Light source driver circuit that uses a low supply voltage and is capable of operating at a high bandwidth

A light source driver circuit is provided that has at least first and second current source circuits that are electrically coupled to a node of the driver circuit. The first and second current source circuits source first and second fractions, respectively, of a total current needed to drive a light source into a node of the driver circuit. The driver circuit uses a sum of the first and second fractions of the total current in combination with a modulation current to drive the light source. By incorporating at least first and second current source circuits into the driver circuit, each of the current sources can be kept sufficiently small in size that they contribute very little parasitic capacitance, and therefore allow the driver circuit to achieve high-bandwidth operations while also allowing the driver circuit to operate at a low supply voltage.

TECHNICAL FIELD OF THE INVENTION

The invention relates to light source driver circuits. More particularly, the invention relates to a light source driver circuit that uses a low supply voltage and that is capable of operating at high bandwidth.

BACKGROUND OF THE INVENTION

Vertical cavity surface emitting laser diodes (VCSELs) are laser diodes that generate light when driven with an electrical current. In general, an input electrical current drives the anode of the VCSEL causing the light emitting region, or aperture, of the VCSEL to emit a laser light beam. A VCSEL driver circuit is a circuit, typically an integrated circuit (IC), that receives a high-speed electrical voltage signal, converts this electrical voltage signal into an electrical current signal, and drives the anode of the VCSEL with a static direct current (DC) bias current that modulated with the converted high-speed current signal.

FIG. 1illustrates a typical final output stage of a typical laser driver IC1that uses a 4.5-volt voltage supply. The anode2of the VCSEL3is driven with a bias current of 8 milliampere (mA) along with a modulation current of 10 mA (+/−5 mA). A modulator circuit4comprises an emitter-coupled NPN bipolar junction transistor (BJT) differential pair comprising first and second emitter coupled BJTs5aand5b, respectively, a load resistor6and current tail7. The NPN BJT differential pair senses the polarity of the electrical input signal, VINP and VINM, and sinks either 0 mA or 10 mA from the output node8of the VCSEL driver IC1. A P Metal Oxide Semiconductor Field Effect Transistor (PMOSFET) current source9sources a current equal to the sum of the bias current and one-half of the modulation current. PMOSFETs11comprise a current mirror. The difference between the current sourced by the PMOSFET current source9and the current sunk by the NPN BJT differential pair5a,5bis sent to the output node8.

The PMOSFETs used in the PMOSFET current source9generally have a poor current-to-device dimension ratio compared to NPN BJTs for the same voltage across the device. Consequently, a PMOSFET current source presents more parasitic capacitance at its output terminals, resulting in poor high-bandwidth performance. In order to keep the parasitic capacitance contribution of the PMOSFETs to a minimum, they are typically permitted to have a large voltage across them, so that they can be kept small in size. Furthermore, the PMOSFET current source9is cascoded to keep the output resistance of the laser driver IC1high; which nearly doubles the required voltage headroom across the PMOSFET current source9. These considerations mandate the need for a 4.5-volt voltage supply, which typically needs to be generated using a boost regulator (not shown) in a 3.3-volt system. The need for a boost generator has negative implications in terms of product area, power consumption, and cost.

Attempts have been made to replace the 4.5-volt voltage supply with a 3.3-volt voltage supply that remove the cascoded arrangement in the PMOSFET current source and increase the size of the PMOSFET current source to account for the loss of 1.2 volts of current source headroom.FIG. 2illustrates a final output stage of a laser driver IC12that has been modified as such to use a 3.3-volt voltage supply by eliminating the cascoded arrangement in the PMOSFET current source13and increasing the size of the PMOSFET current source13to allow the supply voltage to be reduced. One disadvantage of the laser driver IC12is that the increase in the size of the PMOSFET current source13results in a significant increase in parasitic capacitance, which, in turn, results in an unacceptable loss in bandwidth.

