JP4655457B2 - Light quantity control device and image forming apparatus using the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to drive control of a light emitting element, and more particularly to a light amount control apparatus suitable for use in driving a laser element used as a light source in laser xerography.
[0002]
[Prior art]
In the field of laser xerography using a laser element as a light source, there is an increasing demand for higher resolution and higher speed. There is a limit to the speed (hereinafter referred to as modulation speed) at which on / off control of the driving of the laser element is performed according to the input image data. In the case where the number of laser beams is one, if the resolution in the sub-scanning direction is increased as well as the resolution in the main scanning direction, the modulation speed must be sacrificed. Therefore, the only way to increase the resolution in the sub-scanning direction without increasing the modulation speed is to increase the number of laser beams. For example, when the number of laser light beams is four, the resolution in the main scanning / sub-scanning direction can be doubled on the assumption that the modulation speed is the same as in the case of one.
[0003]
A semiconductor laser used as a light source for laser xerography includes an edge-emitting laser element (hereinafter referred to as an edge-emitting laser) having a structure in which laser light is extracted in a direction parallel to the active layer, and the laser light is perpendicular to the active layer. It is roughly classified into surface emitting laser elements (hereinafter, referred to as surface emitting lasers) having a structure that can be taken out in any direction. Conventionally, in laser xerography, an edge emitting laser is generally used as a laser light source.
[0004]
However, from the viewpoint of increasing the number of laser light beams, edge-emitting lasers are technically difficult, and structurally, surface-emitting lasers increase the number of laser light beams more than edge-emitting lasers. It is advantageous. For these reasons, in order to meet the demand for higher resolution and higher speed in the field of laser xerography in recent years, an apparatus using a surface emitting laser capable of emitting a large number of laser light beams as a laser light source. Development is underway.
[0005]
By the way, in a semiconductor laser driving device, an automatic light control (APC) circuit that detects the light amount of the semiconductor laser with a light receiver and automatically controls the light amount based on the detected light amount is used. In controlling the amount of light, in the case of a surface emitting laser, a part of the emitted light is separated by an optical system including a half mirror because of the structural restriction that the laser light is emitted in a direction perpendicular to the active layer. A configuration is adopted in which the amount of light of the surface emitting laser is detected by causing light to enter the light receiver as monitor light.
[0006]
As described above, when the surface emitting laser, the optical system, and the light receiving element are assembled, the positional accuracy between the elements is poor, and the monitor light can be reliably received under such a situation. Since it is necessary to set the light receiving area of the light receiver to be large, the parasitic capacitance of the light receiver becomes very large. For this reason, in a circuit system that controls the light quantity by receiving the detection output of the light receiver, the responsiveness required for the light quantity control cannot be secured unless the detection output of the light receiver is received with low impedance.
[0007]
In addition, in the case of a surface emitting laser, the output current (photocurrent) of the light receiver itself is very small because an optical system including a half mirror is interposed between the surface emitting laser and the light receiver. Is a weak current of about several μA while the received light current is about 100 μA. If such a weak photocurrent is converted into a voltage with a load having a low resistance value, the light amount detection voltage of the surface emitting laser becomes two orders of magnitude smaller than that of the edge emitting laser, and only the gain of the subsequent differential amplifier In this case, the gain at the time of negative feedback necessary for the automatic light amount control is insufficient, and the accuracy of the light amount control is lowered.
[0008]
By the way, as a circuit for amplifying a photocurrent, a configuration in which a plurality of operational amplifiers are connected in multiple stages is known (see, for example, Patent Document 1). The multi-stage connection is for obtaining a high gain while maintaining high speed.
[0009]
Patent Document 2 discloses a method of amplifying current with a current mirror circuit. In principle, if the output impedance can be increased to suppress the output parasitic capacitance, a high gain can be obtained in one stage.
[0010]
[Patent Document 1]
JP 59-90242 A
[Patent Document 2]
JP-A-8-293742
[0011]
[Problems to be solved by the invention]
However, the conventional technique according to Patent Document 1 has the following problems. When multiple stages are connected, the offset of the first stage amplifier is amplified and input to the next stage amplifier as compared with the case where a necessary gain is obtained in one stage, so some offset adjustment is required.
[0012]
Further, the conventional technique according to Patent Document 2 has the following problems.
[0013]
The problem of cancellation error: Since the current mirror circuit has low responsiveness when the current is small, in Patent Document 2, the bias current is added to the light receiving current, and the current mirror circuit amplifies the current and then adds it. By subtracting this current, the amplified photocurrent is extracted, and only the amplified current is extracted regardless of the bias current. However, in practice, the larger the current amplification factor, the greater the cancellation error when adding and amplifying the bias current and then subtracting the current that has amplified only the bias current. If the cancellation error becomes large, an error current is included in the amplified photocurrent. Therefore, an offset is added when the next stage differential amplification is performed, and the operation becomes impossible beyond the worst dynamic range.
[0014]
Maximum current problem: Also, there is an optimum value for the current that flows per transistor size (per the ratio of width to length in the case of MOS, per emitter area in the case of bipolar). As a result, a voltage drop that is too large increases and the dynamic range cannot be obtained, making the design difficult.
[0015]
The problem of optimum input impedance: To detect photocurrent from a photoreceiver with a large parasitic capacitance at high speed, it is necessary to receive it at low impedance. The bias current must be increased and the bias current must be increased. As a result, the input parasitic capacitance of the current mirror increases, and the superimposed bias current decreases the DC offset accuracy in the subsequent circuit in order to lower the input impedance. For this reason, there is an optimum value for the input impedance of the current mirror according to the parasitic capacitance and photocurrent of the photoreceiver, and the size and bias current of the transistor of the current mirror circuit to which the photoreceiver output is connected according to the parasitic capacitance of the photoreceiver. It is desirable that it can be changed. However, in the technique described in Patent Document 2, the sizes of all the transistors included in the current mirror circuit must be changed at the same ratio, and the transistor layout does not lose central symmetry in order to ensure the relative accuracy of the current mirror. It is very difficult to lay out.
[0016]
The present invention has been made in view of the above problems, and an object of the present invention is to provide a light amount control apparatus having a large dynamic range with little offset error.
[0017]
  The present invention as described in claim 1A light receiving element for receiving light from the light emitting element,First amplification means for amplifying an input signal from the light receiving element;A first compensation current source for providing a first compensation current to an input of the first amplification means when the light emitting element is off;A second amplifying means for amplifying a non-input signal, having a main part having a common configuration with the main part of the first amplifying means, in order to remove the offset generated in the first amplifying means;A second compensation current source for providing a second compensation current to the input of the second amplification means;Error amplifying means for generating a control signal for automatic light quantity control based on output signals of the first and second amplifying means, and driving means for driving the light emitting element based on the output signal of the error amplifying means;,ComprisingThe first and second compensation currents have a value equal to the light receiving current from the light receiving element corresponding to the target light amount, and the main part converts to an impedance higher than the input impedance, and the converting means Current amplifying means for amplifying the output of the light source, wherein the converting means makes the input impedance smaller than a predetermined automatic light amount control time To / impedance determined by the stray capacitance Co of the light emitting element, and the converting means and the current The amplifying means includes a bias current source for supplying a bias current and a current mirror circuit on which the bias current is superimposed on the input side, and the bias current of the current amplifying means is the amplification factor of the current mirror circuit of the current amplifying means. Determined according toThis is a light quantity control device. Since the second amplifying means has a main part having the same configuration as the main part of the first amplifying means, the offsets included in the output signals of the first and second amplifying means are the same. By generating a control signal for automatic light quantity control based on the output signal, it is possible to remove the offset generated in the first amplification means.Further, the drive can be performed at high speed by satisfying the condition that the input impedance is made smaller than the predetermined automatic light amount control time To / impedance determined by the stray capacitance Co of the light emitting element. Further, by supplying a bias current to the current mirror circuit, the input impedance of the current mirror circuit can be lowered and the responsiveness can be improved. Furthermore, since the bias current is set according to the amplification factor of the current mirror circuit, the current mirror circuit can be operated in an optimum state, and a large dynamic range can be secured with little error.
[0021]
In the light amount control device, the current mirror circuit of the current amplifying means has an input-side transistor having a control terminal to which a control potential is applied, and a control terminal connected in common with the control terminal of the input-side transistor, A plurality of output-side transistors connected in parallel to each other, and a current source commonly connected in series to the plurality of output-side transistors, the amplification factor of the current amplification means is the number of the plurality of output-side transistors It can be set as the structure determined by. The amplification factor of the current mirror circuit can be easily changed according to the number of transistors on the output side.
