U.S. patent application number 09/949592 was filed with the patent office on 2003-03-13 for optical source driver with bias circuit for controlling output overshoot.
Invention is credited to Fischer, Jonathan H..
Application Number | 20030048820 09/949592 |
Document ID | / |
Family ID | 25489295 |
Filed Date | 2003-03-13 |
United States Patent
Application |
20030048820 |
Kind Code |
A1 |
Fischer, Jonathan H. |
March 13, 2003 |
OPTICAL SOURCE DRIVER WITH BIAS CIRCUIT FOR CONTROLLING OUTPUT
OVERSHOOT
Abstract
A driver circuit for a laser diode or other optical source
includes a differential circuit having first and second inputs for
receiving differential input data, a current generator circuit for
generating modulation current for the optical source in response to
the input data, and a variable bias circuit for applying a variable
bias to the differential circuit. The current generator is
preferably adapted to establish the modulation current for
application to one of a first output and a second output of the
driver circuit in accordance with the differential data applied to
the first and second inputs of the differential circuit. The
variable bias circuit may be configured such that the variable bias
current generated thereby for application to the differential
circuit is a function of the modulation current, thereby
controlling an output overshoot of the driver circuit.
Inventors: |
Fischer, Jonathan H.;
(Longmont, CO) |
Correspondence
Address: |
Ryan, Mason & Lewis, LLP
90 Forest Avenue
Locust Valley
NY
11560
US
|
Family ID: |
25489295 |
Appl. No.: |
09/949592 |
Filed: |
September 10, 2001 |
Current U.S.
Class: |
372/38.02 |
Current CPC
Class: |
H01S 5/042 20130101;
H01S 5/0427 20130101 |
Class at
Publication: |
372/38.02 |
International
Class: |
H01S 003/00 |
Claims
What is claimed is:
1. A driver circuit for an optical source, the driver circuit
comprising: a differential circuit having first and second inputs;
a current generator circuit adapted to establish a modulation
current for application to one of a first output and a second
output of the driver circuit in accordance with differential data
applied to the first and second inputs of the differential circuit;
and a variable bias circuit having an input coupled to a bias
current output of the current generator circuit and an input
coupled to a bias terminal of the differential circuit, the
variable bias circuit being operative to generate a variable bias
current for application to the differential circuit so as to
control an output overshoot of the driver circuit.
2. The driver circuit of claim 1 wherein the optical source
comprises a laser diode.
3. The driver circuit of claim 1 wherein the differential circuit
comprises an input stage differential pair of the driver
circuit.
4. The driver circuit of claim 1 wherein the differential data is
generated from a single-ended input data signal.
5. The driver circuit of claim 4 wherein the single-ended input
data signal is processed internally to the driver circuit to
generate the differential data.
6. The driver circuit of claim 1 wherein the differential data
comprises a differential input data signal.
7. The driver circuit of claim 1 further comprising an input stage,
an intermediate stage and an output stage, the input stage
comprising the differential circuit, the output stage comprising
the first and second outputs of the driver circuit.
8. The driver circuit of claim 7 wherein the intermediate stage
comprises a push-pull stage having a top portion which is driven by
outputs of the input stage and drives the output stage, and a
bottom portion which is driven by the differential data.
9. The driver circuit of claim 1 wherein the first and second
outputs of the driver circuit are associated with respective
collector terminals of corresponding transistors of an output stage
differential pair of the driver circuit.
10. The driver circuit of claim 1 wherein the variable bias circuit
is configured such that the variable bias current generated thereby
is a function of the modulation current.
11. The driver circuit of claim 1 wherein the current generator
circuit generates at the bias current output thereof a scaled
version of the modulation current, the variable bias circuit being
configured to process the scaled version of the modulation current
to generate the variable bias current.
12. The driver circuit of claim 1 wherein the differential circuit
comprises first and second transistors configured as a differential
pair, the first and second inputs of the differential circuit each
corresponding to a base terminal of the respective first and second
transistors, the bias current being applied to a common emitter
terminal of the first and second transistors, a collector terminal
of each of the first and second transistors being associated with a
signal operative to select a particular one of the first and second
outputs of the driver circuit for application of the modulation
current thereto.