Accordingly, a need exists for a laser driver circuit that is capable of operating at a low supply voltage and that is capable of achieving high-bandwidth performance.

DETAILED DESCRIPTION

In accordance with illustrative embodiments, a light source driver circuit is provided that has a modulation circuit, at least first and second current source circuits that are electrically coupled to a node of the driver circuit. The driver circuit has an output terminal that is electrically coupled, either directly or indirectly, to the node and that is disposed to be electrically coupled to an electrode of a light source. The first current source circuit sources a first fraction of a total current needed to drive a light source into the node. The second current source circuit sources a second fraction of the total current needed to drive the light source into the node. The driver circuit uses substantially a sum of the first and second fractions of the total current to drive the light source. The drive current of the light source is determined by the first fraction, the second fraction and the current that dissipates through the modulation circuit. However, the current that dissipates through the modulation circuit is generally negligible compared to the first and second fractions of the total current. The first current source circuit is coupled to the node through via a transistor. The second current source circuit is coupled to the node through a resistor. By incorporating the transistor-based first current source circuit, and the resistor-based second current source circuit into the driver circuit, each of the current sources can be kept sufficiently small in size that they contribute very little parasitic capacitance, and therefore allow the driver circuit to achieve high-bandwidth operations. The small size of the current sources also ensures that the voltage drop across them is sufficiently small that laser driver circuit can operate at a low supply voltage.

Exemplary embodiments will now be described with reference to the figures, in which like reference numerals represent like components, elements or features. It should be noted that features, elements or components in the figures are not intended to be drawn to scale, emphasis being placed instead on demonstrating inventive principles and concepts.

In the following detailed description, for purposes of explanation and not limitation, exemplary, or representative, embodiments disclosing specific details are set forth in order to provide a thorough understanding of inventive principles and concepts. However, it will be apparent to one of ordinary skill in the art having the benefit of the present disclosure that other embodiments according to the present teachings that are not explicitly described or shown herein are within the scope of the appended claims. Moreover, descriptions of well-known apparatuses and methods may be omitted so as not to obscure the description of the exemplary embodiments. Such methods and apparatuses are clearly within the scope of the present teachings, as will be understood by those of skill in the art.

The terminology used herein is for purposes of describing particular embodiments only, and is not intended to be limiting. The defined terms are in addition to the technical, scientific, or ordinary meanings of the defined terms as commonly understood and accepted in the relevant context. For example, when the word PMOS is used, a person skilled in the art would understand that the circuit may be changed into a configuration to change all PMOS transistors with NMOS transistors. Therefore, a similar circuit having NMOS transistors is also within the inventive principles and concepts. Other suitable types of transistors such as bipolar transistors using similar circuitry topology may be considered as well. Similarly, when the term “a signal” is used herein, it is understood that the signal may be a differential signal that requires two input and output terminals. When the term “two input terminals” is used, it is understood that the term may include having a differential input signal which requires two input terminals. The term “voltage supply” may include the positive node (VDD) or the negative node (GND) of the power supply.

The terms “a,” “an” and “the” include both singular and plural referents, unless the context clearly dictates otherwise. Thus, for example, “a device” includes one device and plural devices. The term “substantially” means to within limits or degrees acceptable to those of skill in the art. For example, the term “substantially parallel to” means that a structure or device may not be made perfectly parallel to some other structure or device due to tolerances or imperfections in the process by which the structures or devices are made. The term “approximately” means to within an acceptable limit or amount to one of ordinary skill in the art. Where a first device is said to be directly connected or directly coupled to a second device, this encompasses examples where the two devices are connected together without any intervening devices other than electrical connectors (e.g., wires). Where a first device is said to be electrically coupled to a second device, this encompasses examples where the two devices are directly connected together without any intervening devices other than electrical connectors (e.g., wires) and examples where the first and second devices are electrically connected to one another via one or more intervening devices (e.g., one or more resistors).