[0022]
In the light amount control device, in the first and second amplifying means, based on the output voltage of the current mirror circuit of the current amplifying means in the second amplifying means and the switching means connected in series to the plurality of transistors. Control means for controlling the potential of the control terminal of the bias current source of the current amplifying means, and the switch means corresponding to the output side transistor to be selected according to the degree of amplification is controlled based on an external control signal. It can be set as a structure. Since the number of transistors on the output side can be changed from the outside, the amplification factor of the current mirror circuit can be easily changed.
[0023]
In the above-described configuration, the plurality of output-side transistors may have the same parameter regarding the amplification factor. Since the parameters are equal, the amplification degree can be changed and set with high accuracy and ease.
[0024]
In the above configuration, the plurality of output-side transistors may be unipolar transistors, and the parameter relating to the amplification degree may be a ratio of a channel width to a channel length.
[0025]
In the light quantity control device, the first and second amplifying units may have individual loads for each output, and each output may be connected to a differential input of the error amplifying unit.
[0026]
In the light quantity control device, the outputs of the first and second amplifying means may be connected to the differential input of the error amplifying means and have a common load between the differential inputs.
[0027]
In the light amount control apparatus, the first amplifying unit includes: a first current mirror circuit that receives the input signal; and a second current mirror circuit that amplifies the output current of the first current mirror circuit. It can be set as the structure which has. Since two current mirror circuits are used, the setting of the amplification degree and the bias current according to the transistor size can be easily set and changed.
[0028]
In the light amount control device, the first amplifying unit has an output corresponding to the input signal and a first bias current, and the second amplifying unit has a second current of the same magnitude as the first bias current. The error amplifying means outputs a constant output corresponding to the bias current of 2 and an output corresponding to the difference between the two outputs.
Thereby, a large dynamic range can be obtained without an offset error.
[0029]
In the above configuration, it is preferable that the values of the first and second bias currents are set so that the outputs of the first amplifying unit and the second amplifying unit are maintained below a certain level. The output level can be easily controlled by setting the bias current.
[0030]
  In the light quantity control device, a reference current flowing from a reference current source instead of the first and second compensation current sourcesInThe third compensation currentto addThird compensationA current source,Third compensationWhen changing the target light quantity by turning on and off the current source,Third compensationCurrentSaidSet to a constant ratio with respect to the reference currentIt can be configured. compensationBy setting the current to a constant ratio with respect to the reference current, even when a light amount different from the target light amount (for example, a light amount that is a constant ratio lower than the target light amount) is required,compensationEasily set the current to a constant ratio with respect to the reference currentcompensationThe current value can be set.
[0031]
  In the above invention,Third compensationThe current source was turned on and off with a double-throw switch such as a differential transistor, and turned off when turned off.Third compensationA configuration in which the potential is almost the same as that before the output of the current source is turned off can be employed. ThiscompensationNoise generated when the current source is turned on / off can be reduced.
[0032]
  The invention also claims14A plurality of light emitting elements, a photosensitive member, an optical system for irradiating the photosensitive member with a light beam from the plurality of light emitting elements, and a light amount control for controlling a light amount of the plurality of light emitting elements. And the light quantity control device is an image forming apparatus configured as described above.
[0033]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the accompanying drawings.
[First Embodiment]
FIG. 1 is a diagram showing an overall configuration of a light amount control apparatus according to the first embodiment of the present invention. The light quantity control device includes a current amplifier 300 that is an amplifier, loads 500 W and 500 D of the current amplifier 300, an APC circuit 600, a driving unit 100, and a light receiver 11.
The light quantity control device has a function of automatically controlling the light emission quantity of light emitting elements LD1 and LD2 described later so as to become the target light quantity.
[0034]
The driving unit 100 drives the light emitting elements LD1 and LD2. Although only two light emitting elements LD1 and LD2 are shown in FIG. 1 for convenience, a plurality of light emitting elements are actually connected to the driving unit 100. The light emitting elements LD1 and LD2 are, for example, surface emitting diodes (VCSEL). The light receiver 11 is provided in common to the plurality of light emitting elements LD1 and LD2. The light receiver 11 is formed of, for example, a photodiode, receives light emitted from the light emitting elements LD1 and LD2, and outputs a photocurrent corresponding to the light amount on the output line L. In the output line L, the parasitic capacitance (mainly, the depletion layer capacitance of the photodiode) Co of the light receiver 11 exists.
[0035]
The current amplifier 300 functions as a reception signal amplifier, and includes two amplification units 300W and 300D. The amplification unit 300W is connected to the output line L, and the amplification unit 300D functions as a dummy amplifier. The amplifying unit 300D receives the current I1 from the current source 332 connected to the power supply VDD, and supplies a corresponding output current to the load 500D. The current I1 has a value equal to the light reception current from the light receiver 11 corresponding to the target light amount. Hereinafter, the current I1 is referred to as a compensation current. When the light emitting elements LD1 and LD2 are turned off by the switch SWP, the compensation current I1 is supplied from the current source 331 connected to the power source VDD to the input of the amplifying unit 300W, and at least one of the light emitting elements LD1 and LD2 is turned on. When light is received, a light receiving current is applied from the light receiver 11. As a result, the potential fluctuation of the output line L is suppressed and the convergence of APC is improved. The amplifying unit 300W supplies an output current corresponding to the input to the load 500W. The amplification units 300W and 300D have a similar configuration (substantially the same part). Therefore, the APC circuits (differential amplifiers) 600 that receive these output currents can cancel each other because the offset currents included in each of them are equal. The APC circuit 600 thus cancels the offset current and generates an output voltage corresponding to the difference between the photocurrent and the reference current.
[0036]
The switch 110 and the capacitors C111 and C112 constitute a sample and hold circuit. When the light emitting element LD1 is driven, the switch 110 is switched to the capacitor C111 side, whereby the error voltage detected by the APC circuit 600 is held in the capacitor C111. Similarly, when the light emitting element LD2 is driven, the switch 110 is switched to the capacitor C112 side, whereby the control voltage detected by the APC circuit 600 is held in the capacitor C112. Drive circuit 113₁, 113₂Constitutes a drive unit 100 for driving the light emitting elements LD1, LD2. Capacitors C111 and C112, and further switch 110 may be defined as drive unit 100. Drive circuit 113₁, 113₂Drives the light emitting elements LD1 and LD2 according to the hold voltages of the capacitors C111 and C112, respectively. Drive circuit 113₁, 113₂Between each of the light emitting elements LD1 and LD2, and a switch 114 that is turned on (closed) at the timing of driving the light emitting elements LD1 and LD2.₁, 114₂Is provided.
[0037]
As described above, the light quantity control device shown in FIG. 1 includes the light emitting elements LD1 and LD2, the light receiver 11 that receives light from the light emitting elements, and the first amplification unit 300W that amplifies the input signal from the light receiver 11. In order to remove the offset generated in the first amplifying unit 300W, a second amplifying unit 300D means for amplifying a non-input signal having a main part of the same configuration as the main part of the first amplifying part 300W, The APC circuit 600 functioning as an error amplifying means for generating a control signal for automatic light quantity control based on the output signals of the first and second amplifiers 300W and 300D, and the light emitting elements LD1 and LD2 based on the output signals And a driving unit 100 for driving.
[0038]
The amplification unit 300W includes an impedance conversion circuit 311W, a current source 312W of a bias current I2, a current source 313W of a reference current I1, a current amplification circuit 314W of an amplification factor n, and a current source 315W of a current n × I2. Similarly, the amplification unit 300D includes an impedance conversion circuit 311D, a current source 312D of a bias current I2, a current source 313D of a reference current I1, a current amplification circuit 314D with an amplification factor n, and a current source 315D with a current n × I2. That is, the amplification unit 300D has the same or similar configuration as the amplification unit 300W.
[0039]
The impedance conversion circuits 311W and 311D are circuits that receive an input at a low impedance and output at a high impedance. This impedance conversion enables high-speed operation with good response. In this embodiment, as will be described later, the impedance change circuits 311W and 311D are formed by a current mirror circuit. Since the bias current I2 is applied to the current mirror circuit, the input impedance can be set sufficiently small. As a result, the impedance conversion circuit 311W can receive the photocurrent from the output line L with low impedance and output it with high impedance.
