JPH06270462A - Quantity-of-light control device of image forming apparatus - Google Patents

Abstract

PURPOSE:To enable a wide range of a gain change and to miniaturize a quantity-of-light control device by providing a feedback circuit applying negative feedback to an operational amplifier through the variable resistor incorporated between the output terminal of the operational amplifier and the ground. CONSTITUTION:A feedback circuit having a variable resistor 6 incorporated therein is connected across the output terminal of the operational amplifier 5 of a monitor output amplifying circuit and the ground through a resistor 4 having a resistance value R3 and the positive terminal thereof is earthed. By changing the center position (a) of the variable resistor 6, non-linear output voltage VOUT is obtained. Especially, in the case of a 0, extremely large output near to infinity can be obtained and, in the case of a=1, an output value R3I and a wide range of an output voltage value can be obtained. Since the large change of a resistance value (r) can be reduced as compared with a conventional one, the effect in fluctuations of offset voltage or the change of input impedance is reduced and, since one variable resistor will suffice, cost reduction and miniaturization can be achieved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image of a laser beam printer, a digital copying machine or the like for forming an image by an electrophotographic process by irradiating an image carrier with a light beam emitted from a light emitting element such as a semiconductor laser. The present invention relates to a light quantity control device of a forming apparatus, and more particularly to improvement of a monitor output amplifier circuit that amplifies a monitor current for controlling the light quantity.

[0002]

2. Description of the Related Art In this type of image forming apparatus, the relationship between the current supplied to a laser diode, which is a laser beam generation source, and the optical output varies from laser diode to laser diode and also due to the heat generated by the laser diode itself.
It is not possible to emit laser light by simply controlling open loop constant current. Therefore, it is necessary to monitor the light output of the laser diode with a photodiode and control so as to obtain a desired light output level. This control is generally called APC (automatic light amount control).

FIG. 1 shows a configuration example of a conventional laser control circuit for performing APC. This laser control circuit is a constant current circuit 11
And a switching circuit 12 and a monitor output amplifier 13. The constant current circuit 12 is a voltage-current converter, and supplies a current I ₁ according to the light amount signal 14 from the control device (CPU) 10. The switching circuit 12 is a circuit that switches this current with the laser lighting signal (image signal) 15. The laser diode 16 emits light in response to the ON operation of the switching circuit 12. This light emission amount is taken out as a current value by the photodiode 17, and this current value is converted into a voltage signal by the series grounding resistor 18. The light emission amount extracted as the voltage value is amplified by the monitor output amplifier 13 and becomes a light emission amount signal 19. The control unit 10 raises or lowers the level of the light amount signal 14 while monitoring the light emission amount signal 19 to adjust it to a constant value.

The monitor output amplifier 13, which is the feedback system of the laser control circuit, is conventionally shown in FIG. 2 or 3.
It was composed of an electric circuit as shown in. This electric circuit is a negative feedback negative-phase amplifier having an operational amplifier (op amp) 5 and resistors 2 to 4, and the voltage drop R ₁ I generated by the resistor having the resistance value R ₁ is multiplied by (1 + R ₃ / R ₂ ). It amplifies. The monitor output amplifier of FIG. 2 changes the voltage drop R ₁ I by the variable resistor 2, and the monitor output amplifier of FIG. 3 changes the gain by the variable resistors 3 and 4, both of which are shown in FIGS. As shown in, the resistance value (R ₁ or R
₂ and R ₃ ) changes, the output V _OUT changes linearly. Therefore, in the circuit of FIG. 2, the resistor 2 (R ₁ )
3 is made up of fine adjustment and coarse adjustment, and in the circuit of FIG. 3, resistance 3 (R ₂ ): fine adjustment, resistance 4 (R ₃ ): coarse adjustment or resistance 3 (R ₂ ): coarse adjustment, resistance 4 ( R ₃ ): Consists of fine adjustment and responds to small and large changes in the output V _OUT .

Further, in the conventional circuit, as shown in FIGS. 6 and 7, the voltage drop R ₁ I and the voltage gain (1 + R ₃ / R ₂ )
There is a case where both are changed. In this configuration, the output V _OUT is guaranteed by configuring the voltage drop R ₁ I for fine adjustment and the voltage gain (1 + R ₃ / R ₂ ) for coarse adjustment (or vice versa).

[0006]

However, in the above conventional example, the resistance value R ₁ or the resistance values R ₂ , R _{2 is set} .
_{When 3} is changed, there is no problem when the change range is small, but when the gain is to be changed greatly, the change range of the resistance value R ₁ or the resistance values R ₂ and R ₃ must be made very large. The large changes in the resistance values R ₁ , R ₂ and R ₃ cause a change in offset voltage and a change in input impedance, which has a drawback of affecting the output V _OUT .

Further, conventionally, since two variable resistors for fine adjustment and coarse adjustment are required, there is a drawback that the cost reduction and the miniaturization are hindered.

In view of the above points, an object of the present invention is to
An object of the present invention is to provide a light amount control device for an image forming apparatus, which enables a wide range of gain changes, and can be reduced in price and downsized.

[0009]

To achieve the above object, the present invention provides a light amount of an image forming apparatus for irradiating a light beam emitted from a light emitting element onto an image carrier to form an image by an electrophotographic process. In the control device, detection means for detecting the light amount of the light beam emitted from the light emitting element,
It is characterized by comprising an operational amplifier for amplifying the output of the detection means, and a feedback circuit for applying a negative feedback to the operational amplifier via a variable resistor incorporated between the output terminal of the operational amplifier and the ground.