Exemplary embodiments will now be described with reference to the figures, in which like reference numerals represent like components, elements or features. It should be noted that features, elements or components in the figures are not intended to be drawn to scale, emphasis being placed instead on demonstrating inventive principles and concepts.

FIG. 3illustrates a system-level block diagram of the low-power, high-bandwidth laser driver circuit50in accordance with an exemplary embodiment. A voltage supply51supplies a low supply voltage to a modulation circuit52and to first and second current source circuits53and54, respectively, of the laser driver circuit50. The first and second current sources53and54, respectively, generate first and second fractions, respectively, of a total bias current needed to bias a laser or a light source (not shown). An output terminal55of the laser driver circuit50is disposed for connection to a cathode or anode of a laser, such as a VCSEL, for example. The first and second fractions of the total bias current are summed together at an output node56of the laser driver circuit50to obtain the total bias current. Following Kirchoff's Current Law, the current flowing to the output terminal55is the sum of the first and second fractions, as well as the current that dissipates through the modulation circuit52. However, the current that dissipates through the modulation circuit52is relatively small compared to the first and second fractions of the total bias current. In one embodiment, the first fraction may be approximately 10 mA, the second fraction may be approximately 8 mA, the current that flows to the output terminal55is approximately 18 mA, but the current that dissipates through the modulation circuit52may be less than one hundred micro amperes. For this reason, the sum of the first and second fractions of the total bias current are considered to be substantially equal to the total bias current needed to drive at least a light source coupled to the output terminal55.

The modulation circuit52receives first and second input data signals of opposite polarity, VIN+ and VIN−, at first and second terminals, respectively, of the modulation circuit52and generates a modulation current that is summed with the total bias current at the output node56. The first and second input data signals of opposite polarity, VIN+ and VIN−, may be referred to as a differential signal. In one embodiment, the modulation circuit may receive a single ended signal with one input terminal. In another embodiment, the modulation circuit may receive two single ended signals with two input terminals. Therefore, when it is mentioned that a modulation circuit52has an input terminal to receive an input data signal, all variations explained above are all included.

The sum of the modulation current and the total bias current drives a laser (not shown) electrically coupled with the output terminal55. The first and second current sources53and54, respectively, are configured to operate at the low supply voltage provided by the voltage supply51and to contribute very little parasitic capacitance, thereby enabling the laser driver circuit50to operate at very high bandwidths associated with an input data signal labeled as VIN+ and VIN− to indicate the differential signal. The first current source circuit53may be coupled to the output terminal55through a transistor. The transistor may be a PMOS transistor biased at an operating state to provide a predetermined current. The second current source circuit54may be coupled to the output terminal55through a resistor. Generally the resistor is a low-resistance resistor having a resistance value lower than approximately 90 ohms. The first current source circuit53may be configured to provide a first fraction of the total bias current needed to drive the laser (not shown). The second current source circuit54may be configured to provide a second fraction of the total bias current needed to drive the laser (not shown). The second fraction is approximately equal to or less than the first fraction.

Intelligently splitting the total bias current into the first and second fractions in the first current source circuit53and the second current source circuit54may enable the drive current to be generated at high speed with a low supply voltage. More specifically, the first current source circuit53that is coupled to the output terminal55is configured to provide the first fraction of the total bias current. On the other hand, the second current source circuit54that is coupled to the output terminal55to provide the second fraction of the total bias current without adding parasitic capacitance, which enables the driver circuit50to operate at high speed. The second current source circuit54may cause power to be dissipated through the resistor.

In one embodiment, the second fraction is approximately 40% to 50% of the total bias current. The voltage level at the output terminal55may change when the input signal changes. Changes of the voltage level at the output terminal55may affect the value of the second fraction. In some embodiments, the second current source circuit54may comprise a control circuit operable to maintain the second fraction within the 40% to 50% range. The first and second current source circuits53and54, respectively, may respond to the changes of the data input signal at different rates. The first current source circuit53reacts to a change of the data input signal at a first rate and the second current source circuit54reacts to the change of the data input signal at a second rate faster than the first rate. Therefore, when there is a change in the data input signal that requires a change of the drive current at the output terminal55, the changes of the drive current due to the change of the data input signal is substantially drawn from the second current source circuit54relative to the first current source circuit53.

