KR20110005069A - The apparatus for driving light source and the optical transmitter - Google Patents

Abstract

PURPOSE: An apparatus for driving a light source and an optical transmitter is provided to control a tail current and a modulation current by using a plurality of a PMOS transistor. CONSTITUTION: An apparatus for driving a light source comprises a current source and a controller(210). The current source outputs a current of a certain level. The controller controls the output current of the current source. The controller comprises a first transistor array(230), a second transistor array(240), and the controller(220). The first transistor array comprises a plurality of PMOS transistors. The second transistor array has a symmetric structure with the first transistor array. The controller controls the resistance value of the first and second transistor array.

Description

Light source driver and optical transmitter {THE APPARATUS FOR DRIVING LIGHT SOURCE AND THE OPTICAL TRANSMITTER}

The present invention relates to a light source driver and a transmitter, and more particularly, to a transmitter including a light source driver and a light source driver in which a pulse width distortion problem is alleviated.

Recently, as the data usage increases, there is a great interest in an optical communication system capable of transmitting a large amount of data quickly and accurately. The optical cable used for optical communication is much more economical than copper wire because there is little interference from the outside, so long distance communication is possible without a repeater. Therefore, optical communication systems using optical cables are well suited for transmitting large amounts of data at high speed.

In addition, recently, a vertical cavity surface emitting laser (VCSEL) has been manufactured, and light transmission using VCSEL instead of a laser diode or a light emitting diode has been performed. The VCSEL is one of the light emitting devices 170 capable of low cost because of high reliability, high-speed driving, large-scale arrangement, and mass production.

In order to use the VCSEL diode, a transmitter of an optical communication system requires a VCSEL light source driver, and the VCSEL light source driver sends a bias current and a modulation current to drive the VCSEL diode to output optical power to the output. The optical power transmitted through the VCSEL diode is received through the PD (photodiode) of the receiver. Therefore, in order to be able to determine '0' and '1' of the received signal well without increasing the BER (Bit Error Rate), the VCSEL light source driver 130 should output the maximum optical power. At this time, since only the modulation current determines the optical power of the VCSEL diode, the magnitude (peak-to-peak) of the output optical power is determined according to the modulation current.

In the conventional VCSEL light source driver, since the modulation current is controlled by the tail current of the VCSEL light source driver, it is inevitable to use a very large transistor in order to obtain the maximum modulation current.

Therefore, when a signal passing through the input buffer stage is immediately passed to the VCSEL light source driver, a sudden big load is applied, and a bandwidth is suddenly reduced, which causes the VCSEL light source driver to not operate at a desired speed. In addition, the use of larger transistors introduces a problem of pulse width distortion (PWD) when changing the modulation current at low supply voltages.

In particular, the smaller the modulation current, the more severe the pulse width distortion occurs. Pulse-width distortion means that the vertical intersection of the eyes in the eye diagram is located above or below the center point. This pulse width distortion causes timing jitter. Since timing jitter is noise of a signal transmitted to a receiver, when timing jitter becomes severe, the noise figure of the signal also worsens, which increases the bit error rate (BER) of the receiver.

The present invention provides a transmitter including a light source driver and a light source driver for adjusting the modulation current while maintaining a constant tail current to alleviate the pulse width distortion problem to solve the above problems.

According to the present invention for achieving the above object, the present light source driver, a current source for outputting a constant magnitude current; And an adjusting unit which adjusts a current output from the current source using a transistor array and supplies the light to the light emitting device.

The control unit may include: a first transistor array including a plurality of PMOS transistors connected in parallel; A second transistor array including the same number of PMOS transistors as the first transistor array, and having a symmetrical structure with the first transistor array; And a controller for controlling resistance values of the first and second transistor arrays.

The controller may adjust the resistance values of the first and second transistor arrays by turning on or off respective PMOS transistors included in the first and second transistor arrays.

In addition, the switching unit for switching the current output from the current source based on the differential input signal; preferably further includes.

In the first transistor array, each gate of the PMOS transistor may be connected to the controller, each source may be connected to a V _DD voltage terminal, and each drain may be connected to the light emitting device and the switching unit. .

