Driver circuit for a laser

A driver circuit for a laser includes a first differential amplifier with a first controlled current source, and at least one second differential amplifier with an associated second controlled current source. The at least one second differential amplifier is connected in parallel with the first differential amplifier so that the laser is a common load of the first and the at least one second differential amplifiers. A control circuit controls the current sources based on an external control voltage.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a driver circuit for a laser used as an 
electrical-to-optical transducer in optical communication systems. 
2. Background Information 
To modulate the laser, use is commonly made of a differential amplifier. 
Below the so-called threshold current, the laser acts like a 
light-emitting diode, and the optical output power in this current range 
is very low. Above the threshold current, lasing occurs, and the optical 
output power P.sub.opt increases linearly with increasing modulating 
current I.sub.mod. The rise of the function P.sub.opt =f(I.sub.mod) is 
dependent on the laser type and specimen used and varies with temperature 
and aging. To obtain the typical optical output powers specified in the 
data sheet, a modulating current of I.sub.mod =5 mA is required for a 
0.8-.mu.m laser of high quantum efficiency and low temperature, for 
example; for 1.55-.mu.m lasers of moderate quantum efficiency, I.sub.mod 
must be 40 mA. 
Prior art driver circuits for lasers employ a differential amplifier which 
is adapted to the properties of the respective laser and optimized for 
maximum current, cf. H. M. Rein, "Multi-Gigabit-Per-Second Silicon Bipolar 
IC's for Future Optical Fiber Transmission Systems", IEEE Journal of 
Solid-State Circuits, Vol. 23, No. 23, June 1988, pages 664 to 675, and K. 
Yamashita et al, "Master-Slice Monolithic Integration Design and 
Characteristics of LD/LED Transmitter for 100-400 Mbit/s Optical 
Transmission Systems", Journal of Lightwave Technology, Vol. LT-4, No. 3, 
March 1986, pages 353 to 359. In the solutions described, the optimization 
for maximum current was performed because the transit frequency of a 
silicon bipolar transistor depends on the emitter-current density, cf. 
H.-M. Rein, R. Ranft, "Integrierte Bipolarschaltungen", Springer-Verlag 
Berlin Heidelberg 1980, pages 100 et seq. The transit frequency decreases 
with decreasing emitter-current density, and the depletion-layer 
capacitances and collector-substrate capacitances of the transistor 
increase with increasing transistor areas. As a result, the optimum 
bandwidth of the differential amplifier is achieved only within a small 
current range which depends essentially on the emitter area and 
corresponds approximately to operation at maximum emitter-current density. 
If, in such driver circuits, the emitter-current density is reduced by a 
factor of 10, for example, the influence of the parasitic capacitances on 
the pulse response will increase so that the bandwidth will be greatly 
limited and the pulse shape will be substantially degraded. Therefore, 
such driver circuits are only suitable for lasers with constant properties 
if no disadvantages in the transient response are to result. 
SUMMARY OF THE INVENTION 
It is the object of the invention to provide a driver circuit whose pulse 
response is essentially independent of the modulating-current range. 
This object is attained by providing an arrangement with at least one 
additional differential amplifier. By distributing the modulating current 
to several differential amplifiers, the individual differential amplifier 
can be optimized so that it is always operated near the maximum 
emitter-current density. In this manner, a pulse response in a required 
current range is achieved which is better than in the known prior art, 
i.e., higher bit rates are transmissible, or for a specified pulse 
response, the adjustable current range can be considerably extended. The 
driver circuit according to the invention can thus be used to drive 
different laser types, e.g., a 0.8-.mu.m laser or a 1.55-.mu.m laser, 
without having to reduce the transmission rate. Adaptation to changes in 
characteristics of the lasers due to temperature changes and aging of the 
lasers is also possible. The number of differential amplifiers required is 
determined by the respective application. The individual differential 
amplifiers are activated via a control circuit depending on the current 
swing to be processed. The control voltage can be digital, but it is also 
possible to operate with an analog control voltage and vary the current 
swing continuously. The driver circuit can be readily implemented with 
different types of differential amplifiers. For example, the transistor 
stages of the differential amplifiers may consist of single transistors 
connected in parallel; a feedback circuit in the emitter branch is also 
possible. With the solution according to the invention, a universally 
applicable driver circuit is provided which is suitable for monolithic 
integration.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
As shown in FIG. 1, the circuit consists of a first differential amplifier 
T01, T02 with a laser diode LD in an output line and further differential 
amplifiers T11, T12; T21, T22; T31, T32, which are connected in parallel 
with the first differential amplifier. The emitter lead of each of the 
differential amplifiers contains a series combination of a current-source 
transistor T0, T1, T2, T3 and a resistor, this series combination forming 
a current source. The control input of each of the current sources with 
the currents I0, I1, I2, I3 is connected to one output of a control 
circuit ST. Also indicated is a circuit which supplies the laser diode LD 
with a bias current I.sub.v and input voltage U.sub.ein applied to the 
inputs of each differential amplifier T11, T12; T21, T22; T31, T32. 
At a small modulating current I.sub.mod, e.g., I.sub.mod .ltoreq.5 mA, only 
the first current source I0 of the first differential amplifier T01, T02 
is switched on. Thus, at this minimum modulating current I.sub.mod, only 
one differential amplifier T01, T02 is operated, whose transistors can be 
optimally adapted to this current. For 5 mA.ltoreq.I.sub.mod .ltoreq.17 
mA, the second current source I1 of the second differential amplifier T11, 
T12 is switched on, which is controllable within the range 
0.ltoreq.I1.ltoreq.12 mA, the desired current I1 being determined via the 
control circuit ST by the magnitude of the control voltage U.sub.st. At 
small currents I1 delivered by the second differential amplifier T11, T12, 
e.g., in the range 0.ltoreq.I1.ltoreq.5 mA, the pulse response of the 
driver circuit is determined essentially by the first differential 
amplifier T01, T02 with the first current source I0. In the upper current 
range of the second current source I1 of the second differential amplifier 
T11, T12, i.e., in the range 5 mA.ltoreq.I1.ltoreq.12 mA, this amplifier 
is operated near the optimum operating range of the transistors, since the 
latter are designed to carry the current I1.sub.max =12 mA. 
As modulating-current requirements increase, the third current source I2, 
associated with the third differential amplifier T21, T22, and the fourth 
current source I3, associated with the fourth differential amplifier T31, 
T32, are switched on, so that at the maximum modulating current I.sub.mod, 
all current sources I0, I1, I2, I3 are switched on. 
The current is thus distributed to the four differential amplifiers. 
Already with a distribution of the modulating current to two differential 
amplifiers, the pulse response of the circuit is better than that of the 
prior art circuits. 
FIG. 2 shows the dependence of the modulating current I.sub.mod on the 
control voltage U.sub.st with the current shares of the individual current 
sources switched on under control of the control voltage U.sub.st. As an 
example, FIG. 2 shows I0=5 mA and the three further current sources 
I1=I2=I3=12 mA, so that the maximum modulating current I.sub.mod =41 mA. 
The adaptation of the differential amplifiers for optimum current can be 
implemented in various ways. For example, the modulating current I.sub.mod 
can also be distributed to the individual differential amplifiers 
nonuniformly. The first current source I0 need not be adjusted to the 
minimum modulating current but may also cover a larger current range and 
be controllable. It is also possible to provide a control circuit for IO 
to In which changes the current share of each current source with small 
control-voltage changes, in which case the control ranges of the 
individual current sources may overlap or coincide.