[0040]
More specifically, the impedance settings of the amplifiers 300W and 300D may be such that the input impedance of the impedance converters 311W and 311d is made smaller than a predetermined automatic light amount control time To / impedance determined by the stray capacitance Co of the light emitting element. preferable. In the light amount control of the surface emitting laser, the light receiving control light receiver cannot be mounted in the surface emitting laser package, and the light receiving area has to be increased because it is installed outside. For this reason, the output capacity of the light receiver increases in proportion to the light receiving area. On the other hand, the light amount control must be performed in a period excluding the image region in the laser beam scanning area (see FIG. 11 described later), and there is a time restriction. In particular, when the amount of light is sequentially controlled for a large number of lasers such as a surface emitting laser, the time allowed per beam is short. For example, when the paper feed speed is 40 inches / sec and the number of beams is 32 at a resolution of 2400 DPI, the scanning period of the laser beam is 32 / (2400 * 40) = 333 μsec. If the effective image area is 60%, 40% can be used for light amount control, and the time is 133 μsec. For example, if two levels of light quantity control are performed for 32 beams, the time allowed for the light quantity control per one beam is 133/32 * 2 = 2 μsec. Therefore, the light amount control is not completed within 2 μsec unless the response time constant of the current amplifiers 300W and 300D is shorter than at least 2 μsec. In addition to the stray capacitance of the photodiode, the output capacitance of the light receiver 11 includes the capacitance of the wiring. The stray capacitance of the photodiode is about 100 pF even if a sufficient bias voltage is applied to reduce the depletion layer capacitance if the light receiving area is about 1 cm square, and the wiring capacitance is about 10 pF when the wiring capacitance is routed several tens of cm. If the sum is roughly 100 pF and the input impedance of the impedance conversion circuits 300D and 300W is R, the time constant consisting of the input stray capacitance and input impedance is 100 pF × R. Control does not converge. Therefore, a condition of at least 100 pF × R <2 μsec is derived, and from this, a condition of R <2 μsec / 100 pF (parasitic capacitance) is derived.
[0041]
A current source 313W of the reference current I1 is provided between the output of the impedance conversion circuit 311W and the ground, and the compensation current I1 or the light receiving current supplied by the current source 331 is subtracted. For the same purpose, a current source 313D for the compensation current I1 is provided between the output of the impedance conversion circuit 311D and the ground, and the compensation current I1 supplied by the current source 332 is subtracted. A current source 312W having a current (bias current) I2 larger than the compensation current I1 is connected between the power supply VDD and the input of the impedance conversion circuit 311W. The bias current I2 from the current source 312W is added to the input current of the impedance conversion circuit 311W to convert the impedance. The current amplifier circuit 314W is formed of a current mirror circuit and has an amplification factor n corresponding to the transistor size. That is, the current amplifier circuit 314W outputs a current obtained by multiplying the photocurrent and the bias current I2 by n. The current source 315W connected between the output terminal of the current amplifier circuit 314W and the ground subtracts n × I2 current from the output current. The amplifying unit 300D has the same configuration, and a voltage generated when an output current corresponding to the same cancellation error flows to the load 500D is supplied to the non-inverting terminal of the APC circuit 600 as a reference voltage. Since the cancellation error occurs equally at the input of the amplifier 600, it is canceled.
[0042]
Thus, according to the present embodiment, the light receiver 11 when the light emitting elements LD1 and LD2 are turned on and off by adding the compensation current I1 and the bias current I2 to the impedance conversion circuits 311W and 311D formed by the current mirror circuit. The voltage variation on the output line L is suppressed, and the input impedance of the impedance conversion circuit 311W is lowered to improve the responsiveness. The current amplifier 300 includes two amplifying units 300W and 300D. The amplifying unit 300D has a constant output corresponding to the bias current I2. The amplifying unit 300W outputs an output corresponding to the light reception signal and the bias current I2. The APC circuit 600 outputs a differential error signal corresponding to the difference between these two outputs, so that the offset current can be canceled.
[0043]
FIG. 2 is a diagram showing a detailed circuit configuration of FIG. Note that the current sources 332 and 313D are omitted here for simplification, but when the compensation current I1 is smaller than the bias current I2, the accuracy drop is small and the influence of the simplification is small. The impedance conversion circuit 311W is a current mirror circuit composed of transistors Q11 and Q12. Transistors Q11 and Q12 have a size of 1: 1. Similarly, the impedance conversion circuit 311D is a current mirror circuit composed of transistors Q15 and Q16. Transistors Q15 and Q16 have a size of 1: 1. The current source 331 and 312W are connected to the drain of the transistor Q11 constituting the input of the current mirror circuit through the switch SWP, and the current source 312D is connected to the drain of the transistor Q15 constituting the output of the current mirror circuit. A current source 313W is connected to the output of the current mirror circuit forming the impedance conversion circuit 311W. On the other hand, a current source is not connected to the output of the current mirror circuit forming the impedance conversion circuit 311D.
[0044]
The current amplifier circuit 314W is a current mirror circuit of the transistors Q13 and Q14. In the configuration shown in the figure, since the size of the transistors Q13 and Q14 is 1: 1, the bias current I2 flows through the current source 315W (n = 1). Similarly, the current amplifier circuit 314D is a current mirror circuit of transistors Q17 and Q18. In the configuration shown in the figure, since the size of the transistors Q17 and Q18 is 1: 1, the bias current I2 flows through the current source 315D (n = 1). As a result, equal cancellation error voltages are generated in the loads 500W and 500D and canceled by the amplifier 600, and the difference between the received light current and the reference current I1 is amplified.
[0045]
Next, the circuit operation of the light quantity control device shown in FIG. 2 will be described.
[0046]
First, the dummy amplifying unit 300D generates a cancellation error voltage by subtracting the bias current I2 supplied from the current source 315D from the bias current I2 supplied from the current source 312D, and provides this as a non-inverting input of the APC circuit 600. Based on the reference voltage Vdmy, the light amount control is performed by the following series of operations so that the light amounts of the light emitting elements LD1 and LD2 become the target light amount corresponding to the compensation current I1.
[0047]
During this light amount control, the compensation current I1 is not supplied from the current source 331 to the output line L. At this time, a photocurrent corresponding to the light amounts of the light emitting elements LD1 and LD2 is output from the light receiver 11 to the output line L. This photocurrent flows through transistor Q11. The current source 313W cancels the reference current I1 and the remaining bias current flows to the transistor Q13. The reason why the current direction of the current source 313W is opposite to that in FIG. 1 is that the current mirror sucks both input and output. A photocurrent which is a received light signal and a bias current I2 flow through the transistor Q11 of the current mirror circuit biased by the current source 312W, and the photocurrent amplified by the action of the current source 313W flows through the transistor Q14 on the output side. A difference from the reference current I1 plus a bias current I2 flows. From this, the bias current I2 is canceled by the current source 315W, this current flows through the load 500W, and the generated voltage (referred to as the light amount detection voltage Vdet) is applied to the inverting input terminal of the APC circuit 600.
[0048]
The APC circuit 600 compares the reference voltage Vdmy and the light amount detection voltage Vdet, and outputs an error voltage corresponding to the difference. This error voltage is sampled by the sample hold circuit 110 and held in the capacitors C111 / C112. Then, the drive circuit 113₁/ 113₂Drives LD1 and LD2 according to the hold voltage of the capacitors C111 / C112. By this series of light amount control, the light amounts of the light emitting elements LD1 and LD2 converge to the target light amount corresponding to the compensation current I1.
[0049]
When the light amount control is completed, no photocurrent is supplied from the light receiver 11 to the output line L. However, when the light emitting elements LD1 and LD2 are off, the compensation current I1 of the current source 331 is supplied to the output line L. Here, since the photocurrent at the time of light amount control converges to the compensation current I1, there is no voltage fluctuation of the output line L.
[0050]
As described above, when the photocurrent is not output from the light receiver 11, the compensation current I1 corresponding to the target light amount is supplied from the current source 331 to the output line L. Even when the current is not output, the compensation current I1 having the same current value as the photocurrent at the time of light quantity control flows through the transistor Q11. Therefore, the potential of the output line L, that is, the input potential of the circuit system that performs the light quantity control. Variation can be suppressed. Thereby, the convergence of the light quantity control can be improved, and the light quantity control can be performed in the same time as the conventional edge emitting laser even if the LD1 and LD2 are surface emitting lasers, for example.