When in use, the output terminal55is coupled to a circuit branch comprising at least one light source. The output resistance seen at the output terminal55may be halved or significantly reduced because the resistor at the second current source circuit54is in parallel arrangement with the resistance seen at the output circuit branch having the at least one light source. Therefore, the resistance seen at the output circuit branch may be parasitic resistance and/or the resistance seen at the at least one light source.

Adding the resistor at the second current source circuit54may contribute to additional power consumption. However, by providing the second current source circuit54to supply a substantial portion of the total drive current, the size of the transistor in the first current source circuit53may be reduced and this effectively enables the driver circuit50to operate at a high speed. The additional power consumption can be trade-off to obtain the high speed performance, but combining the first and second current source circuits53and54, respectively, intelligently may improve the overall performance of the driver circuit50and yield a desirable technical effect. Examples (non-exhaustive) of the manner in which the first and second current sources53and54, respectively, may be configured to achieve these goals are described below in detail with reference toFIGS. 4-7.

It should be noted that driver circuit is described herein as a laser driver circuit for driving a laser, such as a VCSEL, the inventive principles and concepts apply to any driver circuit for driving a light source. The light source can be any type of light source including, for example a P-intrinsic-N (PIN) diode, a light emitting diode (LED), etc. For exemplary purposes, the driver circuit is described herein as a laser driver circuit and the light source is described herein as a laser, and in particular, a VCSEL.

FIG. 4illustrates a block diagram of a laser driver circuit100in accordance with an exemplary embodiment that uses a low supply voltage and that is capable of achieving high-bandwidth operations. In accordance with a first exemplary embodiment, the laser driver circuit100uses a 3.3-volt voltage supply and the first and second current source circuits comprise a transistor-based current source circuit110and a resistor-based current source circuit120, respectively. In accordance with this exemplary embodiment, the transistor-based current source circuit110comprises a PMOSFET current source111and a PMOSFET current mirror112and the resistor-based current source circuit120comprises a resistor121and a voltage regulator circuit122. The transistor-based current source circuit110and the resistor-based current source circuit120are electrically in parallel with one another and each is electrically coupled to the 3.3-volt voltage supply123and to node124. The laser driver circuit100includes a modulator circuit101comprising first and second BJTs102and103, respectively, a load resistor104and a current tail105. The modulator circuit101may be identical to, and operate in the same manner as, the modulator circuit4shown inFIGS. 1 and 2.

In accordance with this exemplary embodiment, a total bias current of 18 mA is sourced by the parallel combination of the transistor-based and resistor-based current sources110and120, respectively, into node124. Each of the transistor-based and resistor-based current sources110and120, respectively, sources one-half of the total bias current into node124, although the fractions of the total bias current that are sourced by the current sources110and120can be varied to fractions other than one-half, as will be described below in more detail. A laser125having an anode126that is electrically coupled to an output node127of the laser driver circuit100is driven by the total bias current plus the modulation current produced by the modulation circuit101at the output node127. The output node127is electrically coupled to node124, and therefore node124may also be considered the output node of the laser driver circuit100. In accordance with this exemplary embodiment, the laser125is a VCSEL, although the inventive principles and concepts are not limited to the laser125being any particular type of laser.

The division of the total bias current sourcing requirements between the transistor-based and resistor-based current source circuits110and120, respectively, ensures that the current through the PMOSFET current source111is small enough that the PMOSFET current source111can be small enough in size to have a very low parasitic capacitance. The division of the total bias current sourcing requirements between the transistor-based and resistor-based current source circuits110and120, respectively, also ensures that the voltage drop across the resistor121is small enough to allow the output voltage of the voltage regulator122to be less than 3.3 volts. The reduced parasitic capacitance allows the laser driver circuit100to achieve high-bandwidth performance while the reduced output voltage requirement of the voltage regulator122allows the laser driver circuit100to have a low supply voltage, which is a 3.3-volt voltage supply in this exemplary embodiment.

