JPH0541568Y2 - - Google Patents

Description

[Detailed Description of the Invention] "Industrial Application Field" This invention relates to a semiconductor laser drive circuit used in a laser beam printer that uses a laser beam of a semiconductor laser.

"Prior art" A laser beam modulated based on print input information such as characters, figures, symbols, etc. is exposed and scanned on a charged photoreceptor to form an electrostatic latent image, which is then developed.
BACKGROUND ART So-called laser beam printers, which are configured to obtain a hard copy of printed input information through printing processes such as transfer and fixing, are becoming popular, and printers of this type that utilize a laser beam from a semiconductor laser have already been put into practical use.

FIG. 3 is a block diagram showing the schematic configuration of a laser beam printer that utilizes a laser beam from a semiconductor laser.

In this figure, 11 is a feedback circuit (or constant current circuit) including a photodetector 12, 13 is a modulation circuit,
14 is a semiconductor laser (laser diode), and a laser beam LB modulated by the print input information MD is outputted from the semiconductor laser 14 through each of these circuits.

The print input information MD is directly from the computer.
Alternatively, the signal is input to the modulation circuit 13 from an interface via a storage medium such as a magnetic tape or a magnetic (or optical) disk.

The modulation circuit 13 performs on/off current modulation, and the semiconductor laser 14 generates a laser beam LB using the modulated current.

The photodetector 12 detects the output state of the laser beam LB, and when the laser beam LB does not match the predetermined output value, the feedback circuit 11 is activated to control the output value toward the predetermined value. In other words, the feedback circuit 11 and the photodetector 12 perform an auto power function that keeps the output value of the laser beam LB constant.
Control (APC) is configured.

Further, in FIG. 3, 15 is a laser beam scanning section, 16 is an electrophotographic process, and 17 is a hard copy output section.

The laser beam scanning unit 15 includes a polygon mirror rotated at a constant speed by a drive motor, and reflects the laser beam LB to scan the surface of the photoreceptor drum.

The electrophotographic process 16 includes a charged (statically charged) photoreceptor drum, a charger, a static eliminator, a developer, etc., and a laser beam that scans the surface of the photoreceptor drum.
The LB forms an electrostatic latent image, and the developing device develops the electrostatic latent image.

The hard copy output section 17 includes a transfer device, a paper feed/discharge device, a fixing device, etc., and prints the developed image on the photosensitive drum on paper to create a hard copy.

``Problems that the invention aims to solve'' Printers that use laser beams from semiconductor lasers have the advantage of being able to provide small, low-cost products. There is a problem in that the output value of the laser beam becomes unstable because it is easily affected by changes in temperature.

This problem will be explained in detail below with reference to FIGS. 4 and 5.

Figure 4 shows the feedback circuit 11 and photodetector 1 described above.
2 is a circuit diagram specifically showing the modulation circuit 13 and the semiconductor laser 14. FIG.

The feedback circuit 11 includes a pin photo diode 12a with a fast response speed, and this diode 12a.
An operational amplifier (hereinafter referred to as an operational amplifier) 11a whose differential input voltage is the detection signal Vs from the processing circuit 11c based on the detection result and the reference signal Vr, and an amplification operation whose base input is the output of the operational amplifier 11a. A constant current circuit is formed of a transistor 11b.

The processing circuit 11c is used to adjust the density of printing, adjust the laser beam output that has changed due to ambient temperature, etc.

The modulation circuit 13 is formed of a transistor 13b for switching operation which inputs print input information MD via an inverter 13a.
A laser diode 14a is used as the semiconductor laser, and is connected via a resistor 19 to the collector of the transistor 11b whose emitter is grounded via a resistor 18.

FIG. 5 shows an example of the output characteristics of the laser diode 14a, with the drive source I on the horizontal axis and the laser beam output P on the vertical axis. Note that this output characteristic is related to an ambient temperature of 25°C, and when the temperature falls below that temperature, the output curve shifts to the left, and when it rises, the output curve shifts to the right.

Now, in the case of the above circuit, considering the drive current I flowing through the laser diode 14a, this drive current is determined by I=(Vcc-Vs)/R.

Note that R is the resistance value of the resistor 18. As can be seen from the above calculation formula, the drive current I changes as the power supply voltage fluctuates;
This is because the reference signal Vr of a depends on the power supply voltage. Generally, such power supply voltage is supplied from a constant voltage circuit, so it is unlikely to fluctuate greatly.
However, due to the characteristics of the laser diode 14a, a small change in the drive current I appears as a large change in the laser beam output P, which is not preferable.