[0051]
Further, when the light emitting elements LD1 and LD2 are surface emitting lasers, as described above, the light receiving area of the light receiver 11 needs to be set wide due to the problem of the optical system of the surface emitting laser. Since the parasitic capacitance Co to be generated is very large, the responsiveness required for the light amount control cannot be secured unless the photocurrent output from the light receiver 11 is received with low impedance. On the other hand, in the light quantity control device according to the present embodiment, the photocurrent supplied through the output line L is received by the impedance conversion circuit 311W, the impedance is increased by the impedance conversion circuit 311W, and then the voltage is converted by the load 500W. Since the light quantity detection voltage Vdet is obtained, the time constant is shortened to improve the convergence. Furthermore, since the photocurrent is received by the one-stage current mirror and the input potential can be made close to the ground potential, a bias voltage to the photoreceiver 11 can be secured, and the photoreceiver speed is increased.
[0052]
Furthermore, when an amplifier is used instead of the current mirror as in Patent Document 1, since the frequency band of the amplifier is close to the frequency band necessary for the light amount control, the light amount control becomes unstable due to interference during the light amount control. When the current mirror is used together with the bias current, the instability of the light quantity control can be suppressed because the band of the current mirror is wider than the band required for the light quantity control.
[0053]
In the light quantity control device according to the present embodiment, further, in the subsequent stage of the current amplifier circuit 314W, a current having a current value equal to the bias current I2 is subtracted from the output current, and the remaining current flows through the load 500W. Only a small current flows through the load 500W. As a result, the light amount detection voltage Vdet obtained at both ends of the load 500W can be brought close to a certain level (here, the GND level) regardless of the bias and the target light amount, so that the APC circuit 600 can be designed without worrying about the dynamic range. In addition, since the output terminal voltage of the light receiver 11 can be brought close to a certain level (here, the GND level), the bias voltage of the light receiver 11 can be secured.
[0054]
Further, in the light amount control apparatus according to the present embodiment, the dummy amplification unit 300D is provided, and the reference voltage Vdmy generated by the dummy amplification unit 300D is given as a non-inverting input of the APC circuit 600. Thereby, the offset can be reliably removed, and the accuracy of the light amount control can be increased.
[0055]
In the present embodiment, the case where the two light emitting elements LD1 and LD2 are driven has been described as an example. However, in laser xerography, for example, when the present invention is applied to driving a surface emitting laser having a large number of light emitting portions as its light source. In this case, a large number of light emitting units are driven. Then, the light amount control is performed for each channel corresponding to a large number of light emitting units. This point will be described later.
[0056]
Thus, for example, in laser xerography, the light amount control time according to the present embodiment is used to drive a surface emitting laser having a large number of light emitting units as its light source, so that the light amount control time in one channel is reduced to the conventional end surface light emission. Scanning efficiency can be improved because the laser light quantity control time can be set. In addition, when controlling the light amount, a light receiving device (for example, a photodiode) having a large parasitic capacitance and a large light receiving area can be used, so that it is not necessary to improve the accuracy of the optical system, which can contribute to cost reduction. Furthermore, since the light quantity control is possible even if the monitor light quantity is small, the ratio of the light quantity to the photoconductor increases, and the surface emitting laser can be used with a low quantity of light, so the reliability of the surface emitting laser is improved accordingly. As well as longer life.
[0057]
Even if the current amplification degree n is increased from 1, it is canceled because the offsets included in both the + and-inputs of the amplifier 600 are equal and are not affected by the offset. However, if the offset increases too much and exceeds the input dynamic range of the amplifier 600, there is a possibility that the amplifier 600 does not operate normally.
[Second Embodiment]
FIG. 3 shows a circuit configuration of a light amount control apparatus according to the second embodiment of the present invention, which improves the problem of the dynamic range of the amplifier 600. In the figure, the same reference numerals are assigned to the same components as those shown in the above-mentioned drawings, and the description thereof is omitted.
[0058]
In the circuit configuration in FIG. 2, two loads 500W and 500D are used, whereas in the circuit configuration in FIG. 3, one load 500 is used. One end of the load 500 is connected to the drain of the transistor Q14, and the other end is connected to the output terminal of the operational amplifier 340. Also, the current sources 315W and 315D in FIG. 2 are replaced with a cascode current mirror circuit including the transistors Q21, Q22, Q23, and Q24 in FIG. The cascode current mirror circuit subtracts the output current of the current amplification circuit 314D from the output current of the current amplification circuit 314W and supplies the remaining current to the load 500. The other end of the load 500 becomes the reference voltage Vdmy corresponding to the bias current I2. In this embodiment, the reference voltage Vdmy is generated from the bias current I2. However, as long as the reference voltage Vdmy can be included in the input dynamic range of the amplifier 600, the reference voltage Vdmy may be generated with a constant current independent of the bias current.
[0059]
Thus, even with the circuit configuration of FIG. 3 in which the load is shared, the same operation and effect as in FIG. 2 can be obtained. According to the second embodiment, the circuit configuration is simplified and the number of parts is reduced. As described above, the bias current I2 must be set so as to obtain an impedance in consideration of the parasitic capacitance of the light receiver 11. Further, when the gain n of the current amplifier circuits 314W and 314D is set large, it is preferable to set an appropriate value in consideration of the cancellation error.
[Third Embodiment]
FIG. 4 shows a circuit configuration of a light amount control apparatus according to the third embodiment of the present invention. In the figure, the same reference numerals are assigned to the same components as those shown in the above-mentioned drawings, and the description thereof is omitted.
[0060]
The third embodiment corresponds to a modification of the second embodiment, in which the bias current I2 of the output current of the current amplifier Q14 is made constant regardless of the gain. The current source 316W shown in FIG. 4 is for making the output current of the transistor Q12 cancel a part of the remaining bias current after the light receiving current is canceled by the current source 313W, and to make the output current of the transistor Q14 constant. The same applies to the current source 316D shown in FIG. Since the operational amplifier 350 controls the current sources 316W and 316D of the current I3 so that the output current of the current amplifier circuit 314D and the bias current I2 of the current source 317D coincide, the bias current superimposed on the photocurrent is current amplified. It becomes a value corresponding to the amplification degree of the circuits (current mirror circuits) 314W and 314D, and the difference current between the output of the current amplifier circuit 314W on the photocurrent side and the bias current 317W becomes only the cancellation error. As described above, the operational amplifier 350 is a circuit that automatically sets the current amplifier output current using the internal current of the dummy amplifying unit 300D. In the third embodiment, since the bias current included in the output current can be made constant, the bias current occupying the main part of the output current is controlled to be constant even when the current amplification factor is increased. Therefore, the transistor can be optimally set in size because it is only necessary to design it in accordance with the controlled current. Rather than amplifying the bias current n times and then canceling with the n times bias current, the bias current is subtracted from the 1 times bias current output regardless of the current amplification factor, so the cancellation error is absolute. The problem that the value increases and the offset in the next stage increases can be prevented.
[0061]
Here, of the two-stage current mirror circuits, the optimization of the transistor size (setting of the amplification factor n) constituting the latter-stage current mirror circuits 314W and 314D is selected by providing a plurality of transistors on the output side, for example. It is sufficient to use it. The configuration shown in FIG. 4 is an example in which a transistor Q25 is connected in parallel to a transistor Q14. The drain of the transistor Q14 on the output side of the current mirror circuit is connected to the load 500 via a transistor Q26 (fixed on) whose gate is grounded. Similarly, the transistor Q25 is connected to the load 500 via a transistor Q27 that functions as a switching means. In the illustrated example, the gate of the transistor Q27 is not grounded and is fixed off. By grounding the gate G of the transistor Q27, which is a control terminal of the switch means, based on the external control signal G1 supplied from the outside, the transistor Q25 is connected in parallel with the transistor Q14, and the amplification degree can be set large. . When the sizes of the transistors Q13, Q14, and Q25 are equal, in other words, when the parameters related to the amplification of these transistors are equal, the amplification is doubled by activating the transistor Q25 in addition to the transistor Q14. The transistors Q13, Q14, and Q25 are unipolar transistors (FETs) represented by MOS transistors. In this case, the parameter related to the amplification factor is the ratio of the channel width to the channel length. The drain current of the FET is proportional to the channel width W / channel length L. Actually, in the current mirror circuit, the channel lengths of a pair of transistors are made equal in order to obtain a desired accuracy. However, if high accuracy is not necessarily required, even if transistors having different channel lengths are combined, W / It is possible to design with L as a guide.
[0062]
Similarly, the transistor Q18 located on the output side of the dummy side is connected to the inverting input terminal of the operational amplifier 350 via the transistor Q29, and the transistor Q28 is connected to the inverting input terminal via the transistor Q30 functioning as a switching means. It is connected. The degree of amplification can be increased by the same amount by grounding the gate of the transistor Q30, which is the control terminal of the switch means, based on the external control signal G1.