13. The driver circuit of claim 1 wherein the variable bias circuit
is configured to provide a first current path having a first
transistor configured to characterize a first output stage
transistor of the driver circuit and a second current path having a
second transistor configured to characterize a second output stage
transistor of the driver circuit, the variable bias circuit being
configured such that current through one of the paths is varied in
accordance with a differential base-emitter voltage of the output
stage, the first and second current paths being configured so as to
have a substantially fixed current ratio therebetween.
14. The driver circuit of claim 13 wherein the substantially fixed
current ratio comprises an approximately 10:1 current ratio.
15. The driver circuit of claim 1 wherein the variable bias circuit
further comprises: first and second transistors configured to
characterize respective first and second output stage transistors
of the driver circuit; first and second current devices, each
coupled between a first terminal of a corresponding one of the
first and second transistors and a first supply voltage, second and
third terminals of the first transistor being coupled to a second
supply voltage, a second terminal of the second transistor being
coupled to the second supply voltage, and a third terminal of the
second transistor being coupled to the second supply voltage via a
resistive element; a differential amplifier having first and second
inputs coupled to the first terminals of the respective first and
second transistors; and a third current device having an input
terminal coupled to an output terminal of the differential
amplifier, the third current device being operative to establish a
current through the resistive element such that an output current
of the variable bias circuit is variable as a function of a
differential emitter-base voltage of the first and second
transistors.
16. An integrated circuit comprising: at least one driver circuit
for an optical source, the driver circuit comprising: a
differential circuit having first and second inputs; a current
generator circuit adapted to establish a modulation current for
application to one of a first output and a second output of the
driver circuit in accordance with differential data applied to the
first and second inputs of the differential circuit; and a variable
bias circuit having an input coupled to a bias current output of
the current generator circuit and an input coupled to a bias
terminal of the differential circuit, the variable bias circuit
being operative to generate a variable bias current for application
to the differential circuit so as to control an output overshoot of
the driver circuit.
17. An apparatus comprising: an optical source; and a driver
circuit coupled to the optical source, the driver circuit
comprising: a differential circuit having first and second inputs;
a current generator circuit adapted to establish a modulation
current for application to one of a first output and a second
output of the driver circuit in accordance with differential data
applied to the first and second inputs of the differential circuit;
and a variable bias circuit having an input coupled to a bias
current output of the current generator circuit and an input
coupled to a bias terminal of the differential circuit, the
variable bias circuit being operative to generate a variable bias
current for application to the differential circuit so as to
control an output overshoot of the driver circuit.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to circuits for
supplying drive current to lasers or other optical sources, and
more particularly to bias control circuits for use in an output
stage of a laser driver or other optical source driver.
BACKGROUND OF THE INVENTION
[0002] Laser diodes and other types of semiconductor lasers are in
widespread use as optical sources in high-speed optical data
transmission applications. Laser diodes are particularly desirable
in such applications due to their high optical output power and
spectral purity. A laser driver circuit, also referred to herein as
simply a "driver," is used to supply appropriate drive current to a
semiconductor 18 laser, so as to control the optical output signal
between an "on" state corresponding to a logic one level and an
"off" state corresponding to a logic zero level, in accordance with
the data to be transmitted.
[0003] Conventional semiconductor laser driver circuits are
described in U.S. Pat. No. 5,883,910, issued Mar. 16, 1999 in the
name of inventor G.N. Link and entitled "High Speed Semiconductor
Laser Driver Circuits," which is incorporated by reference
herein.
