Drive circuit and optical network unit

1. A drive circuit comprising:

a bias current supply circuit for supplying a bias current to a light-emitting device for transmitting an optical signal, the light-emitting device being included in a light-emitting circuit; and

a modulation current supply circuit for supplying the light-emitting device with a modulation current of a magnitude according to a logical value of data to be transmitted, wherein

the modulation current supply circuit includes:

a differential drive circuit for switching whether to supply a current to the light-emitting device according to the logical value of the data; and

a termination resistor connected between differential outputs of the differential drive circuit and AC-coupled to a node without interposing the light-emitting circuit, the node being supplied with a fixed voltage, and,

the differential drive circuit and the light-emitting circuit are DC-coupled to each other, and power supply of the current supplied to the light-emitting device by the differential drive circuit is supplied from the light-emitting circuit.

2. The drive circuit according to claim 1 , wherein

the differential drive circuit includes:

a first transistor having a first conduction electrode and a second conduction electrode, the first conduction electrode being DC-coupled to a first end of the light-emitting device;

a second transistor having a first conduction electrode and a second conduction electrode, the first conduction electrode being DC-coupled to a second end of the light-emitting device, and the second conduction electrode being electrically connected to the second conduction electrode of the first transistor; and

a first termination resistor and a second termination resistor that are connected in series between the first conduction electrode of the first transistor and the first conduction electrode of the second transistor, wherein

a connection node between the first conduction electrode of the first transistor and the first termination resistor is DC-coupled to a connection node between the first end of the light-emitting device and a node to which a direct-current power supply voltage is supplied, and

a connection node between the first conduction electrode of the second transistor and the second termination resistor is DC-coupled to a connection node between the second end of the light-emitting device and the bias current supply circuit.

3. The drive circuit according to claim 2 , wherein

the modulation current supply circuit further includes:

a capacitor connected between a node to which a fixed voltage is supplied and a connection node between the first termination resistor and the second termination resistor.

4. An optical network unit in a communication system where optical signals from a plurality of optical network units to an optical line terminal are time-division multiplexed, the optical network unit comprising:

a light-emitting circuit including a light-emitting device for transmitting the optical signal; and

a drive circuit for driving the light-emitting device, wherein

the drive circuit includes:

a bias current supply circuit for supplying a bias current to the light-emitting device; and

a modulation current supply circuit for supplying the light-emitting device with a modulation current of a magnitude according to a logical value of data to be transmitted, and,

the modulation current supply circuit includes:

a differential drive circuit for switching whether to supply a current to the light-emitting device according to the logical value of the data; and

a termination resistor connected between differential outputs of the differential drive circuit and AC-coupled to a node without interposing the light-emitting circuit, the node being supplied with a fixed voltage, and,

the differential drive circuit and the light-emitting circuit are DC-coupled to each other, and power supply of the current supplied to the light-emitting device by the differential drive circuit is supplied from the light-emitting circuit.