Control circuit of light emitting element

Details Control circuit of light emitting element Control circuit of light emitting element

2001-07-10

2002-10-15

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Plural power supplies

Plural cathode and/or anode load device

C315S291000

Reexamination Certificate

active

06465967

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a control circuit of a light emitting element such as a semiconductor laser diode, particularly to a constitution of monitoring light emitting states of a plurality of light emitting elements by a single light receiving element and controlling individual light emitting elements by monitor current thereof.
2. Description of the Related Art
Conventionally, a discrete element integrated with a light emitting element and a light receiving element, for example, a discrete element integrated with a laser diode and a photodiode, is constructed by a pair constitution of one element of laser diode coupled with one element of photodiode (hereinafter, such an element is referred to as 1LD-1PD element). Therefore, it is general that an auto power control circuit (hereinafter, referred to as APC circuit) of a laser diode carries out light control with respect to 1LD-1PD element. For example, as shown by
FIG. 6
, a laser diode LD is made to emit light by generating drive current ILD from a driver circuit X
1
. When the laser diode LD emits light, monitor current IPD is generated at a photodiode PD and the current is converted into voltage by a current/voltage converting portion X
2
. There is calculated a difference between the voltage and reference voltage by an error amplifier X
3
and a difference voltage component thereof is subjected to voltage to current conversion by the driver circuit X
1
to thereby generate the drive current ILD. By such an APC loop, compensation of error current of the drive current ILD is carried out.
According to the constitution of
FIG. 6
, when a discrete element paired with two or more of laser diodes and one photodiode (hereinafter, referred to as 2LD-1PD element) is controlled, although drive current of two laser diodes cannot be controlled by single monitor current, there has been carried out a trial of an APC circuit for controlling drive current of two laser diodes by single monitor current, for example, there is provided a light amount control apparatus in Japanese Patent Laid-Open No. 209545/1998.
According thereto, as shown by
FIG. 7
, monitor current of a photodiode PD
1
constituting a 2LD-1PD element along with laser diodes LD
1
and LD
2
, is divided uniformly by a current dividing circuit DIV, distributed to amplifiers Y
1
and Y
2
, respectively subjected to current-voltage conversion by the amplifiers Y
1
and Y
2
and output voltage of these is compared with reference voltage by comparators Y
3
and Y
4
in the comparators Y
3
and Y
4
. When output voltage of the amplifiers Y
1
and Y
2
is lower than the reference voltage, the logical level of the output is determined as “L” and when the output voltage of the amplifiers Y
1
and Y
2
is higher than the reference voltage, the logical level is determined as “H”. Outputs of the comparators Y
3
and Y
4
are respectively provided to sample hold control circuits (hereinafter, referred to as SH control circuits) Y
5
and Y
6
. The SH control circuits Y
5
and Y
6
are provided with up/down counters, not illustrated, when an enable state is brought about, the outputs are upcounted or downcounted in accordance with the logical levels of the outputs of the comparators Y
3
and Y
4
and when a disable state is brought about, count values are held. The SH control circuits Y
5
and Y
6
upcount the outputs when the outputs of the comparators Y
3
and Y
4
are at “L” and downcount the outputs when the outputs are “H”. The driver circuits Y
7
and Y
8
generate drive current ILD
1
and ILD
2
in accordance with count values of the SH control circuits Y
5
and Y
6
. Single monitor current is divided in two by the current dividing circuit DIV and there is carried out compensation of error current of the drive current ILD
1
and ILD
2
similar to the constitution shown in
FIG. 6
respectively by an APC loop including the amplifier Y
1
, the comparator Y
3
, the SH control circuit Y
5
and the driver circuit Y
7
and an APC loop including the amplifier Y
2
, the comparator Y
4
, the SH control circuit Y
6
and the driver circuit Y
8
.
However, according to the constitution shown in
FIG. 7
, in the current dividing circuit DIV for dividing the monitor current of the photodiode, emitters of PNP bipolar transistors TR
1
, TR
2
and TR
3
are respectively connected to a common power source terminal via resistors R
1
, R
2
and R
3
, a base and a collector of the transistor TR
1
are connected to the photodiode and bases of the transistors TR
2
and TR
3
are connected to the base of the transistor TR
1
to thereby constitute a current mirror circuit. Bias to the bases of the transistors TR
2
and TR
3
is constituted only by the monitor current of the photodiode and therefore, when the monitor current of the photodiode is reduced, drive function at post stages of the amplifiers Y
1
and Y
2
is deteriorated and an operational frequency band is attenuated. Therefore, it is difficult to make low consumption formation of the APC circuits and high frequency formation of operational frequency band compatible with each other.
Further, the amplifiers Y
1
and Y
2
are respectively provided with operational amplifiers Y
9
and Y
10
, positive inputs thereof are connected to the ground via resistors R
4
and R
5
and connected to the current dividing circuit DIV via diodes D
1
and D
2
and variable resistors RV
1
and RV
2
and negative inputs thereof are connected to the voltage dividing circuit DIV via the diodes D
1
and D
2
and resistors R
6
and R
7
. Input voltage to the operational amplifiers Y
9
and Y
10
is derived from voltage drop by the resistors connected to the ground and current-voltage conversion is carried out with a ground level as a reference. Therefore, it is difficult to fit the positive and negative inputs to input ranges capable of realizing optimum operation of the operational amplifiers Y
9
and Y
10
to thereby hamper high degree APCs. That is, when the inputs are received from the ground, generally, an input stage of the operational amplifier is constructed by a PNP bipolar transistor constitution as shown by FIG.
8
. As a general disadvantage in the case of using such an input constitution, the frequency characteristic is worse than that of an NPN input stage constituting circuit (the function of the transistor is generally superior in the case of NPN). Further, when input or output is at a vicinity of 0 [V], there is a possibility that the transistor is brought into a saturated region and stability with regard to the frequency characteristic is deteriorated.
Further, although gain of the amplifiers Y
1
and Y
2
are adjusted to set by the variable resistors RV
1
and RV
2
connected to the inputs of the operational amplifiers Y
9
and Y
10
, the frequency band and the phase are significantly varied by function of the operational amplifiers Y
9
and Y
10
.
Further, the constitution of
FIG. 7
is the circuit constitution specified to the 2LD-1PD element, the circuit constitution per se cannot be used as an APC circuit using a 1LD-1PD element or an APC circuit using two of 1LD-1PD elements and there is needed an APC circuit for separate use, which gives rise to an increase in the cost. For example, although in a printer apparatus, there is used a laser diode for scanning a printing drum, in a high grade machine, scanning is carried out at high speed and therefore, the machine is dealt with by a 2LD-1PD element or two of 1LD-1PD elements, which constitution is to be selected is determined by price and function of laser diode and in the case of a low grade machine, scanning may be carried out at low speed and the machine is dealt with by a 1LD-1PD element. Therefore, APC circuits need to prepare for different machines, which results in an increase in the cost.
SUMMARY OF THE INVENTION
Hence, according to the invention ,by connecting a current source to a collector of respective bipolar transistor constituting a current mirror for dividing monitor current of a light receiving element and prov

Affiliated with

Also associated with

Rating

Control circuit of light emitting element does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Control circuit of light emitting element, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Control circuit of light emitting element will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2998004