[0004] In order to optimize the performance of a semiconductor
laser based optical system, it is important to precisely control
the drive to the semiconductor laser. More particularly, it is
generally desirable to minimize the overshoot in the drive signal
when the semiconductor laser is driven from the off state to the on
state. Significant overshoot can cause automatic gain control (AGC)
circuitry in a receiver of the optical system to implement a
substantial reduction in the receiver gain, thereby unnecessarily
attenuating the desired signal. This will degrade signal-to-noise
performance and increase bit error rate, while reducing the
distance over which the optical signal can be transmitted at a
given quality level. Typically, a maximum acceptable driver output
overshoot is no more than about 10% of the average peak-to-peak
output signal amplitude.
[0005] A significant problem with conventional semiconductor laser
driver circuits such as those described in the above-cited U.S.
Pat. No. 5,883,910 is that such circuits can cause a driver output
signal to exhibit an overshoot which may exceed the above-noted
maximum acceptable levels.
[0006] A need therefore exists for improved driver circuits, for
use with semiconductor lasers and other optical sources, which are
configured to limit the overshoot of the driver output signal to
acceptable levels.
SUMMARY OF THE INVENTION
[0007] The invention provides improved optical source driver
circuits which meet the above-noted need.
[0008] In accordance with one aspect of the invention, a driver
circuit for a laser diode or other optical source includes a
differential circuit having first and second inputs for receiving
differential input data, a current generator circuit for generating
modulation current for the optical source in response to the input
data, and a variable bias circuit for applying a variable bias to
the differential circuit. The current generator is preferably
adapted to establish the modulation current for application to one
of a first output and a second output of the driver circuit in
accordance with the differential data applied to the first and
second inputs of the differential circuit. The variable bias
circuit may be configured such that the variable bias current
generated thereby for application to the differential circuit is a
function of the modulation current, thereby controlling an output
overshoot of the driver circuit.
[0009] Advantageously, the invention can significantly reduce the
output overshoot of an optical source driver circuit, e.g., to a
level below the previously-mentioned 10% maximum acceptable level,
without adversely impacting other performance parameters of the
circuit.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a plot of laser diode output as a function of
drive current in an illustrative embodiment of the invention.
[0011] FIG. 2 shows a simplified diagram of a laser diode optical
source and an associated laser driver circuit in which the present
invention is implemented.
[0012] FIG. 3 is a schematic diagram of the laser driver of FIG.
2.
[0013] FIG. 4 is a plot of output stage differential base-emitter
voltage (dVbe) as a function of bias current for different
temperature levels in the FIG. 3 laser driver.
[0014] FIG. 5 shows laser driver output as a function of time for
different modulation current levels and a fixed bias current for an
input stage differential pair, illustrating an overshoot problem
that is alleviated by the techniques of the present invention.
[0015] FIG. 6 is a schematic diagram of a dVbe bias circuit for use
in the FIG. 3 laser driver in accordance with the present
invention.
[0016] FIG. 7 shows laser driver output as a function of time for
different modulation current levels utilizing for an input stage
differential pair a variable bias generated by the bias circuit of
FIG. 6, illustrating a reduction in overshoot achieved using the
techniques of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0017] The present invention will be illustrated herein using
exemplary semiconductor laser driver circuits. It should be
understood, however, that the particular circuits shown are by way
of illustrative example only, and the techniques of the invention
are more generally applicable to a wide variety of other optical
source drivers. Moreover, although illustrated using a laser diode
optical source, the invention can of course be utilized with other
types of optical sources.
[0018] The general operating characteristics of an illustrative
embodiment of the invention will initially be described with
reference to the simplified diagrams of FIGS. 1 and 2. More
detailed schematic diagrams showing the particular type of bias
control utilized in the illustrative embodiment will be described
in conjunction with FIGS. 3 and 6.
[0019] FIG. 1 shows laser diode light output as a function of diode
current in an illustrative embodiment of the invention. Point 100
on the plotted output characteristic corresponds to the laser
threshold current. This is the point at which further increases in
current will generate laser light output. It is generally desirable
in high-speed optical data transmission applications for the direct
current (DC) bias of the laser diode to be set at or near this
point. Points 102 and 104 on the plotted characteristic correspond
to respective low and high modulation current levels IMOD(0) and
IMOD(1). These current levels are associated with generation of an
optical logic zero output and an optical logic one output,
respectively. It is assumed for simplicity and clarity of
description that a high level optical output is a logic one and a
low level optical output is a logic zero, although it is to be
appreciated that this is not a requirement of the invention.