In addition, the resistor121, which has a resistance of 90 ohm in the exemplary embodiment, reduces the effective output resistance of the laser driver circuit by a factor of two, assuming the laser125is a VCSEL having a resistance of approximately 80 Ohms. This reduction in the effective output resistance of the laser driver circuit100doubles its bandwidth since the resistor-capacitor (RC) time constant at the output of the laser driver circuit100is cut in half. In order to avoid too large of a voltage drop across the resistor121, the PMOSFET current source111supplies the fraction of the total bias current that the resistor121cannot be used to source. The amount of current flowing through the resistor121is controlled by the voltage regulator122.

In accordance with this exemplary embodiment, the resistor-based current source120also includes a reference resistor131, a reference current source132that drives the reference resistor131, and a lowpass filter circuit133. The lowpass filter circuit133has an input terminal that is electrically coupled to node124and an output terminal that is electrically coupled to a first input terminal of the voltage regulator122. A second input terminal of the voltage regulator122is electrically coupled to a first terminal of the reference resistor131. A second terminal of the reference resistor131is electrically coupled to the reference current source132. An output terminal of the voltage regulator122is electrically coupled to a first terminal of the resistor121. A second terminal of the resistor121is electrically coupled to node124. A supply voltage terminal of the voltage regulator is electrically coupled to the 3.3-volt voltage supply123.

The low pass filter circuit133is configured to monitor, as an input, the voltage of the laser driver circuit100at node124and feeds, as an output, a filtered output voltage to the first input terminal of the voltage regulator122. The voltage regulator122compares the filtered output voltage with the first reference voltage across reference resistor131and regulates the voltage signal output from the output terminal of the voltage regulator122to achieve an intended, or predetermined, voltage drop across the resistor121in order to maintain the fraction of the total bias current sourced by the resistor121into node124at the predetermined, or intended, level. The filtered output voltage may be indicative of an average voltage at the node124.

In essence, the voltage regulator122forces the average voltage drop across the reference resistor131, which has a resistance of 2160 Ohm in the exemplary embodiment, to match with the voltage drop across the resistor121, which, as indicated above, has a resistance of 90 Ohm in this exemplary embodiment. Therefore, if 375 uA is sunk from the reference resistor131, it has a voltage drop across it of 0.81 volts, the resistor121has a voltage drop across it of 0.81 volts, and the average current through the resistor121is 375×(2160/90)=9 mA. As indicated above, in accordance with this embodiment, the total bias current needed to be sourced is 18 mA. To maintain the output voltage outputted from the output terminal of the voltage regulator122at approximately 200 millivolts (mV) below the 3.3 volts supplied by the 3.3-volt voltage supply123, the remaining 9 mA of the total bias current is sourced by the PMOSFET current source111. The parasitic capacitance introduced by the PMOSFET current source111is acceptable because of the reduction in effective output resistance of the laser driver circuit100obtained through the use of the resistor121.

FIG. 5illustrates a block diagram of a laser driver circuit200in accordance with another exemplary embodiment that uses a low supply voltage and that is capable of achieving high-bandwidth performance. As with the exemplary embodiment shown inFIG. 4, in accordance with this exemplary embodiment, the laser driver circuit200uses a 3.3-volt voltage supply123and the first and second current source circuits comprise the transistor-based current source circuit110shown inFIG. 4and described above and a resistor-based current source circuit220, respectively. In accordance with this exemplary embodiment, the laser driver circuit200also includes a comparator circuit230and an NMOSFET differential pair240to adjust the fractions of the total bias current that are sourced by the transistor-based and resistor-based current source circuits110and220, respectively, based on the output voltage outputted from the output terminal of the voltage regulator circuit122to ensure that the output voltage does not get to close to the supply voltage rail.