[0063]
  As described above, the amplification degree of the current mirror circuits 314W and 314D can be controlled by the number of the plurality of transistors on the output side, and is determined by the number to be connected. As a result, the amplification degree of the current mirror circuits 314W and 314D can be adjusted easily and accurately. The bias current becomes a value corresponding to the set amplification degree by feedback control of the operational amplifier 350.
[Fourth Embodiment]
  FIG. 5 is a circuit diagram showing a configuration example of a current source 331 that supplies the compensation current I1 according to the fourth embodiment of the present invention. In the illustrated configuration, a current source 325 is used as a reference current source, and three current sources 331 for supplying a compensation current are provided.compensationCurrent source 331₁331₂331_ThreeIt consists of. These three current sources 331₁331₂331_ThreeIs a current switch 337 which is a double throw switch₁337₂337_ThreeCan be selectively connected to the drain of the transistor Q11. Current source 331₁331₂331_ThreeIs not selected (when off), the function of the transistor 335 connected to the current source 334 causes the output of the current source that is turned off to have substantially the same potential as before the output is turned off. Thereby, switching noise at the time of on / off switching can be prevented.
[0064]
  In the configuration shown in FIG. 5, since the current is converted by the reference current (1 + α) · I1 of the current source 313W after impedance conversion by the current mirror (α is a constant ratio with respect to the reference current), the photocurrent + compensation current I1 is the light emitting element LD1. In total, a current of (1 + α) · I1 flows including the on / off state of LD2. That is, when the light emitting elements LD1 and LD2 are ON, the photocurrent is controlled by the light amount control so that the total current becomes (1 + α) · I1. For example, if the compensation current is (2α) · I1, the photocurrent is controlled to match (1−α) · I1. In this way, by setting the compensation current to a constant ratio α with respect to the reference current, even when a light amount different from the target light amount (for example, a light amount lower than the target light amount) is required, it is easily compensated The current value can be set. Control using the constant ratio α is useful for correcting unevenness in the amount of light between the light emitting elements LD1 and LD2, for example. In addition,compensationCurrent source 331₁331₂331_ThreeThe on / off switch of FIG. 6 has a differential transistor configuration as shown in FIG. 6 and further controls the potential at the time of OFF so that the current source output is substantially equal to that at the time of OFF, thereby reducing switching noise at the time of switching. be able to.
[Fifth Embodiment]
  FIG. 7 is a diagram showing a fifth embodiment of the present invention. In the configuration shown in the figure, a current source 361 for correcting superimposed current (2β · I1).₁And superimposed current β · I1 current source 361₂Is arranged. In addition, a current source of 3β · I1 for subtracting the superimposed current is also provided. As described above, by providing the superimposed current separately from the compensation current, it is possible to perform light amount correction for each light emitting element with a different value separately from light amount correction common to the light emitting elements LD1 and LD2. This can be used, for example, when there is a difference in the amount of light on the photoreceptor due to the optical system in the light emitting element. Naturally, the present invention can be applied not only to the cause of the optical system but also to the case where the image quality can be expected to be improved by adjusting the light amount for each light emitting element as the characteristics of the entire laser xerography. In this way, when the light emitting elements LD1 and LD2 are ON, the photocurrent is controlled by the light amount control so that the total current becomes (1 + α + 3 · β).
[Sixth Embodiment]
  FIG. 8 is a diagram showing a sixth embodiment of the present invention. In the configuration shown in the figure, light amount control is performed for each of the light emitting elements LD1 and LD2. In this case, unlike FIG. 7, the reference current on the output side of the impedance conversion circuits 311W and 311D of the current mirror circuit is also changed correspondingly. This is intended to improve accuracy by reducing the canceling current. In this case, since the current to the input of the impedance conversion circuits 311W and 311D by the current mirror circuit varies depending on the compensation current, it takes time for the potential of the line L of the photocurrent to converge when the setting is changed. However, the entire light amount control that takes time is common to all the light emitting elements and may be performed only once in advance. The light quantity control of the individual laser is controlled by the current source 361.₂(Β · I1) and the current source 361₁If (2β · I1) is performed, the potential of the photocurrent output line L does not fluctuate, so that the convergence of each of the light emitting elements LD1 and LD2 is not sacrificed.
[Seventh Embodiment]
  Next, a light quantity control apparatus according to the seventh embodiment of the present invention will be described with reference to FIGS.
[0065]
FIG. 9 is a diagram showing an overall configuration of a light amount control apparatus according to the seventh embodiment of the present invention. In FIG. 9, the light quantity control device 10 drives a plurality of light emitting elements. In the configuration of FIG. 9, the light quantity control device 10 drives 32 light emitting elements LD1 to LD32. In other words, the light quantity control device 10 has a 32-channel configuration. Each of the light emitting elements LD1 to LD32 is formed of a surface emitting diode (VCSEL: Vertical Cavity Surface Emitting Laser) and arranged in a matrix. The light quantity control device 10 is formed of, for example, an IC chip and includes a circuit described below.
[0066]
The light quantity control device 10 has a driver 100 for each channel, that is, for each of the light emitting elements LD1 to LD32.₁~ 100₃₂Have The light quantity control device 10 includes a common control potential setting circuit 200, a current amplifier 300, a light quantity monitor 400, a forced lighting circuit 500, and an APC circuit 600 as a common control unit for each channel. The current amplifier 300 has the configuration according to the first to third embodiments described above. The forced lighting circuit 500 has a load function of the loads 500W, 500D, and 500 shown in FIGS. 1 to 4, and can adjust the size of the load, and is a light emitting element necessary for determining the drawing start position of the image signal It is equipped with a forced lighting function.
[0067]
Driver 100₁~ 100₃₂Receives a signal from the control unit common to the respective channels via the bus 150, and performs control for driving and controlling the light emitting elements LD1 to LD32. Specifically, the driver 100₁~ 100₃₂Performs APC control for controlling the light amount of each of the light emitting elements LD1 to LD32 and modulation control after the APC control. As described later, in APC control, the driver 100₁~ 100₃₂Controls both the voltage and current applied to the light emitting elements LD1 to LD32. Driver 100 during voltage drive₁~ 100₃₂Is a capacitor Cd connected to the cathode of each of the light emitting elements LD1 to LD32 via each terminal COUT.₁~ Cd₃₂To control. Driver 100 during current drive₁~ 100₃₂Controls the amount of current flowing through each light emitting element LD1 to LD32 via each terminal LDOUT.
[0068]
Driver 100₁~ 100₃₂Are connected in common via a terminal LDCOM and connected to a load 105. In the configuration of FIG.₁~ 100_FourThe LDCOM terminals are connected in common, and one end is connected to the other end of the load 105 connected to the ground. Each driver 100₁~ 100₃₂Outputs a current (complementary output) corresponding to the drive current when the corresponding light emitting element is not driven. By flowing this current through the load 105, a constant current always flows to the light quantity control device 10 without depending on the number of lighting of the light emitting elements, and the operation is stabilized.
[0069]
The light quantity control device 10 performs modulation control after setting the laser light quantity of each of the light emitting elements LD1 to LD32 to an appropriate value by APC control. The outline of APC control is as follows. First, the laser light amount of the light emitting element LD1 is adjusted. Driver 100₁Drives the light emitting element LD1. A current corresponding to the laser light amount of the light emitting element LD1 flows through a light receiver PD (for example, a photodiode, which corresponds to the above-described light receiver 11) provided in common to each of the light emitting elements LD1 to LD32. The current amplifier 300 is configured to switch SWSa (current switch 337 in FIG.₂And a current obtained by adding the addition current from the current source 450 is received with low impedance and amplified. In this case, the switch SWSb (current switch 337 in FIG._ThreeIs turned on, the added current is canceled by the reference current supplied from the current source 460, the remaining current is supplied to the resistor connected to the reference voltage Vref2, and the current output from the current amplifier 300 is set to the voltage. This voltage (referred to as a detection voltage) is output to the APC circuit 600 via the switch SW19. The APC circuit 600 includes a plurality of operational amplifiers 61, a series circuit of one switch (any one of SWfb1 to SWfb32) and a capacitor (any one of Cfb1 to Cfb32). The APC circuit 600 shown in FIGS. 1 to 8 corresponds to the operational amplifier 61 shown in FIG. Each series circuit is connected between the output terminal and the inverting input terminal of the operational amplifier 61. Each series circuit constitutes a sample and hold circuit. One sample and hold circuit corresponds to one light emitting element. For example, a sample and hold circuit including the switch SWfb1 and the capacitor Cfb1 corresponds to the light emitting element LD1. Similarly, a sample and hold circuit including the switch SWfb32 and the capacitor Cfb32 corresponds to the light emitting element LD32.