This relationship can be understood from FIG.
For example, if the drive current I changes slightly by ΔI,
The laser beam output P appears as a greatly magnified variation ΔP.

Changes in the laser beam output P other than the expected value not only degrade the quality of the hard copy but also cause erroneous modulation, so the power supply voltage should be kept as constant as possible to suppress changes in the drive current I. However, since the voltage of the constant voltage circuit itself fluctuates due to the influence of ambient temperature, electrical noise, etc., the above-mentioned problem occurs.

"Means for Solving the Problems" The present invention was developed in view of the above-mentioned problems, and the purpose is to develop a semiconductor laser drive circuit in which the drive current flowing through the semiconductor laser is not affected by fluctuations in the power supply voltage. .

However, in the present invention, in the laser beam printer described above, the drive current of the semiconductor laser that generates the laser beam is controlled by a constant current circuit, and the constant current circuit is configured to control the output of the constant current circuit. We propose a semiconductor laser drive circuit for a laser beam printer, characterized in that it is configured to input a voltage generated by a grounded resistance member through which a corresponding current flows and output a constant current.

"Function" According to the present invention described above, the constant current circuit inputs a predetermined voltage generated in the resistive member with respect to the ground potential and outputs a constant current, so the constant current circuit does not cause the current voltage to fluctuate. As a result, the drive current of the semiconductor laser controlled by the constant current circuit is hardly affected by fluctuations in the power supply voltage.

Embodiment FIG. 1 is a circuit diagram showing a first embodiment of a semiconductor laser drive circuit according to the present invention.

In this figure, 20 is a first transistor for constant current, 21 is a second transistor for bias, these first and second transistors 20 and 21 are connected between the bases, and the second transistor 21 is between the base and the collector. are connected to form a diode-connected first constant current circuit. In addition, 22,
23 is an emitter resistor, and 24 is a negative feedback resistor. In this constant current circuit, the emitter resistor 23 and the second transistor 21 serve as a bias circuit, and the resistor 24
acts as negative feedback against fluctuations in power supply voltage and promotes constant current. Note that it is preferable to use a resistor 24 with a relatively large resistance value. Reference numeral 25 denotes an amplifying transistor connected to the main electrode current path of the second transistor 21 via a resistor 24, and whose emitter is grounded to earth via a resistor 26 for generating a reference signal.

Reference numeral 27 denotes an operational amplifier that inputs the reference signal Vr generated in the resistor 26 and the detection signal Vs described in the conventional example as a differential input voltage.
A feedback circuit is formed such that the output of the transistor 25 is inputted to the base of the transistor 25. That is, the transistor 25 and the operational amplifier 27 form a second constant current circuit configured as a part of the auto power control (APC) described above.

In the second constant current circuit formed in this way, the reference voltage Vr is determined based on the ground potential, so
Almost unaffected by fluctuations in power supply voltage.

Reference numeral 28 denotes a transistor whose base is grounded, which forms a buffer circuit, whose emitter is connected to the first transistor through a resistor 29, and whose base is grounded through a bias power supply 30.

Reference numeral 31 denotes a laser diode as a semiconductor laser, which has a resistor 32 on its anode side and is connected between the collector of the transistor 28 and ground.

33 and 34 are transistors and inverters forming a modulation circuit similar to the conventional example;
inputs print input information MD and turns on and off, modulating the drive current flowing through the laser diode 31. In the semiconductor laser drive circuit described above, the collector current of the second transistor 21 is determined under the control of the auto power control (APC),
The first transistor 20
is biased and supplies constant current. Note that auto power control (APC) works as a negative feedback, and if the laser beam output of the laser diode 31 is higher than the expected value, it reduces the above constant current under the condition of Vs>Vr, and vice versa. When the output is lower than the expected value, the constant current is increased under the condition of Vs<Vr. That is, the laser beam output is automatically controlled so as to be close to the expected value. The laser diode 31 generates a laser beam using a drive current supplied from a constant current circuit,
Further, this laser beam is modulated by a modulation circuit, and becomes a laser beam according to print input information MD. When the laser beam is modulated, the voltage Vo between the collector connection point Q and the ground fluctuates drastically at a frequency of MHz by turning the transistor 33 ON and OFF. 28 and is not transmitted to the constant current circuit.