And a dummy load for consuming all input current as heat, wherein the second transistor array includes a gate of each of the PMOS transistors connected to the control unit, and a source of the second transistor array connected to the V _DD voltage terminal. The drain is preferably connected to the light emitting element and the dummy rod.

In addition, the switching unit includes two NMOS transistors, each source of the two NMOS transistors is coupled to each other and connected to the current source, each drain is connected to the control unit, and each gate has a differential input signal. It is preferable to be input.

On the other hand, according to the present invention for achieving the above object, the transmitter, the optical amplifier including a preamplifier stage, a light source driver and a light emitting element, the preamplifier stage is gain gain; And a negative capacitance circuit for obtaining a bandwidth, wherein the light source driver comprises: a current source for outputting a current having a constant magnitude; And an adjusting unit which adjusts a current output from the current source using a transistor array and supplies the light to the light emitting device.

The control unit may include: a first transistor array including a plurality of PMOS transistors connected in parallel; A second transistor array including the same number of PMOS transistors as the first transistor array, and having a symmetrical structure with the first transistor array; And a controller for controlling the resistance values of the first and second transistor arrays.

According to the present invention, since the modulation current is adjusted while maintaining the tail current constant using a plurality of PMOS transistors, there is an effect that can mitigate the pulse width distortion problem.

In addition, since the transmitter of the optical communication system transmits a signal to which the pulse width distortion problem is alleviated, the receiver may receive a cleaner signal.

Hereinafter, with reference to the drawings will be described the present invention in more detail.

1 is a schematic block diagram of an AC-coupled transmitter applied to an optical communication system according to an embodiment of the present invention. As shown in FIG. 1, the transmitter includes an input buffer stage 110, a preamplifier stage 120, a light source driver 130, a first capacitor 140, a second capacitor 150, a dummy load 160, and light emission. Device 170, bias current source 180.

The input buffer stage 110 is designed in a CML (current-mode-logic) structure having on-chip input termination to enable gigabit-class ultra-fast operation.

The preamplifier stage 120 is a device required to enable the light source driver 130 to operate at a desired speed and a wide input signal range. Therefore, since the preamplifier stage 120 should have sufficient gain and bandwidth, the preamp stage 120 includes three stages of gain to obtain sufficient gain and a negative capacitance circuit to obtain a wide bandwidth.

The light source driver 130 is connected to the light emitting device 170 to drive the light emitting device 170 using a modulation current. A method of adjusting the modulation current by the light source driver 130 will be described later.

The first capacitor 140 and the second capacitor 150 excludes the applied DC current and passes only the AC current.

The dummy load 160 is added for impedance matching and is a device that consumes heat without reflecting an input signal.

The light emitting device 170 receives a modulation current from the light source driver 130 as an input and emits optical power as an output. At this time, the magnitude of the output optical power is determined by the modulation current. The light emitting device 170 is preferably VCSEL, but is not limited thereto.

2 is a block diagram of an optical communication system including a more detailed structure of a light source driver 130 according to an embodiment of the present invention. As shown in FIG. 2, the light source driver 130 may include a controller 220 for adjusting resistance of the first and second transistor arrays 230 and 240, and a first and second transistor array 230 for adjusting load current. 240, a switching unit 250 for switching tail currents, and a tail current source 260 for supplying tail currents. The controller 220 and the first and second transistor arrays 230 and 240 may be referred to as the controller 210 because they adjust the magnitude of the modulation current, which is a current input to the light emitting device 170.

The controller 220 adjusts the resistances of the first and second transistor arrays 230 and 240 by different voltages applied to the first and second transistor arrays 230 and 240. A detailed method of controlling the first and second transistor arrays 230 and 240 by the controller 220 will be described later.

Each of the first and second transistor arrays 230 and 240 includes a PMOS transistor array connected in parallel, and the PMOS transistor arrays of the first and second transistor arrays 230 and 240 are mutually different. It is preferred to be symmetric. In the present embodiment, for convenience of description, it is assumed that each of the first and second transistor arrays 230 and 240 is composed of four PMOS transistors.

Each gate of the PMOS transistors provided in the first and second transistor arrays 230 and 240 is connected to the controller 220, and each source is connected to the V _DD voltage terminal. A drain of each PMOS transistor provided in the first transistor array 230 is connected to the first capacitor 140 and the switching unit 250, and each PMOS transistor provided in the second transistor array 240 is provided. The drain of is connected to the second capacitor 150 and the switching unit 250.