[0070]
The operational amplifier 61 amplifies the difference voltage when the light emitting element LD1 is driven and outputs the amplified voltage to the corresponding signal line of the bus 150. Driver 100₁Changes the drive current applied to the light emitting element LD1 so that the differential voltage becomes zero. As a result, the amount of laser light from the light emitting element LD1 changes, and the amount of current flowing through the light receiver PD changes. A detection voltage corresponding to the current flowing through the light receiver PD is output from the current amplifier 300 to the APC circuit 600. By such feedback control, the added current applied to the input output of the current amplifier 300 is canceled and disappears, and the driving state of the light emitting element LD1 is set so that the laser light quantity corresponds to the reference current generated by the APC reference voltage Vref. Set. The setting of the driving state means that both the driving voltage and the driving current applied to the light emitting element LD1 are adjusted to a value corresponding to the APC reference voltage Vref.
[0071]
While controlling the light emitting element LD1 in this way, only the switch SWfb1 is turned on among the 32 sample hold circuits of the APC circuit 600, and the laser light quantity of the light emitting element LD1 corresponds to the APC reference voltage Vref. The voltage at the time of convergence to the value to be stored in the capacitor Cfb1. Similarly, the APC control is performed on the light emitting elements LD2 to LD32 one by one in order.
[0072]
As will be described later, the APC control is preferably performed twice. In the second APC control, the switch SWSa that was turned on in the first APC is turned off. Since the cancellation current supplied to the output side of the current amplifier 300 is the reference current + addition current, the received light current is controlled by a current corresponding to the reference current + addition current. The 32 sample and hold circuits in the APC circuit 600 can be commonly used in the first and second APC controls, but 32 sample and hold circuits may be newly provided for the second APC control.
[0073]
The light quantity monitor 400 outputs a light quantity monitor signal indicating the laser light quantity of each of the light emitting elements LD1 to LD32 from the current flowing through the current amplifier 300.
[0074]
The forced lighting circuit 500 is a circuit that generates a synchronization signal required before performing APC control. In image processing apparatuses such as copiers, printers, and facsimile machines in which the light quantity control device 10 is incorporated, a light sensor is provided slightly before the drawing start position to accurately determine the position at which the image is drawn, and the light emitting element outputs The drawing start position is determined based on the timing when the light to be crossed the optical sensor.
[0075]
FIG. 11 shows a configuration example of a laser scanning system and output of each sensor in laser xerography, which is an aspect of an image forming apparatus including the light quantity control device of the present invention. The basic configuration of the laser beam scanning system in the laser xerography apparatus is as follows. The laser light emitted from the laser light source 10d is irradiated onto the photosensitive member surface 16 through the lens 15, the polygon mirror 12, and the lenses 13 and 14. As the polygon mirror 12 rotates, the laser beam repeatedly scans the photosensitive member surface 16. Further, part of the laser light emitted from the laser light source 10 d is input to the light receiver 11 via the semi-transmissive mirror 19. In FIG. 11, the output of the photoreceiver 11 at this time is shown as the light amount control sensor output, and the output of the optical sensor provided slightly before the drawing start position is shown as the SOS (Start of Scan) sensor output. The area for APC is provided before and after the scanning area. Reference numeral 18 corresponds to the light quantity control device 10 described above.
[0076]
As described above, since the individual laser light amounts of the light emitting elements LD1 to LD32 are smaller than those of the end face laser, a plurality of the light emitting elements LD1 to LD32 are simultaneously turned on to scan the SOS sensor. In this case, it is preferable to turn on only a plurality of light emitting elements located in the center portion among the light emitting elements arranged in two dimensions. However, in APC control, the light emitting elements are turned on one by one and the conditions are set (gain of feedback loop), so if a predetermined number of light emitting elements are turned on at the same time, the feedback loop of APC control will oscillate. There is a possibility that. Therefore, in order to solve this problem, the forced lighting circuit 500 changes the load size of the current amplifier 300 according to the modulation signal (modulation data). That is, a load corresponding to the number of light emitting elements to be turned on is connected to the output of the current amplifier 300. In the configuration shown in the figure, a plurality of resistors are connected to the output of the current amplifier via a switch. Focusing on the operational amplifier 61, the forced lighting circuit 500 reduces the current-voltage conversion gain in accordance with the number of light emitting elements to be turned on, so that the gain of negative feedback as a whole does not change. With such a configuration, a state equivalent to a state in which only one light emitting element is always turned on can be obtained. In other words, the gain of the feedback loop is a value in a state in which only one light emitting element is turned on. As a result, it is possible to prevent the feedback loop from oscillating.
[0077]
The common control potential setting circuit 200 includes each driver 100.₁~ 100₃₂This circuit generates a control potential necessary for generating various currents required in the circuit. In the configuration of FIG. 8, the common control potential setting circuit 200 includes each driver 100.₁~ 100₃₂A circuit for generating a common potential for setting a bias current flowing therein, and a circuit for generating a common potential for generating an offset current. The bias current and the offset current are typical examples, and each driver 100₁~ 100₃₂Can set the control potential required to generate other currents required for drive and control. The common control potential for setting the bias current is generated by a circuit including an operational amplifier (op-amp) 211, current sources 212 and 213, and loads 214 and 215. A common control potential for offset current setting and other current setting is also generated by the same circuit. The current source 212 supplies the instructed current to the load 214 in response to an external bias current setting signal. The terminal voltage of the load 214 is applied to the plus side terminal of the operational amplifier 211. A constant current source 213 connected to the constant voltage source 216 causes a current corresponding to the output of the operational amplifier 211 to flow through the load 215. The terminal voltage of the load 215 is given to the negative terminal of the operational amplifier 211. The operational amplifier 211 controls the current source 213 so that the current source 213 passes the same current as the bias current set by the bias current setting signal. At this time, the output signal of the operational amplifier 211 is output to the corresponding bus line of the bus 150. On the other hand, the positive voltage of the constant voltage source 216 is output to the corresponding bus line of the bus 150. This bus line is common to each common control potential and each driver 100.₁~ 100₃₂Is common. As described above, the bias current value set from the outside is in the form of a differential voltage via each bus 150 to each driver 100.₁~ 100₃₂To be supplied. Each driver 100₁~ 100₃₂Generates a bias current from the received differential voltage as described later. As a result, even if the power supply voltage of the constant voltage source 216 fluctuates, the potential difference is constant, and the influence of fluctuations in the power supply voltage can be avoided. Note that the output voltage of the operational amplifier 211 and the voltage of the constant voltage source 216 are preferably transmitted by balanced two wires.
[0078]
Next, referring to FIG.₁~ 100₃₂The internal structure of will be described. Each driver 100₁~ 100₃₂Since the configuration is the same, the subscripts 1 to 32 are omitted in the following description, and only the driver 100 will be described.
[0079]
The driver 100 has two multipliers 21 and 22. Multiplier 21 is provided to control current source 30, and multiplier 22 is provided to control a corresponding one of capacitors Cd1 to Cd32 shown in FIG. Hereinafter, for convenience, one corresponding capacitor is denoted by Cd, and is indicated by a broken line in FIG. The capacitor Cd functions as a voltage source for a short time when the drive voltage to the laser rises. The current source 30 generates a current that flows through the corresponding light emitting element LD, and the capacitor Cd that functions as a voltage source supplies a driving voltage to the corresponding light emitting element LD.
[0080]
Here, the relationship between the driving current and the driving voltage (terminal voltage) of the surface emitting laser (voltage-current characteristics) is proportional (linear relationship) in a practical range because the internal resistance of the surface emitting laser is high. Further, the relationship between the drive current and the laser light quantity is also proportional (linear relationship) within a practical range. Based on such characteristics, the current amount of the current source 30 in the first APC control is determined so that the laser light amount of the light emitting element LD becomes the reference light amount (first light amount). The amount of light is determined to be the second amount of light. Similarly, the driving voltage accumulated in the capacitor Cd in the first APC control is determined so that the laser light amount of the light emitting element LD becomes the reference light amount (first light amount), and the laser light amount is the second in the second APC control. The amount of light is determined. By the interpolation or extrapolation process using these two values, the laser light quantity can be corrected to an arbitrary light quantity.
