance value when the value of the first current signal is larger than the predetermined value.
4. The transimpedance amplifier according to claim 1 ,
wherein the first amplifier includes a bipolar transistor,
the bipolar transistor of the first amplifier includes a base electrically connected to the first input terminal, a collector electrically connected to a power supply line, and an emitter electrically connected to the ground,
the first input terminal is biased to a bias voltage including a base-emitter voltage of the bipolar transistor of the first amplifier,
the second amplifier includes a bipolar transistor,
the bipolar transistor of the second amplifier includes a base electrically connected to the second input terminal, a collector electrically connected to the power supply line, and an emitter electrically connected to the ground, and
the second input terminal is biased to a bias voltage including a base-emitter voltage of the bipolar transistor of the second amplifier.
5. The transimpedance amplifier according to claim 1 ,
wherein the variable current source has an alternating-current impedance when the variable current source is seen from the first input terminal, and the alternating-current impedance is set to be greater than an alternating-current impedance of the variable resistance element when the variable resistance element is seen from the first input terminal.
6. The transimpedance amplifier according to claim 2 ,
wherein the control terminal of the FET receives the control signal set to a voltage value greater than a voltage value of the first current terminal and a voltage value of the second current terminal.
7. The transimpedance amplifier according to claim 1 ,
wherein the variable current source is an FET including a first current terminal electrically connected to the input terminal, a second current terminal electrically connected to the ground, and a control terminal, and
the control terminal of the FET receives a control signal generated according to an offset of the differential voltage signal.
8. The transimpedance amplifier according to claim 7 ,
wherein the control signal is set to a voltage value between a voltage value of the first current terminal and a voltage value of the second current terminal.