High-speed electro-optical modulator drivers and method

• US 6,836,185 B1
• Filed: 05/17/2002
• Issued: 12/28/2004
• Est. Priority Date: 05/17/2002
• Status: Active Grant

• Current Assignee: Marvell Asia Pte Limited (Marvell Technology Group Limited)
• Original Assignee: Inphi Corporation
• Inventors: Pobanz, Carl Walter
• Primary Examiner(s): Choe, Henry
• Application Number:
  US10/151,059
• Time in Patent Office: 956 Days
• Field of Search: 330/259, 330/260, 330/290, 330/291, 330/310, 330/311, 330/85, 330/88, 330/99, 330/104
• US Class Current: 330/260
• CPC Class Codes: G02F 1/0327 Operation of the cell; Circuit arrangements G02F1/05 takes precedence
• CPC Class Codes: H03F 2200/36 the amplifier comprising means for increasing the bandwidth
• CPC Class Codes: H03F 2203/45516 the FBC comprising a coil and being coupled between the LC and the IC
• CPC Class Codes: H03F 2203/45521 the FBC comprising op amp stages, e.g. cascaded stages of the dif amp and being coupled between the LC and the IC
• CPC Class Codes: H03F 2203/45554 the IC comprising one or more coils
• CPC Class Codes: H03F 2203/45601 the IC comprising one or more passive resistors by feedback
• CPC Class Codes: H03F 2203/45638 the LC comprising one or more coils
• CPC Class Codes: H03F 2203/45652 the LC comprising one or more further dif amp stages, either identical to the dif amp or not, in cascade
• CPC Class Codes: H03F 2203/45691 the LC comprising one or more transistors as active loading resistors
• CPC Class Codes: H03F 3/45183 Long tailed pairs H03F3/4521, H03F3/45237 take precedence

Abstract

A modulator driver design is disclosed that employs a differential pair amplifier coupled to feedback amplifiers through tuning networks. Each tuning network comprises a set of inductors that enables a broadband response while reducing the loading effect of the feedback amplifier. An active load is placed at the output to serve multiple purposes, including: generating a high output swing, enabling a lower power supply voltage, and allowing the entire bias circuit to be monolithically integrated. A modulator driver comprises: a first amplifier stage (A1) having inputs and outputs; a second amplifier stage (A2), having inputs and outputs, the inputs of the second amplifier coupled to the outputs of the first amplifier; an active load having inputs and outputs, the inputs of the active load coupled to the outputs of the amplifier stage; and a feedback stage (A3) having inputs and outputs, the inputs of the feedback stage coupled to the outputs of the second amplifier stage by means of a tuning network, and the outputs of the feedback stage coupled to the inputs of the second amplifier stage.

Claims

1. A modulator driver, comprising:

a first amplifier stage (A1) having inputs and outputs, the first amplifier stage comprising a first differential pair of transistors;

a second amplifier stage (A2) having inputs and outputs, the second amplifier stage comprising a second differential pair of transistors, the inputs of the second amplifier stage coupled to the outputs of the first amplifier stage;

an active load coupled between a supply voltage and the outputs of the second amplifier stage and which produces an output voltage that varies with the electrical current conducted by said second differential pair of transistors, said active load arranged to provide an AC resistance that is greater than its equivalent DC resistance; and

a feedback stage (A3) having inputs and outputs, the inputs of the feedback stage commonly coupled to the outputs of the second stage amplifier and the inputs of the active load, the outputs of the feedback stage commonly coupled to the inputs of the second stage amplifier and the outputs of the first amplifier stage.

2. The modulator driver of claim 1, wherein the feedback stage comprises a first feedback amplifier and a second feedback amplifier.

3. The modulator driver of claim 2, wherein the inputs of the first stage amplifier comprises positive and negative inputs, and wherein the outputs of the first stage amplifier comprises positive and negative outputs, the negative input having signal voltages and currents that are inverted relative to the positive input, the negative output having signal voltages and currents that are inverted relative to the positive output.