The controller 220 controls load resistance values of the light source driver 130 differently by turning on and off PMOS transistors provided in the first and second transistor arrays 230 and 240. Thus, even if the tail current described later is constant, the modulation current supplied to the light emitting device 170 is different.

The switching unit 250 includes a first NMOS transistor and a second NMOS transistor to switch tail currents based on the differential input signal. The source of the first NMOS transistor and the source of the second NMOS transistor are coupled to each other and connected to the tail current source 260. A drain of the first NMOS transistor is connected to the first transistor array 230 and the first capacitor 140, and a drain of the second NMOS transistor is connected to the second transistor array 240 and the second capacitor 150. It is connected. In addition, differential input signals IN + and IN- are input to gates of each of the two NMOS transistors.

In detail, when one NMOS transistor of two NMOS transistors provided in the switching unit 250 is turned on, the other NMOS transistor is turned off. The tail current therefore flows only to the on-MOS transistor. Thus, the tail current affects the light emitting device 170 only when the second NMOS transistor is turned on.

The following describes in detail how to change the modulation current by changing the load current.

As described above, changing the tail current to change the modulation current causes a pulse width distortion problem. However, in the present invention, the modulation current is controlled by adjusting on and off of the PMOS transistors provided in the first and second transistor arrays 230 and 240 while maintaining the tail current at the maximum modulation current value.

As shown in FIG. 2, the tail current flows divided into the load current and the modulation current, and the ratio of the load current and the modulation current is determined by the resistance value of the second transistor array 240 and the resistance value of the light emitting device 170. Is determined by.

Specifically, it is assumed that the tail current of the light source driver 130 is 20 mA, the resistance value of the light emitting device 170 is 50 Ω, and the resistance value of each PMOS transistor of the second transistor array is 200 Ω.

Then, when only one PMOS transistor provided in the second transistor array 240 is turned on, the load resistance value is 200 Ω, when two PMOS transistors are turned on, the load resistance value is 100 Ω, and when the three PMOS transistors are turned on, the load is The resistance value is 67Ω. When the four PMOS transistors are turned on, the load resistance value is 50?.

For example, when one PMOS transistor of the second transistor array 240 is turned on, the ratio of the load current I _load and the modulation current I _mod becomes one to four, and thus is applied to the second transistor array 240. The current is 4 mA and the modulation current applied to the light emitting device 170 is 16 mA. In addition, when four PMOS transistors of the second transistor array 240 are turned on, the load current applied to the second transistor array 240 since the ratio of the load current I _load and the modulation current I _mod becomes one to one. Is 10mA and the modulation current applied to the light emitting device 170 is 10mA.

As described above, even though the tail current value is constant, the load current is set differently so that the modulation current input to the light emitting device 170 is different.

Although the number of four PMOS transistors has been described in the present embodiment, the number of PMOS transistors may be changed as necessary, and the resistance value of the PMOS transistor itself may also be changed.

As a result, the load resistance value of the light source driver 130 is adjusted by changing the number of PMOS transistors provided in the second transistor array 240 and the self resistance value, and the load current according to the load resistance value is changed, so that the light emitting device It can be seen that the modulation current input to 170 may also be changed. As such, the tail current is kept constant, so that the conventional pulse width distortion problem is alleviated.

3 is a graph of an eye diagram measured using the light source driver 130 of the present invention. 3A to 3D are graphs of eye diagrams measured when modulation currents applied to the light emitting device 170 are 5 mA, 8 mA, 12 mA, and 15 mA, respectively, and it can be confirmed that the pulse width distortion problem is alleviated.

4 is a detailed block diagram of preamplifier stage 120 with sufficient gain and bandwidth of a signal in accordance with one embodiment of the present invention.

As shown in FIG. 4, the preamplifier stage 120 includes a gain stage 320 for obtaining a sufficient gain and a negative capacitance circuit 340 connected to the gain stage 320 and for obtaining a wide bandwidth.

In detail, the gain stage 320 includes a first resistor, a second resistor, a third NMOS transistor M3, a fourth NMOS transistor M4, and a first current source I1.