[0020] It should also be noted that the particular output
characteristic as shown in FIG. 1 is illustrative only, and the
invention can be used with optical sources having other types of
output characteristics.
[0021] FIG. 2 shows a portion of an optical system transmitter in
accordance with the invention. The portion of the transmitter as
shown includes a laser driver circuit 200 and a laser diode D1. A
DC bias current IDC is applied to the laser diode D1 as indicated
by an associated DC bias circuit (not shown). The laser driver
circuit 200 includes a data input and positive and negative outputs
denoted OUTN (terminal 202) and OUTP (terminal 204), respectively.
The applied data in this simplified diagram serves to control the
position of switch 205 such that the low modulation current IMOD(0)
is applied to the laser diode D1 when the data is at a logic low
level, and the high modulation current IMOD(1) is applied to the
laser diode D1 when the data is at a logic high level. This occurs
through direction of the modulation current IMOD via switch 205 and
the OUTN terminal 202 to supply voltage VCC when the input data is
at a logic low level, and via switch 205 and the OUTP terminal 204
to the anode of laser diode DI when the input data is at a logic
high level.
[0022] The description herein assumes that the IMOD(0) and IMOD(1)
levels as shown in FIG. 1 are normalized to the applied DC bias
current, such that when IMOD(0) or IMOD(1) is indicated as being
applied to the laser diode D1, the total applied current is the sum
of the DC bias current IDC and the particular modulation current
IMOD(0) or IMOD(1). It should be noted that the IMOD(0) current may
be zero, i.e., points 100 and 102 in FIG. 1 may be the same, such
that the applied current in this case is only the DC bias current
IDC.
[0023] The laser driver 200 is particularly well-suited for use in
an optical system that includes multiple laser driver modules, each
supplying drive current for a corresponding laser diode. In such an
application, there may be significant advantages in minimizing the
overall system power in order to allow higher integration. One
possible technique is to configure a given laser driver module so
as to drive the corresponding laser diode with just enough current
to meet the system optical power specification. When the laser
diode is new, little current is needed to meet this specification.
However, as the laser diode ages, more current is needed. To handle
expected production variation and laser diode aging, the
above-described modulation current (IMOD) may be specified, e.g.,
over a 12:1 range (such as 5 mA to 60 mA). The system may also
require a particular ratio of the "on" state current (Ion) to "off"
state current (Ioff) for the laser diode, e.g., an Ion:Ioff ratio
of 10:1 or greater. With reference to FIG. 1, the laser diode D1 is
considered to be in the on state upon application of the high
modulation current IMOD(1) and in the off state upon application of
the low modulation current IMOD(0).
[0024] FIG. 3 shows a more detailed view of one possible
implementation of the laser driver 200 of FIG. 2 in accordance with
the invention. The laser driver 200 as shown includes a
differential base-emitter voltage (dVbe) bias circuit 300, a
current generator circuit 302, an output stage differential pair
comprising transistors Q0 and Q1, a push-pull stage comprising
transistors Q2 through Q5, and an input stage differential pair
comprising transistors Q6 and Q7. The top half (Q2, Q3) of the
push-pull stage (Q2-Q5) drives the output stage differential pair
(Q0, Q1), while the bottom half (Q4, Q5) of the push-pull stage is
driven directly by differential data inputs IP and IN. The
differential data inputs also drive the input stage differential
pair (Q6, Q7). The output stage differential pair corresponds
generally to switch 205 of FIG. 2.
[0025] The current generator circuit 302 generates the modulation
current IMOD that is applied via one of the transistors Q0 or Q1 of
the output stage differential pair to respective output terminal
OUTP 204 or OUTN 202, in accordance with the differential data
inputs. The current generator circuit 302 also generates a first
scaled modulation current A.times.IMOD for application as a bias
current to the differential pair Q4, Q5, and a second scaled
modulation current K.times.IMOD for application to an IIN input of
the dVbe bias circuit 300. The dVbe bias circuit 300 utilizes the
scaled modulation current K.times.IMOD to generate a bias current
IB1 for application to a common emitter terminal of the input stage
differential pair Q6, Q7.