4. The modulator driver of claim 3, wherein the inputs of the second stage amplifier comprises positive and negative inputs, and wherein the outputs of the second stage amplifier comprise positive and negative outputs, the negative input having signal voltages and currents that are inverted relative to the positive input, the negative output having signal voltages and currents that are inverted relative to the positive output.

29. The modulator driver of claim 1, wherein said active load comprises at least one depletion mode field-effect transistor (FET) having its gate and source connected together.

5. A modulator driver, comprising:

a first amplifier stage (A1) having inputs and outputs, the first amplifier stage comprising a first differential pair of transistors, wherein the inputs of the first amplifier stage comprise positive and negative inputs having signal voltages and currents that are inverted with respect to each other and the outputs of the first amplifier stage comprise positive and negative outputs having signal voltages and currents that are inverted with respect to each other;

a second amplifier stage (A2) having inputs and outputs, the second amplifier stage comprising a second differential pair of transistors, the inputs of the second amplifier stage coupled to the outputs of the first amplifier stage, wherein the inputs of the second amplifier stage comprise positive and negative inputs having signal voltages and currents that are inverted with respect to each other and the outputs of the second amplifier stage comprise positive and negative outputs having signal voltages and currents that are inverted with respect to each other;

an active load having inputs and outputs, the inputs of the active load coupled to the outputs of the second amplifier stage;

a feedback stage (A3) having inputs and outputs, the inputs of the feedback stage commonly coupled to the outputs of the second stage amplifier and the inputs of the active load, the outputs of the feedback stage commonly coupled to the inputs of the second stage amplifier and the outputs of the first amplifier stage, said feedback stage comprising a first feedback amplifier and a second feedback amplifier; and

a first tuning network, coupled between the negative output of the second stage amplifier and the input of the first feedback stage, and a second tuning network, coupled between the positive output of the second stage and the input of the second feedback stage.

6. The modulator driver of claim 5, further comprising a first summing node which couples the output of the first feedback stage to the positive input of the second amplifier stage and the negative output of the first amplifier stage, and a second summing node which couples the output of the second feedback stage and the negative input of the second amplifier stage and the positive output of the first amplifier stage.

7. The modulator driver of claim 6, further comprising a predriver amplifier, having differential outputs coupled to the differential inputs of the first amplifier stage.

8. The modulator driver of claim 5, wherein the first tuning network comprises a first inductor (L1) having a first end and a second end, a second inductor (L2) having a first end and a second end, and a third inductor (L3) having a first end and a second end, the first end of the first inductor coupled to the negative output of the second amplifier stage, the second end of the first inductor commonly coupled to the first end of the second inductor and the first end of the third inductor, the second end of the second inductor coupled to a negative input of the active load, the second end of the third inductor coupled to the input of the first feedback stage.

9. The modulator driver of claim 8, further comprising a first resistor (R1) and a fourth inductor (L4), coupled in series between the output of the first feedback stage and the first summing node.

10. The modulator driver of claim 9, wherein the second tuning network comprises a fifth inductor (L5) having a first end and a second end, a sixth inductor (L6) having a first end and a second end, and a seventh inductor (L7) having a first end and a second end, the first end of the fifth inductor coupled to the positive output of the second amplifier stage, the second end of the fifth inductor commonly coupled to the first end of the sixth inductor and the first end of the seventh inductor, the second end of the sixth inductor coupled to a positive input of the active load, the second end of the seventh inductor coupled to the input of the second feedback stage.

11. The modulator driver of claim 10, further comprising a second resistor (R2) and eighth inductor (L8), coupled in series between the output of the second feedback stage and the second summing node.

12. The modulator driver of claim 11, wherein each of the first, second, third, fourth, fifth, sixth, seventh, and eighth inductor comprises a transmission line.

13. The modulator driver of claim 7, wherein the predriver comprises a differential amplifier that exhibits an output impedance lower than its input impedance.

14. The modulator driver of claim 7, wherein the predriver comprises a pair of transistors configured as voltage followers.