In other words, since the base-grounded transistor 28 has high output impedance and excellent high-frequency characteristics, it works effectively as a buffer between the constant current circuit and the modulation circuit, and can sufficiently supply a constant current regardless of fluctuations in the voltage Vo. Make it.

Note that the voltage V of the base power supply 30 of the transistor 28 is determined to be V≧V _BC +V ₁ +V ₂ . In this equation, V _BC is the voltage between the collector and base of the transistor 28, and V ₁ and V ₂ are the voltages generated in the resistor 32 and the laser diode 31. As can be seen from the description of the above embodiment, since the first and second constant current circuits are hardly affected by fluctuations in the power supply voltage, the drive current of the laser diode 31 is not affected by fluctuations in the power supply voltage.

In the above embodiment, the drive circuit for the current injection type (cathode grounded type) laser diode 31 was explained. However, when using a current emission type (anode grounded type) laser diode, the circuit configuration shown in FIG. 2 is used. The same effect can be expected by doing so.

FIG. 2 shows a second embodiment of the present invention.

In this embodiment, on the cathode side of the laser diode 35, a buffer circuit consisting of a transistor 36 with a common base and a constant current circuit similar to the second constant current circuit consisting of a transistor 37 and an operational amplifier 38 are provided. Note that reference numerals 39 and 40 in this drawing are transistors and inverters forming a modulation circuit, and 41 is a bias power supply. In the above embodiment, the fluctuation of the power supply voltage is the modulation voltage
Since it is buffered together with Vo by the transistor 36, the drive current flowing through the laser diode 35 is determined by the resistor 42. Since the operational amplifier 38 receives the voltage generated by the resistor 42 with reference to the ground potential as the reference signal Vr, it is not affected by fluctuations in the power supply voltage, so the driving current of the laser diode 35 is Almost no change.

"Effect of the invention" As described above, in the semiconductor laser drive circuit according to the invention, the constant current circuit inputs a predetermined voltage generated in the resistive member with respect to the ground potential and outputs a constant current. The driving current of the semiconductor laser determined by the circuit is almost unaffected by fluctuations in the power supply voltage.

Therefore, according to the present invention, it is possible to provide a semiconductor laser drive circuit that can sufficiently cope with fluctuations in power supply voltage, and has great practical value as a semiconductor laser drive circuit for a laser beam printer.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a semiconductor laser drive circuit showing a first embodiment of the present invention, FIG. 2 is a circuit diagram of a semiconductor laser drive circuit showing a second embodiment of the present invention, and FIG. 3 is a circuit diagram of a semiconductor laser drive circuit showing a second embodiment of the present invention. FIG. 4 is a circuit diagram showing a conventional example of a semiconductor laser drive circuit, and FIG. 5 is a diagram showing an example of the output characteristics of a laser diode. 20, 21...first and second transistors forming the first constant current circuit, 25, 27...transistor and operational amplifier forming the second constant current circuit,
26...Resistor that generates the reference signal Vr of the operational amplifier 27, 28...Transistor forming a buffer circuit, 31...Laser diode, 33, 34
...Transistors and inverters forming a modulation circuit, 37, 38...Transistors and operational amplifiers forming a constant current circuit, 42...Resistors that generate a reference signal Vr for the operational amplifier 38.

Claims (1)

[Scope of Claim for Utility Model Registration] (1) A laser beam modulated based on print input information such as characters or images is scanned on an electrostatic charged photosensitive surface, visualized by an electrophotographic process, and printed on paper. In a laser beam printer that outputs the print input information as a hard copy through a process such as transfer, the drive current of the semiconductor laser that generates the laser beam is controlled by a constant current circuit, and the constant current circuit is A semiconductor laser drive circuit for a laser beam printer, characterized in that the circuit is configured to output a constant current by inputting a voltage generated by a grounded resistance member through which a current flows in accordance with the output of the constant current circuit. (2) The constant current circuit includes an operational amplifier, an amplification transistor that inputs the output of the operational amplifier as a base input, and a resistor connected between the emitter of the transistor and the ground, and the operational amplifier The setting voltage to be set and
2. A semiconductor laser drive circuit in a laser beam printer according to claim (1), which is configured to input a predetermined voltage generated in the resistor member with respect to a ground potential as a differential input voltage.