[0026] Example values for the above-noted scaling factors A and K
in the FIG. 3 laser driver are 0.11 and 2.1.times.10.sup.-3,
respectively. Other values can also be used, as will be appreciated
by those skilled in the art. The IMOD current and the first and
second scaled versions thereof may be generated in a
straightforward manner in the current generator circuit 302, using
well-known techniques. The term "current generator circuit" as used
herein is intended to include a single circuit which generates each
of the above-noted currents, as well as portions or combinations of
multiple circuits each of which generates a particular one of the
currents.
[0027] The differential data applied to the input terminals IP and
IN may comprise, e.g., approximately 200 to 300 mV peak
differential logic signals, such that the differential pairs Q6, Q7
and Q4, Q5 each switch their corresponding bias current to one side
of the pair or the other.
[0028] The FIG. 3 circuit further includes resistors R1 and R2
coupled between respective collector terminals of the input
differential pair transistors Q7 and Q6 and the supply voltage VCC.
These transistors may be configured as 220 ohm resistors, although
other values could also be used.
[0029] The effects of bias current and applied drive voltage on a
differential pair will now be described in greater detail.
[0030] For ideal bipolar transistors, the collector current (Ic)
and base-emitter voltage (Vbe) relationship in forward bias is
Ic=Is.times.e**(Vbe/Vt),
[0031] where Is is the reverse saturation current and Vt is the
thermal voltage. The thermal voltage Vt is 26 mV at room
temperature, and is given by
Vt=kT/q,
[0032] where k is Boltzmann's constant, q is the charge on an
electron, and T is absolute temperature in degrees Kelvin.
Additional details can be found in A. B. Grebene, "Bipolar and CMOS
Analog Integrated Circuit Design," John Wiley & Sons, 1984.
ISBN 0-471-08529-4, which is incorporated by reference herein. The
base-emitter voltage Vbe for a given current is
Vbe=Vt ln(Ic/Is).
[0033] The difference between the base-emitter voltages for
identical transistors operating at different collector currents
is
dVbe=Vt ln(Ic1/Is)-Vt ln(Ic2/Is)=Vt ln(Ic1/Ic2).
[0034] For a 10:1 current ratio (Ic1=10.times.Ic2),
dVbe=Vt ln(10)=60 mV at room temperature.
[0035] Therefore, in theory, the differential drive across the
base-emitter junctions of transistors Q0 and Q1 in the output stage
differential pair of FIG. 3 need only be 60 mV at a 25.degree. C.
junction temperature. However, this drive needs to scale with
absolute temperature (the Vt term above) while also being
independent of bias current.
[0036] FIG. 4 illustrates that non-ideal effects can cause the
output stage drive to be bias current dependent. The figure plots
dVbe voltage as a function of the current Ic1 for three different
junction temperatures, i.e., Tj=-125.degree. C., Tj=25.degree. C.
(room temperature), and Tj=40.degree. C. It can be seen that the
output stage drive (dVbe) varies as a function of the current Ic1.
If the transistors were ideal, each of the dVbe responses shown in
the figure would have been parallel to the horizontal (Ic1) axis.
It is possible to address this dependence of drive on bias current
and temperature by setting the output swing of the input stage Q6,
Q7 differential pair (IB1.times.R1) to be large enough for the full
IMOD range and vary it only with temperature. However, this
approach can lead to excessive overshoot of the type previously
described herein, as will be illustrated in FIG. 5.
[0037] FIG. 5 shows the laser drive response (output drive current
as a function of time) for a fixed junction temperature when IMOD
is varied from 5 to 60 mA with the bias to the Q6, Q7 differential
pair fixed at the bias needed for proper operation when IMOD=60 mA.
