Driver circuit and optical transmitter

1. A driver circuit comprising a plurality of amplifiers cascade-connected so as to sequentially amplify an input signal,

the plurality of amplifiers each including a differential amplifier that comprises a positive-side power supply terminal, a negative-side power supply terminal, and a tail current source formed by a transistor having a gate terminal connected to a fixed potential and a source terminal connected to one of the positive-side power supply terminal and the negative-side power supply terminal,

wherein the plurality of amplifiers except for a final-stage amplifier among the plurality of amplifiers is connected to a positive-side power supply line and a negative-side power supply line in one of a first connection form and a second connection form,

the final-stage amplifier has the negative-side power supply terminal connected to the negative-side power supply line,

the first connection form is a form in which the positive-side power supply terminal of a first-stage amplifier is connected to the positive-side power supply line, the negative-side power supply terminal of an amplifier immediately preceding the final-stage amplifier is connected to the negative-side power supply line, and, as for amplifiers from the first-stage amplifier to the amplifier immediately preceding the final-stage amplifier, the negative-side power supply terminal of a preceding-stage amplifier and the positive-side power supply terminal of an amplifier succeeding the preceding-stage amplifier are connected to each other, and

the second connection form is a form in which the negative-side power supply terminal of the first-stage amplifier is connected to the negative-side power supply line, the positive-side power supply terminal of the amplifier immediately preceding the final-stage amplifier is connected to the positive-side power supply line, and, as for the amplifiers from the first-stage amplifier to the amplifier immediately preceding the final-stage amplifier, the positive-side power supply terminal of the preceding-stage amplifier and the negative-side power supply terminal of the amplifier succeeding the preceding-stage amplifier are connected to each other.

2. The driver circuit according to claim 1 , wherein a power supply voltage supplied between the positive-side power supply terminal and the negative-side power supply terminal of each of the amplifiers from the first-stage amplifier to the final-stage amplifier increases in an order from the first-stage amplifier to the final-stage amplifier.

3. The driver circuit according to claim 1 , further comprising a level shifter configured to adjust a level of the signal on at least one of paths of the signal between the amplifiers from the first-stage amplifier to the amplifier immediately preceding the final-stage amplifier,

wherein in the first connection form, a positive-side power supply terminal of the level shifter is connected to the positive-side power supply terminal of an amplifier immediately preceding the level shifter, and a negative-side power supply terminal of the level shifter is connected to the negative-side power supply terminal of an amplifier immediately succeeding the level shifter, and

in the second connection form, the positive-side power supply terminal of the level shifter is connected to the positive-side power supply terminal of the amplifier immediately succeeding the level shifter, and the negative-side power supply terminal of the level shifter is connected to the negative-side power supply terminal of the amplifier immediately preceding the level shifter.

4. The driver circuit according to claim 1 , wherein the first-stage amplifier is formed from a group of a plurality of cascade-connected amplifiers.

5. The driver circuit according to claim 1 , wherein

at least two of the amplifiers from the first-stage amplifier to the amplifier immediately preceding the final-stage amplifier are each formed from a group of a plurality of cascade-connected amplifiers, and

the number of stages of the amplifiers forming the preceding-stage group is equal to or greater than the number of stages of the amplifiers forming the succeeding-stage group.

6. The driver circuit according to claim 4 , wherein a current flowing into each of the amplifiers forming the group increases toward the later-stage amplifier.

7. The driver circuit according to claim 1 , wherein

at least one of the amplifiers from the first-stage amplifier to the amplifier immediately preceding the final-stage amplifier is a variable gain amplifier, and

the variable gain amplifier includes

a first transistor for amplification to which a non-inverting input voltage signal is input, including an output that is connected to an inverting output terminal of the variable gain amplifier,

a second transistor for amplification to which an inverting input voltage signal is input, including an output that is connected to a non-inverting output terminal of the variable gain amplifier,

a first variable resistor connected between the positive-side power supply line and the inverting output terminal,

a second variable resistor connected between the positive-side power supply line and the non-inverting output terminal,

a first series circuit connected between the positive-side power supply line and the inverting output terminal, and including a first resistor and a first inductor, and

a second series circuit connected between the positive-side power supply line and the non-inverting output terminal, and including a second resistor and a second inductor.

8. The driver circuit according to claim 7 , further comprising:

a third inductor between the inverting output terminal of the variable gain amplifier and a terminal on a side of the inverting output terminal of the first series circuit, and a terminal on a side of the inverting output terminal of the first variable resistor; and

a fourth inductor between the non-inverting output terminal of the variable gain amplifier and a terminal on a side of the non-inverting output terminal of the second series circuit, and a terminal on a side of the non-inverting output terminal of the second variable resistor.