The NMOSFET differential pair240comprises first and second NMOSFETs241and242, respectively, and a current tail243. In accordance with this exemplary embodiment, the current tail243generates a total reference current of 750 microamperes (μA). The gate terminals of the NMOSFETs241and242are electrically coupled to first and second output terminals, respectively, of the comparator circuit230, which may be an operational amplifier. The drain terminal of the NMOSFET241is electrically coupled to the second terminal of the reference resistor131of the resistor-based current source circuit220. The drain terminal of the NMOSFET242is electrically coupled to the source terminals of the PMOSFETs111and112of the transistor-based current source circuit110.

The 750 uA reference current generated by the current tail243is divided into first and second reference current fractions based on the result of the comparison made by the comparator circuit230, which compares the output voltage outputted from the output terminal of the voltage regulator with a second reference voltage that is generated by a suitable reference circuit (not shown), such as a bandgap circuit, for example. The sizes of the first and second reference current fractions will vary depending on how close the output voltage of the voltage regulator122is to the 3.3-volt supply voltage rail.

If the output voltage of the voltage regulator122exceeds the second reference voltage, then the comparator circuit230adjusts the input voltage of the NMOSFET differential pair240such that first reference current fraction of the 750 uA total reference current is driven into the transistor-based current source circuit110is larger than the second reference current fraction that is driven into the resistor-based current source circuit220. This ensures more of the total bias current is sourced by the transistor-based current source circuit than is sourced by the resistor-based current source circuit220, which results in a smaller voltage drop across the resistor121, and, in turn, a decrease in the output voltage of the voltage regulator output122to bring the output voltage back down to a value that is nearer to the second reference voltage and farther from the 3.3-volt supply voltage rail.

This feedback loop for controlling the allocation of the reference current among the transistor-based and resistor-based current source circuits110and220, respectively, is particularly desirable in cases where there are relatively large fluctuations in temperature due to the dependence of VCSEL forward bias voltage on temperature. The lower the temperature, the larger the forward bias voltage of a VCSEL is. At lower temperatures, PMOSFETs are capable of supplying more current for the same device size, so it makes sense to push a larger portion of the bias current through the PMOSFET current source111than through the resistor121at low temperatures in order to keep the voltage drop across the resistor121relatively small and maintain the output voltage of the voltage regulator122within acceptable limits.

In all other respects, the laser driver circuit200shown inFIG. 5operates in the same manner as the laser driver circuit100shown inFIG. 4operates. The division of the total bias current sourcing requirements between the transistor-based and resistor-based current source circuits110and220, respectively, ensures that the current through the PMOSFET current source111is small enough that the PMOSFET current source111can be small enough in size to reduce its parasitic capacitance. The division of the total bias current sourcing requirements between the transistor-based and resistor-based current source circuits110and220, respectively, also ensures that the voltage drop across the resistor121is small enough to allow the output voltage of the voltage regulator122to be less than 3.3 volts. The reduced parasitic capacitance allows the laser driver circuit200to achieve high-bandwidth performance while the reduced output voltage requirement of the voltage regulator122allows the laser driver circuit100to have a low supply voltage, which is a 3.3-volt voltage supply in this exemplary embodiment.

While the exemplary embodiments described above with reference toFIGS. 4 and 5depict the first and second current source circuits as being transistor-based and resistor-based current sources, respectively, the inventive principles and concepts are not limited with respect to the devices or circuit element configurations that are used in the current source circuits. Also, the feedback loops that are included in the second current sources110and120shown inFIGS. 4 and 5, respectively, are not needed in all cases. For example, with reference toFIG. 4, the voltage regulator122could be eliminated in some cases and the first terminal of the resistor121could be electrically coupled to the voltage supply123, in which case the first current source circuit110would source whatever portion of the total bias current the resistor121is not capable of sourcing. Examples of other current source circuits that may be used for these purposes will now be described with reference toFIGS. 6 and 7.