The plotted curves, shown for IMOD=5 mA, IMOD=30 mA and IMOD=60 mA,
have been normalized such that 1.0 represents the intended output
current for driving the laser diode to the on state and 0.0
represents the intended output current for driving the laser diode
to the off state. It can be seen from the FIG. that an overshoot of
about 20% results when IMOD=5 mA. The overshoot is the result of
over driving the differential output stage at low bias
currents.
[0038] In accordance with the invention, the overshoot of the laser
driver 200 is controlled by adjusting the bias current IB1 applied
to the Q6, Q7 differential input stage so that this input stage
drives the top half (Q2, Q3) of the push-pull stage (Q2-Q5) in a
manner that avoids over driving the differential output stage as
IMOD is varied. The bias current IB1 also preferably tracks
absolute temperature to match the output stage transistor parameter
variations. The above-described adjustments in the bias current IB1
are provided by the dVbe bias circuit 300.
[0039] FIG. 6 shows one possible implementation of the dVbe bias
circuit 300 in accordance with the invention. Transistors Q0' and
Q1' serve to model or characterize the output stage differential
devices Q0 and Q1 in the laser driver 200 of FIG. 3. The dVbe bias
circuit further includes Metal-oxide-semiconductor (MOS) devices
M0, M1, M2, M3 and M4 arranged as shown. Source terminals of these
devices are coupled to a supply voltage VSS. Devices M1 and M2 are
configured to provide currents I and 10I which differ by a factor
of 10. A differential amplifier 304 forces nodes N1 and N2 to be
equal by adjusting the current in device M3 to set the voltage
across resistor R3 to be the difference between the base-emitter
voltages of Q0' and Q1', so as to generate a voltage proportional
to absolute temperature (VPTAT).
[0040] The MOS devices M0, M1, M2, M3 and M4 may have width/length
dimensions of 40/4, 100/4, 10/4, 20/0.5 and 80/0.5, respectively,
all in micrometers (.mu.m). These dimensions are examples only, and
not requirements of the invention.
[0041] In accordance with the invention, the above-noted scaled
version of IMOD (K.times.IMOD) drives device M0 and sets the
current in devices M1 and M2. Biasing these reference transistors
with the scaled version of IMOD causes the bias dependence to be
included in the difference in base-emitter voltage (dVbe) as
generated across R3. The voltage across R3 sets the voltage across
R1 and R2 in the laser driver 200 of FIG. 3 by setting the Q6, Q7
differential pair bias current IB1. The resistor R3 is preferably
of the same type and width as resistors R1 and R2, such that
resistor process and temperature variation cancels out. In this
embodiment, R3 may be selected as 1 kohm, although other values can
be used, as will be appreciated by those skilled in the art.
[0042] FIG. 7 shows the laser driver response using the dVbe bias
circuit 300 of FIG. 6 rather than the fixed bias used in the case
of FIG. 5. Again, IMOD is varied from 5 to 60 mA, and the plotted
curves, shown for IMOD=5 mA, IMOD=30 mA and IMOD=60 mA, have been
normalized such that 1.0 represents the intended output current for
driving the laser diode to the on state and 0.0 represents the
intended output current for driving the laser diode to the off
state. It can be seen from the figure that the overshoot, which was
about 20% in the fixed bias case of FIG. 5, has been substantially
reduced to a level of less than about 10% through the use of the
variable bias circuit 300 of FIG. 6. The FIG. 6 bias circuit thus
operates to prevent the over driving of the differential output
stage at low bias currents.
[0043] The particular embodiments of the invention as described
herein are intended to be illustrative only. For example, as
previously indicated, different device types and transistor
technologies may be used in other embodiments. In addition,
although illustrated using multiple differential circuits, the
invention can also be implemented using one or more single-ended
circuits. In such an embodiment, a single-ended input data signal
applied to the driver circuit may be converted to a differential
data signal within the driver circuit. These and numerous other
alternative embodiments within the scope of the following claims
will be readily apparent to those skilled in the art.
* * * * *