9. The driver circuit according to claim 1 , wherein

at least one of the amplifiers from the first-stage amplifier to the amplifier immediately preceding the final-stage amplifier is a variable gain amplifier, and

the variable gain amplifier includes

a first transistor for amplification to which a non-inverting input voltage signal is input, including an output that is connected to an inverting output terminal of the variable gain amplifier,

a second transistor for amplification to which an inverting input voltage signal is input, including an output that is connected to a non-inverting output terminal of the variable gain amplifier,

a variable resistor connected between the inverting output terminal and the non-inverting output terminal,

a first series circuit connected between the positive-side power supply line and the inverting output terminal, and including a first resistor and a first inductor, and

a second series circuit connected between the positive-side power supply line and the non-inverting output terminal, and including a second resistor and a second inductor.

10. The driver circuit according to claim 9 , further comprising:

a third inductor between the inverting output terminal of the variable gain amplifier and a terminal on a side of the inverting output terminal of the first series circuit; and

a fourth inductor between the non-inverting output terminal of the variable gain amplifier and a terminal on a side of the non-inverting output terminal of the second series circuit.

11. The driver circuit according to claim 1 , wherein the final-stage amplifier has the positive-side power supply terminal connected to the positive-side power supply line.

12. An optical transmitter comprising:

a driver circuit defined in claim 1 ; and

an optical modulator configured to be driven by the driver circuit,

wherein an output terminal of a final-stage amplifier is connected to a positive-side power supply line via the optical modulator.

13. The driver circuit according to claim 2 , further comprising a level shifter configured to adjust a level of the signal on at least one of paths of the signal between the amplifiers from the first-stage amplifier to the amplifier immediately preceding the final-stage amplifier,

wherein in the first connection form, a positive-side power supply terminal of the level shifter is connected to the positive-side power supply terminal of an amplifier immediately preceding the level shifter, and a negative-side power supply terminal of the level shifter is connected to the negative-side power supply terminal of an amplifier immediately succeeding the level shifter, and

in the second connection form, the positive-side power supply terminal of the level shifter is connected to the positive-side power supply terminal of the amplifier immediately succeeding the level shifter, and the negative-side power supply terminal of the level shifter is connected to the negative-side power supply terminal of the amplifier immediately preceding the level shifter.

14. The driver circuit according to claim 2 , wherein the first-stage amplifier is formed from a group of a plurality of cascade-connected amplifiers.

15. The driver circuit according to claim 2 , wherein

at least two of the amplifiers from the first-stage amplifier to the amplifier immediately preceding the final-stage amplifier are each formed from a group of a plurality of cascade-connected amplifiers, and

the number of stages of the amplifiers forming the preceding-stage group is equal to or greater than the number of stages of the amplifiers forming the succeeding-stage group.

16. The driver circuit according to claim 5 , wherein a current flowing into each of the amplifiers forming the group increases toward the later-stage amplifier.

17. The driver circuit according to claim 2 , wherein

at least one of the amplifiers from the first-stage amplifier to the amplifier immediately preceding the final-stage amplifier is a variable gain amplifier, and

the variable gain amplifier includes

a first transistor for amplification to which a non-inverting input voltage signal is input, including an output that is connected to an inverting output terminal of the variable gain amplifier,

a second transistor for amplification to which an inverting input voltage signal is input, including an output that is connected to a non-inverting output terminal of the variable gain amplifier,

a first variable resistor connected between the positive-side power supply line and the inverting output terminal,

a second variable resistor connected between the positive-side power supply line and the non-inverting output terminal,

a first series circuit connected between the positive-side power supply line and the inverting output terminal, and including a first resistor and a first inductor, and

a second series circuit connected between the positive-side power supply line and the non-inverting output terminal, and including a second resistor and a second inductor.

18. The driver circuit according to claim 17 , further comprising:

a third inductor between the inverting output terminal of the variable gain amplifier and a terminal on a side of the inverting output terminal of the first series circuit, and a terminal on a side of the inverting output terminal of the first variable resistor; and

a fourth inductor between the non-inverting output terminal of the variable gain amplifier and a terminal on a side of the non-inverting output terminal of the second series circuit, and a terminal on a side of the non-inverting output terminal of the second variable resistor.

19. The driver circuit according to claim 2 , wherein

at least one of the amplifiers from the first-stage amplifier to the amplifier immediately preceding the final-stage amplifier is a variable gain amplifier, and

the variable gain amplifier includes

a first transistor for amplification to which a non-inverting input voltage signal is input, including an output that is connected to an inverting output terminal of the variable gain amplifier,

a second transistor for amplification to which an inverting input voltage signal is input, including an output that is connected to a non-inverting output terminal of the variable gain amplifier,

a variable resistor connected between the inverting output terminal and the non-inverting output terminal,

a first series circuit connected between the positive-side power supply line and the inverting output terminal, and including a first resistor and a first inductor, and

a second series circuit connected between the positive-side power supply line and the non-inverting output terminal, and including a second resistor and a second inductor.

20. The driver circuit according to claim 19 , further comprising:

a third inductor between the inverting output terminal of the variable gain amplifier and a terminal on a side of the inverting output terminal of the first series circuit; and

a fourth inductor between the non-inverting output terminal of the variable gain amplifier and a terminal on a side of the non-inverting output terminal of the second series circuit.