[0081]
Multipliers 21 and 22 can use 4-quadrant analog multipliers, and capacitors can be used as voltage sources to be connected to the multipliers. The inputs of the multipliers 21 and 22 have a differential configuration. When two differential inputs represented by + and − of each multiplier 21 and 22 are V1a, V1b and V2a and V2b, respectively, each multiplier 21 and 22 having a differential configuration has Iout = α (V1a−V1b). The current described by (V2a-V2b) is output. Where α is a constant.
[0082]
In such a laser driving device, a correction signal is input to one input terminal (multiplier terminal) of each multiplier 21 and 22, and a control voltage is input to the other input terminal (multiplier terminal). When the + side output of the complementary output of a multiplier that is normally configured as a differential is used, there is an offset current, but even if there is an offset in each of the multipliers 21 and 22, the capacitors C1 and C2 connected to the output are used. The offset is canceled during APC. The correction signal takes into consideration the situation where the amount of laser light varies depending on the scanning position of the laser beam, and has a control voltage corresponding to the scanning position of the laser beam.
[0083]
First, by the first APC control, the first light amount (set as a reference value) is set as follows. Switch SWSa is on, SWSb is off, SW1 is off, SW2 is off, SW3 is off, SW5-1 is on, SW5-2 is off, SW5-3 is off, SW5-4 is on, SW6-1 is on SW6-2 is off, SW6-3 is off, SW6-4 is on, SW7 is off, SW8 is on, SW11 is on, SW11-1 is on, SW11-2 is off, SW12 is off, SW13 is on , SW15-1 is turned off, SW15-2 is turned on, SW16 is turned off, and the switch SWSa is turned on. Further, when setting the first light quantity, a 0 V correction signal is applied to the multiplier terminals of the multipliers 21 and 22. In this state, since the multiplier is 0, each multiplier 21 and 22 outputs an offset voltage regardless of what control voltage is input to the multiplicand terminal. Further, the first APC reference voltage Vref1 is applied to the operational amplifier 61 of the APC circuit 600 shown in FIG. The operational amplifier 61 outputs a control voltage so that the laser light amount of the light emitting element LD becomes the first APC reference voltage Vref1. This control voltage is supplied to the current source 30 through the switch SW8, the operational amplifier 26, the inverter 28 and the switch SW11 of FIG. The current source 30 supplies a current corresponding to the received control voltage to the light emitting element LD. The control voltage output from the operational amplifier 26 is stored in the capacitor C3-1 of the sample and hold circuit. Since the correction signal is set to 0V, the multiplier 21 outputs an offset voltage. Therefore, the capacitor C1 is charged with a difference voltage between the control voltage and the offset voltage output from the multiplier 21. On the other hand, the control voltage output from the operational amplifier 61 in FIG. 8 is supplied to the capacitor C2 and stored in the capacitor C4-1 of the sample and hold circuit. Since the correction signal is set to 0V, the multiplier 22 outputs an offset voltage. Therefore, the capacitor C2 is charged with a difference voltage between the control voltage and the offset voltage of the multiplier 22.
[0084]
Then, the second light quantity (this is called a correction light quantity) is set as follows by the second APC control. Switch SWSa off, SWSb off, SW1 off, SW2 off, SW3 off, SW5-1 off, SW5-2 on, SW5-3 on, SW5-4 off, SW6-1 off SW6-2 is on, SW6-3 is on, SW6-4 is off, SW7 is off, SW8 is off, SW11 is off, SW11-1 is on, SW11-2 is off, SW12 is off, SW13 is on , SW15-1 is off, SW15-2 is off, SW16 is off, and SWSa is off. Further, when setting the second light quantity, a correction signal having a predetermined voltage is applied to the multiplier terminals of the multipliers 21 and 22. Further, since the switch SWSa is off, the operational amplifier 61 outputs a control voltage for the first APC control so that the amount of light from the light receiver PD increases by the amount of current added by the current source 450. This control voltage is supplied to the current source 30 through the switch SW8, the operational amplifier 26, the inverter 28, the switches SW5-2 and SW5-3, the multiplier 21, the resistor R11, and the capacitor C1 in FIG. The current source 30 changes the current from the light receiver PD from the reference current to a current obtained by adding the addition current to the reference current in accordance with the received control voltage. The control voltage output from the operational amplifier 26 is stored in the capacitor C3-2 of the sample and hold circuit. The capacitor C1 is charged with a voltage difference between the control voltage and the output of the multiplier 21. If the current applied to the light emitting element LD in the first APC control is I, the current applied to the light emitting element LD in the second APC control can be described as I + ΔI. On the other hand, the control voltage output from the operational amplifier 61 of FIG. 9 is supplied to the capacitor C2 and stored in the capacitor C4-2 of the sample and hold circuit. The capacitor C2 is charged with a voltage difference between the control voltage and the output of the multiplier 22. If the voltage stored in the capacitor C2 in the first APC control is V, the voltage stored in the capacitor C2 in the second APC control can be described as V + ΔV.
[0085]
Here, the switches SW6-1 and SW6-4 are turned on, and the SW6-2 and SW6-3 are turned off. However, in the second and subsequent APCs, SW6-3 and SW6-1 are turned on, and SW6-2 and SW6-4 are turned off. Since this is the same condition as during modulation, an improvement in accuracy can be expected.
[0086]
At the time of modulating the light emitting element LD, a correction voltage corresponding to the light amount correction amount corresponding to the scanning position of the laser light is input to the multiplier terminals of the multipliers 21 and 22. Thereby, both the drive voltage applied to the surface emitting laser from the voltage source constituted by the multiplier 22, the capacitor C2, and the operational amplifier 26 and the drive current supplied from the current source 30 to the light emitting element LD are simultaneously controlled. The light emitting element LD emits light with a light amount corrected according to the scanning position of the laser light.
[0087]
A resistor R11 is connected in series with the capacitor C1. That is, in the present embodiment, the sample and hold circuit including the capacitor C1 is configured with a low-pass filter. Thereby, the high frequency noise which generate | occur | produces when switching on / off of switch SW11 can be suppressed. In addition, a capacitor C11 is connected in parallel to this low-pass filter. This can prevent the phase of the negative feedback loop from being delayed due to the time constant of the low-pass filter. Similarly, by connecting a resistor R21 in series with the capacitor C2, a sample and hold circuit including this is constituted by a low-pass filter. Thereby, the high frequency noise which generate | occur | produces when switching on / off of switch SW8 can be suppressed. Further, a capacitor C21 for preventing a phase delay of the negative feedback loop is connected in parallel to a low-pass filter composed of the capacitor C2 and the resistor R21, thereby preventing oscillation in the negative feedback loop.
[0088]
The voltage application time adjustment circuit 800 controls the switch SW2 to adjust the time for applying a voltage to the light emitting element LD. This voltage is a voltage accumulated in the capacitor Cd. As described above, in this embodiment, the light emitting element LD is driven by controlling both the voltage and current applied to the light emitting element LD. When the light emitting element LD is driven, it is first driven with a voltage and then with a current. By making the voltage application time for voltage drive adjustable, the voltage application time according to the mounting state of the light emitting element LD can be appropriately set as in the case where the wiring from the LDOUT end to the laser in FIG. Can be set.
[0089]
The voltage application time adjustment circuit 800 includes two sets of a delay circuit 81 and an exclusive OR circuit 82. The two delay circuits 81 are connected by an inverter 83 as illustrated. The delay circuit 81 receives the voltage application time signal and the modulation signal, and delays the modulation signal according to the voltage application time signal. The exclusive OR of the output signal of one delay circuit 81 and the modulation signal is taken, and the switch SW2 is turned on by the output signal. As a result, the output signal has a first pulse that rises at the rise of the modulation signal, a first pulse that falls at the rise of the delayed modulation signal, and a second pulse that rises at the fall of the modulation signal and falls at the fall of the delayed modulation signal. appear. That is, the voltage is applied at the rise and fall of the modulation signal with the same pulse width as the delay time of the delay circuit 81. In this way, it is possible to set an appropriate voltage application time. Similarly, by controlling the switch SW1 by the action of the other delay circuit 81 and the exclusive OR circuit 82 to supply an OFF bias, the operation of the light emitting element LD from on to off is controlled (speeded up). .