15. The modulator driver of claim 5, wherein the first tuning network comprises a first single inductor having a first end and a second end, the first end of the first single inductor coupled to the negative output of the second amplifier stage, and the second end of the first single inductor coupled to the input of the first feedback network.

16. The modulator driver of claim 15, wherein the first single inductor comprises a transmission line.

17. The modulator driver of claim 16, wherein the second tuning network comprises a second single inductor having a first end and a second end, the first end of the second single inductor coupled to the positive output of the second amplifier stage, and the second end of the second single inductor coupled to the input of the second feedback network.

18. The modulator driver of claim 17, wherein the second single inductor comprises a transmission line.

20. The modulator driver of claim 7, wherein the gain of the predriver amplifier is electronically adjustable.

21. The modulator driver of claim 7, wherein the gain of the predriver amplifier is controlled by a variable current-source (CS3).

19. A modulator driver, comprising:

a first amplifier stage (A1) having inputs and outputs, the first amplifier stage comprising a first differential pair of transistors, wherein the inputs of the first amplifier stage comprise positive and negative inputs having signal voltages and currents that are inverted with respect to each other and the outputs of the first amplifier stage comprise positive and negative outputs having signal voltages and currents that are inverted with respect to each other;

a second amplifier stage (A2) having inputs and outputs, the second amplifier stage comprising a second differential pair of transistors, the inputs of the second amplifier stage coupled to the outputs of the first amplifier stage, wherein the inputs of the second amplifier stage comprise positive and negative inputs having signal voltages and currents that are inverted with respect to each other and the outputs of the second amplifier stage comprise positive and negative outputs having signal voltages and currents that are inverted with respect to each other;

an active load having inputs and outputs, the inputs of the active load coupled to the outputs of the second amplifier stage; and

a feedback stage (A3) having inputs and outputs, the inputs of the feedback stage commonly coupled to the outputs of the second stage amplifier and the inputs of the active load, the outputs of the feedback stage commonly coupled to the inputs of the second stage amplifier and the outputs of the first amplifier stage, said feedback stage comprising a first feedback amplifier and a second feedback amplifier;

wherein the first stage amplifier has a negative output coupled to a positive input by means of a third resistor (R3), and a positive output coupled to a negative input by means of a fourth resistor (R4).

22. A method for operating a modulator driver, comprising:

in a first arm of a differential modulator driver;

amplifying a positive input voltage signal by a first amplifier to generate a first output current signal;

amplifying the first output current signal by a second amplifier to generate a first driver output signal;

providing a negative feedback of the first driver output signal to the positive input of the second amplifier by a first feedback amplifier;

wherein between the amplifying and the providing steps, tuning the first driver output signal by separating the second amplifier and the first feedback amplifier by means of an inductor network; and

in a second arm of a differential modulator driver;

amplifying a negative input voltage signal by a first amplifier to generate a second output current signal;

amplifying the second output current signal by a second amplifier to generate a second driver output signal;

providing a negative feedback of the second driver output signal to the negative input of the second amplifier by a second feedback amplifier;

wherein between the amplifying and the providing steps, tuning the second driver output signal by separating the second amplifier and the second feedback amplifier by means of an inductor network.

23. The method of claim 22, wherein the input signal is applied only to one of the inputs of the first amplifier.

24. The method of claim 22, wherein the input signal is applied differentially to both inputs of the first amplifier.

25. The method of claim 22, further comprising producing a high output amplitude from the first driver output signal by a first active load (AL1).

26. The method of claim 25, further comprising producing a high output amplitude from the second driver output signal by a second active load (AL2).

27. The method of claim 22, further comprising:

equalizing the frequency response of a first negative feedback signal;

summing the first negative feedback signal with the first output current signal from a first amplifier; and

providing a first summing signal to a first input of the second amplifier.

28. The method of claim 27, further comprising:

equalizing the frequency response of a second negative feedback signal;

summing the second negative feedback signal with the second output current signal from a first amplifier; and

providing a second summing signal to a second input of the second amplifier.