The third NMOS transistor has a drain connected to a first resistor and an output terminal, a gate connected to an input terminal, and a source connected to a source of the first current source I1 and the fourth PMOS transistor. In addition, the fourth NMOS transistor M4 also has a drain connected to the second resistor and the output terminal, a gate connected to the input terminal, and a source connected to the first current source I1 and the third NMOS transistor M3. .

The negative capacitance circuit 340 includes a fifth NMOS transistor M5, a sixth NMOS transistor M6, a third capacitor C3, a second current source I2, and a third current source I3.

The fifth NMOS transistor M5 has a drain connected to a gate of the first resistor, an output terminal, and a sixth NMOS transistor M6, a gate connected to a drain of the sixth NMOS transistor M6, and a source of the fifth NMOS transistor M5. It is connected to the second current source I2 and the third capacitor C3. In addition, the sixth NMOS transistor M6 has a drain thereof connected to the second resistor, the gate and the output terminal of the fifth NMOS transistor M5, and the gate thereof is connected to the drain of the fifth NMOS transistor M5. The source is connected to the third capacitor C3 and the fourth current source. The negative capacitance circuit 340 according to the present embodiment provides a negative capacitance value and increases the bandwidth of the gain stage 320 by canceling the capacitance of the parasitic capacitor 360 of the output node.

In addition, although the preferred embodiment of the present invention has been shown and described above, the present invention is not limited to the specific embodiments described above, but the technical field to which the invention belongs without departing from the spirit of the invention claimed in the claims. Of course, various modifications can be made by those skilled in the art, and these modifications should not be individually understood from the technical spirit or the prospect of the present invention.

1 is a schematic block diagram of an AC-coupled transmitter applied to an optical communication system according to an embodiment of the present invention.

2 is a block diagram of an optical communication system including a more detailed structure of a light source driver according to an embodiment of the present invention;

3 is a graph of an eye diagram measured using a light source driver of the present invention, and

4 is a detailed block diagram of a preamplifier stage having sufficient gain and bandwidth of a signal according to an embodiment of the present invention.

Explanation of symbols on the main parts of the drawings

110: input buffer stage 120: preamp stage

130: light source driver 160: dummy load

170: light emitting device 180: bias current source

210: control unit 220: control unit

230: first transistor array 240: second transistor array

250: second transistor array 260: tail current source

320: gain stage 330: negative capacitor

Claims (9)

A current source for outputting a constant current; And And a controller for controlling a current output from the current source using a transistor array and supplying the light to the light emitting device. The method of claim 1, The control unit, A first transistor array including a plurality of PMOS transistors connected in parallel; A second transistor array including the same number of PMOS transistors as the first transistor array, and having a symmetrical structure with the first transistor array; And And a controller for controlling resistance values of the first and second transistor arrays. The method of claim 2, The control unit, And controlling the resistance values of the first and second transistor arrays by turning on or off each of the PMOS transistors included in the first and second transistor arrays. 3. The method of claim 2, And a switching unit for switching the current output from the current source based on the differential input signal. The method of claim 4, wherein The first transistor array is Wherein each gate of the PMOS transistor is connected to the controller, each source is connected to a V _DD voltage terminal, and each drain is connected to the light emitting element and the switching unit. The method of claim 4, wherein And a dummy load for consuming all input current as heat. The second transistor array is Wherein each gate of the PMOS transistor is connected to the control unit, each source is connected to a V _DD voltage terminal, and each drain is connected to the light emitting element and the dummy rod. The method of claim 4, wherein The switching unit includes two NMOS transistors, Wherein each source of the two NMOS transistors is coupled to each other to be connected to the current source, each drain is connected to the control unit, and a differential input signal is input to each gate. In the optical transmitter comprising a preamp stage, a light source driver and a light emitting element, The preamp stage may include a gain stage for obtaining a gain; And a negative capacitance circuit connected to the gain stage. The light source driver includes a current source for outputting a current having a constant magnitude; And a controller which adjusts a current output from the current source using a transistor array and supplies the light to the light emitting device. The method of claim 8, The control unit, A first transistor array including a plurality of PMOS transistors connected in parallel; A second transistor array including the same number of PMOS transistors as the first transistor array, and having a symmetrical structure with the first transistor array; And And a controller for controlling resistance values of the first and second transistor arrays.

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