[0090]
The current generation circuit 700 receives the differential voltage for each current output from the common control potential setting circuit 200 shown in FIG. 8, and generates a current corresponding to the differential voltage. The operational amplifier 34 and the constant current source 32 of the current generation circuit 700 receive a differential voltage formed by the reference common potential and the reference offset potential, and generate an offset current corresponding to the differential voltage. The offset current flows to the load 24 via the switch SW16. The terminal potential of the capacitor C2 is determined in accordance with the offset current, whereby the driving voltage applied to the light emitting element LD by the capacitor C2 functioning as a voltage source can be adjusted. By adjusting the drive voltage, you can overshoot the drive pulse and follow the laser to a short pulse width to improve the reproducibility of highlights, and set the drive voltage slightly larger to make the image outline It can also be used for image quality adjustment by appropriately setting these according to the image, such as emphasis. The operational amplifier 35 and the current source 31 receive the differential voltage formed by the reference common potential and the reference bias potential via the switch 750, and generate a bias current according to the differential voltage. Further, the current source 31 that has received the OFF bias voltage set by the voltage source in the drawing connected to the switch 750 generates a laser drive current corresponding to the OFF bias voltage. The bias current generated here is a test current for determining the OFF bias voltage, and a period for determining the OFF bias voltage is provided before or after the APC. The laser beam is supplied to each laser, and an OFF bias voltage common to all lasers is determined based on the laser terminal voltage at that time. Further, the value of the voltage source in the figure connected to the switch 750 is set based on the current that flows when the determined common OFF bias voltage is applied to the laser terminal voltage, and the OFF bias is applied by the voltage source in this figure during modulation. The current is controlled.
[0091]
As described above, the light quantity control device according to the seventh embodiment has the functions and effects according to the first to sixth embodiments, that is, has a large dynamic range with little offset error, and various other features described above. It is an apparatus provided with.
[0092]
【The invention's effect】
As described above, according to the present invention, it is possible to provide a light amount control apparatus having a large dynamic range with little offset error.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of a light amount control apparatus according to a first embodiment of the present invention.
FIG. 2 is a circuit diagram of the light quantity control device shown in FIG.
FIG. 3 is a circuit diagram showing a configuration of a light amount control apparatus according to a second embodiment of the present invention.
FIG. 4 is a circuit diagram showing a configuration of a light amount control apparatus according to a third embodiment of the present invention.
FIG. 5 is a diagram showing a configuration of a light amount control apparatus according to a fourth embodiment of the present invention.
6 is a diagram showing the configuration of FIG. 5 in more detail.
FIG. 7 is a diagram showing a configuration of a light amount control apparatus according to a fifth embodiment of the present invention.
FIG. 8 is a diagram showing a configuration of a light amount control apparatus according to a sixth embodiment of the present invention.
FIG. 9 is a block diagram showing an overall configuration of a light amount control apparatus according to a seventh embodiment of the present invention.
10 is a circuit diagram showing an internal configuration of the driver shown in FIG. 9;
FIG. 11 is a diagram illustrating a configuration example of a laser scanning system and output of each sensor in laser xerography, which is an aspect of an image forming apparatus including the light amount control device of the present invention.
[Explanation of symbols]
10 Light quantity control device LD1-LD32 Light emitting element
100₁~ 100₃₂  Driver 200 Common control potential setting circuit
211 Operational Amplifier (Op Amp) 212, 213 Constant Current Source
214, 215 Load 216 Constant voltage source
300 Current amplifier 400 Light intensity monitor
500 Forced lighting circuit 600 APC circuit
61 Operational amplifier SWfb1 to SWfb32 switch
Cfb32 to Cfb32 capacitors
Vref, Vref1, Vref2 APC reference voltage
150 Bus COUT terminal Cd₁~ Cd₃₂  Capacitor
LDOUT terminal LDCOM terminal 11 Load
PD receiver SW19 Switch 21, 22 Multiplier
30 Current source 26 Operational amplifier 28 Inverter
800 Voltage application time adjustment circuit 81 Delay circuit
82 Exclusive OR circuit 700 Current generation circuit
34 operational amplifier 32 constant current source 24 load
35 operational amplifier 900 bias circuit
31 Current source R11, R21 Resistor C11,
Cd, C1, C2, C3-1, C4-1, C3-2, C4-2 capacitors
SW1, SW2, SW3, SW5-1, SW5-2, SW5-3, SW5-4, SW6-1, SW6-2, SW6-3, SW6-4, SW7, SW8, SW11, SW11-1, SW11- 2, SW12, SW13, SW15-1, SW15-2, SW16 switch

Claims (14)

A light receiving element for receiving light from the light emitting element, a first amplifying means for amplifying an input signal from the light receiving element,
A first compensation current source for providing a first compensation current to an input of the first amplification means when the light emitting element is off;
A second amplifying means for amplifying a non-input signal, having a main part having a common configuration with the main part of the first amplifying means, in order to remove the offset generated in the first amplifying means;
A second compensation current source for providing a second compensation current to the input of the second amplification means;
Error amplifying means for generating a control signal for automatic light quantity control based on output signals of the first and second amplifying means;
Driving means for driving the light emitting element based on an output signal of the error amplifying means ;
Comprising
The first and second compensation currents have a value equal to a light receiving current from the light receiving element corresponding to a target light amount,
The main part includes conversion means for converting the input impedance to an impedance higher than the input impedance, and current amplification means for current-amplifying the output of the conversion means. The conversion means converts the input impedance to a predetermined automatic light amount control time To / Smaller than the impedance determined by the stray capacitance Co of the light emitting element,
The converting means and the current amplifying means include a bias current source for supplying a bias current, and a current mirror circuit on which the bias current is superimposed on the input side,
The light amount control device according to claim 1, wherein the bias current of the current amplifying means is determined in accordance with an amplification degree of a current mirror circuit of the current amplifying means . The current mirror circuit of the current amplifying means has an input-side transistor having a control terminal to which a control potential is applied, and a control terminal commonly connected to the control terminal of the input-side transistor, and are connected in parallel to each other. A plurality of output-side transistors and a current source commonly connected in series to the plurality of output-side transistors, and the amplification factor of the current amplification means is determined by the number of the plurality of output-side transistors. The light quantity control device according to claim 1 , wherein Based on the output voltage of the current mirror circuit of the current amplifying means in the second amplifying means and the switching means connected in series to the plurality of transistors, the bias of the current amplifying means in the first and second amplifying means Control means for controlling the potential of the control terminal of the current source, and the switch means corresponding to the output-side transistor to be selected according to the amplification degree is controlled based on an external control signal. The light quantity control device according to claim 2 . 4. The light quantity control device according to claim 2, wherein the parameters relating to the amplification degrees of the plurality of output side transistors are equal. 5. The light quantity control device according to claim 4, wherein the plurality of output-side transistors are unipolar transistors, and the parameter relating to the amplification degree is a ratio between a channel width and a channel length. The light quantity control device according to claim 1, wherein the first and second amplifying units have individual loads for each output, and each output is connected to a differential input of the error amplifying unit. 2. The light quantity control device according to claim 1, wherein outputs of the first and second amplifying means are connected to a differential input of the error amplifying means, and a common load is provided between the differential inputs.   The first amplifying means includes a first current mirror circuit that receives the input signal and a second current mirror circuit that amplifies the output current of the first current mirror circuit. The light quantity control device according to claim 1.   The first amplifying means has an output corresponding to the input signal and a first bias current, and the second amplifying means is responsive to a second bias current having the same magnitude as the first bias current. 2. The light quantity control device according to claim 1, wherein the error amplification means outputs an output corresponding to a difference between the two outputs. Claim 9, characterized in that the output of the first amplifying means and the second amplifying means to be maintained below a certain level, the value of the first and second bias current is set The light quantity control apparatus of description. Instead of the first and second compensation current sources, a third compensation current source for adding a third compensation current to the reference current flowing from the reference current source is provided, and the third compensation current source is turned on / off. light quantity control device when changing the target amount of light, according to claim 1, wherein the third compensation current, characterized in that set to a constant ratio relative to the reference current by. The third compensation current source is turned on and off by a double throw switch, and at the time of turning off, the third compensation current source that is turned off has substantially the same potential as before the output of the third compensation current source is turned off. The light quantity control device according to claim 11 . The light quantity control device according to claim 12, wherein the double throw switch is a differential transistor. A plurality of light emitting elements, a photosensitive member, an optical system for irradiating the photosensitive member with a light beam from the plurality of light emitting elements, and a light amount control device for controlling the light amount of the plurality of light emitting elements, the image forming apparatus light quantity control device which is a light quantity control device according to any one of claims 13 claim 1.

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