FIG. 6illustrates a block diagram of laser driver circuit300in accordance with another exemplary embodiment that has a low supply voltage and is capable of achieving high-bandwidth performance. The laser driver circuit300includes a modulator circuit comprising first and second emitter-coupled BJTs301and302, respectively, that form a differential pair, a current tail303and a load resistor304. The laser driver circuit300also includes an amplifier305, a reference resistor306, a first current source circuit307and an output node, or terminal308disposed for connection with an anode309of a laser311, which may be, for example, a VCSEL.

The amplifier305has first and second input terminals that are electrically coupled to first terminals of the load and reference resistors304and306, respectively, and has an output terminal that is electrically coupled to second terminals of the load and reference resistors304and306, respectively. The feedback loop comprising the amplifier305, the load resistor304and the reference resistor306comprises a second current source circuit of the laser driver circuit300. The load resistor304has a resistance of R0and the reference resistor306has a resistance of nR0, where n is the ratio of the resistance of the reference resistor306to the resistance of the load resistor304. The electrical coupling of the output terminal of the amplifier305to the second terminals of the load and reference resistors304and306, respectively, ensures that the same voltage drop occurs over each of them.

The amplifier305operates as a voltage shifter that shifts the output voltage of the modulator circuit at the output terminal308up or down based on the voltage drops across the load and reference resistors304and306, respectively. Shifting the output voltage of the modulator circuit in this manner causes the current flowing through the load resistor304to remain nearly constant. The gain of the amplifier305should be large enough to ensure that the voltage drops across the load and reference resistors304and306, respectively, are approximately equal and that the current flowing through the load resistor304is approximately equal to the current flowing through the reference resistor306multiplied by n.

The modulation current produced by the modulation circuit at the output terminal308is added together with first and second fractions of the total bias current sourced into the output terminal308by the first and second current source circuits, respectively. The first fraction of the total bias current is the current sourced by the first current source circuit307into the output terminal308. The second fraction of the total bias current is the current sourced into the output terminal308by the feedback loop comprising the amplifier305, the load resistor304and the reference resistor306, which together comprise the second current source circuit. The total bias current plus the modulation current drive the laser311to cause it to generate an optical output signal.

The first current source circuit307is typically a transistor-based current source circuit similar or identical to the first current source circuits9,13or110shown inFIGS. 1, 2 and 4, respectively. However, because the first current source circuit307only provides the first fraction of the total bias current rather than the entire total bias current, the size of the transistor or transistors that comprise the first current source circuit307can be reduced without requiring an increase in the supply voltage, which may be, for example, a 3.3-volt supply voltage. This ensure that the first current source circuit307will contribute very little parasitic capacitance to allow the laser driver circuit300to achieve high-bandwidth performance. Because the second current source circuit that sources the second fraction of the total bias current uses passive resistors that have very little parasitic capacitance, it also allows the laser driver circuit300to achieve high-bandwidth performance. In addition, the output voltage outputted from the output terminal of the amplifier305can easily be below the 3.3-volt supply voltage rail.

FIG. 7illustrates a block diagram of laser driver circuit400in accordance with another exemplary embodiment that has a low supply voltage and is capable of achieving high-bandwidth performance. The laser driver circuit400is similar to the laser driver circuit300shown inFIG. 6except for modifications to allow the laser driver circuit400to drive the cathode rather than the anode of the laser411. Depending on the laser design, sometimes the laser needs to be driven at cathode. The laser driver circuit400is configured to provide the same benefits and advantages of the laser driver circuit300shown inFIG. 6, but drives the cathode409rather than the anode408of the laser411. The laser driver circuit400includes a modulator circuit comprising first and second emitter-coupled BJTs301and302, respectively, that form a differential pair, a current tail303and a load resistor304. The laser driver circuit400also includes an amplifier305, a reference resistor306, a first current source circuit307and an output terminal413disposed for connection with the cathode409of the laser411, which may be, for example, a VCSEL. The output terminal413functions as a current summing node of the laser driver circuit400.

The amplifier305has first and second input terminals that are electrically coupled to first terminals of the load and reference resistors304and306, respectively, and has an output terminal that is electrically coupled to second terminals of the load and reference resistors304and306, respectively. The feedback loop comprising the amplifier305, the load resistor304and the reference resistor306comprises a second current source circuit of the laser driver circuit400. The load resistor304has a resistance of RO and the reference resistor306has a resistance of nR0, where n is the ratio of the resistance of the reference resistor306to the resistance of the load resistor304. The electrical coupling of the output terminal of the amplifier305to the second terminals of the load and reference resistors304and306, respectively, ensures that the same voltage drop occurs over each of them.

The amplifier305operates as a voltage shifter that shifts the output voltage of the modulator circuit at the output terminal413up or down based on the voltage drops across the load and reference resistors304and306, respectively. Shifting the output voltage of the modulator circuit in this manner causes the current flowing through the load resistor304to remain nearly constant. The modulation current produced by the modulation circuit at the output terminal413is added together with first and second fractions of the total bias current sourced into the output terminal413by the first and second current source circuits, respectively. The first fraction of the total bias current is the current sourced by the first current source circuit307into the output terminal413. The second fraction of the total bias current is the current sourced into the output terminal413by the feedback loop comprising the amplifier305, the load resistor304and the reference resistor306, which together comprise the second current source circuit. The total bias current plus the modulation current drive the laser411to cause it to generate an optical output signal.

As indicated above, although the first current source circuit307is typically a transistor-based current source circuit similar or identical to the first current source circuits9,13or110shown inFIGS. 1, 2 and 4, respectively, it only provides the first fraction of the total bias current rather than the entire total bias current. Consequently, the size of the transistor or transistors that comprise the first current source circuit307can be reduced without requiring an increase in the supply voltage, which may be, for example, a 3.3-volt supply voltage. This ensure that the first current source circuit307will contribute very little parasitic capacitance to allow the laser driver circuit400to achieve high-bandwidth performance. Because the second current source circuit that sources the second fraction of the total bias current uses passive resistors that have very little parasitic capacitance, it also allows the laser driver circuit400to achieve high-bandwidth performance. In addition, the output voltage outputted from the output terminal of the amplifier305can easily be below the 3.3-volt supply voltage rail.

FIG. 8illustrates a flow diagram that represents a method in accordance with an exemplary embodiment for reducing a voltage supply that is needed in a laser driver circuit that drives at least one laser while allowing the laser driver circuit to achieve high-bandwidth performance. As indicated by block501, a laser driver circuit is provided with a voltage supply. As indicated by block502, the laser driver circuit is provided with first and second current source circuits that are electrically coupled to a current summing node of the laser driver circuit having an output terminal that is electrically coupled to the summing node and to an electrode of the laser. As indicated by block503, the first current source circuit provides a first fraction of a total bias current needed to bias the laser into the current summing node. As indicated by block504, the second current source circuit provides a second fraction of the total bias current needed to bias the laser into the summing node. As indicated by block505, the laser driver circuit uses a sum of the first and second fractions of the total bias current to bias the laser.

It should be noted that the inventive principles and concepts have been described with reference to a few exemplary embodiments. Persons of skill in the art will understand how the principles and concepts of the invention can be applied to other embodiments not explicitly described herein. It should also be noted that the laser driver circuits and methods described above with reference toFIGS. 3-7are merely examples of suitable circuit configurations and methods that demonstrate the inventive principles and concepts. As will be understood by those skilled in the art in view of the description being provided herein, many modifications may be made to the embodiments described herein while still achieving the goals of described herein, and all such modifications are within the scope of the invention.