[0010]

According to the present invention, since an amplifier circuit whose gain changes non-linearly with respect to a change in the value of the variable resistance is provided as a monitor output amplifier circuit, it is possible to change the gain in a wide range. That is, the gain G of this circuit is a when the position of the middle point of the variable resistance is a.

[0011]

[Equation 1] G = −R ₃ / R ₂ [1+ (1−a) / a + (1−a) · r / R ₃ ], which allows nonlinear changes.

Further, since it is not necessary to provide variable resistors for fine adjustment and coarse adjustment as in the conventional example, it is possible to reduce the cost and the size.

[0013]

Embodiments of the present invention will now be described in detail with reference to the drawings.

(Basic Structure) FIG. 8 shows a basic circuit structure of the light quantity control device of the present invention. In FIG. 8, A is a laser light source as an exposure unit that forms an image using laser light. Reference numeral B is a laser light detection means (including a monitor output amplification circuit) for monitoring the output of the laser light. C is a laser light amount control means for controlling the laser light source A so as to obtain a predetermined light output level based on the output of the laser light detection means B. The light output level control means C ₁ and the light output level setting means C Having ₂ . In D, the timing of execution by the laser light amount control means C is executed in the area excluding the maximum recordable area in the main scanning direction and in the area excluding the image guarantee areas at the front and rear ends in the recording medium in the sub scanning direction. Thus, the timing control means controls the timing of the laser light amount control means C.

FIG. 9 shows the basic structure of the monitor output amplifier circuit which constitutes the above laser beam detecting means B. This circuit includes a resistor having a resistance value R _1, a resistor having a resistance value R _2, a resistor having a resistance value R ₃ , an operational amplifier (op amp) 5, and a variable resistor 6 having a variable resistance value r. The negative terminal of the operational amplifier 5 is connected to a photodiode (not shown) via the resistor 3, and a feedback circuit incorporating a variable resistor 6 between its output and ground is connected via the resistor 4. The positive terminal of the operational amplifier 5 is grounded via the resistor 2. Therefore,
Assuming that the position of the middle point of the variable resistor 6 is a, the gain G of this circuit is

[0016]

[Equation 2] G = −R ₃ / R ₂ [1+ (1−a) / a + (1−a) · r / R ₃ ], and the relationship between the resistance value r and the gain G is as shown in FIG. It becomes a non-linear change.

(First Embodiment) FIG. 11 shows a circuit configuration of a monitor output amplifier circuit according to a first embodiment of the present invention.
In the operational amplifier 5 of this circuit, its negative terminal is connected to the anode of the photodiode 1, and a feedback circuit incorporating a variable resistor 6 between its output and ground has a resistance value R ₃
Connected via a resistor 4 of which the positive terminal is grounded.

The circuit of this embodiment is of an anode type, and a monitor current I is generated by the photodiode 1 receiving a laser beam from a laser diode (not shown). When the center position of the variable resistor 6 is a, the output voltage V _{OUT of the} circuit is given by the following equation.

[0019]

[Equation 3] V _OUT = −R ₃ I (1+ (1−a) / a + (1−a) · r / ar) From this equation, by changing the center position a of the variable resistor 6, a nonlinear output is obtained. It can be seen that the voltage V _OUT is obtained.
Particularly, when a≈0, a very large output value close to infinity can be obtained, and when a = 1, an output value R ₃ I and a wide range of output voltage values can be obtained.

Further, since the large change in the resistance value r is less than that in the conventional example, it is less affected by the change in the offset voltage and the change in the input impedance, and further, only one variable resistor is required. It is possible to achieve price reduction and size reduction.

(Second Embodiment) FIG. 12 shows a circuit configuration of a monitor output amplifier circuit according to a second embodiment of the present invention.
The negative terminal of the operational amplifier 5 of this circuit is connected to the anode of the photodiode 1 via the resistor 3 having the resistance value R ₂ , and the resistance value R is provided between the anode of the photodiode 1 and the ground.
Resistor 2 of ₁ is connected. Other configurations are shown in FIG.
Is the same as the first embodiment of the present invention.

The circuit of this embodiment is of an anode type, and a monitor current I is generated by the photodiode 1 receiving a laser beam from a laser diode (not shown). The current I, the voltage IR ₁ results flows through resistor 2 of the resistance value R _1. When the center position of the variable resistor 6 is a, the output voltage V _{OUT of the} circuit is given by the following equation.

[0023]

[Formula 4] V _OUT = −R ₁ I · R ₃ / R ₂ [1+ (1−a) / a + (1−a) · r / R ₃ ] From this formula, the central position a of the variable resistor 6 It can be seen that by varying, a non-linear output voltage V _OUT can be obtained.
In particular, if a≈0, a very large output value close to infinity can be obtained, and if a = 1, an output value R ₁ I and a wide range of output voltage values can be obtained. is there.

Further, since a large change in the resistance value r is required as compared with the conventional example, there is little influence on the change in the offset voltage and the change in the input impedance, and since only one variable resistor is required, the price is low. Downsizing and downsizing can be achieved.

(Third Embodiment) FIG. 13 shows a circuit configuration of a monitor output amplifier circuit according to a third embodiment of the present invention.
This circuit is different from the circuit of FIG. 12 of the above-described embodiment in that the anode of the photodiode 1 is grounded, and the cathode thereof is connected to the positive terminal of the operational amplifier 5 and the power supply voltage via the resistor 2. The negative terminal of 5 is connected to the power supply voltage via the resistor 3.

The circuit of this embodiment is of the cathode type, and the cathode of the laser diode and the photodiode 1
Both anodes are of the type grounded to ground. The monitor current I is generated by the photodiode 1 receiving the laser light from the laser diode. The current I, the voltage IR ₁ results flows through resistor 2 of the resistance value R _1. This voltage IR ₁ is amplified by the circuit formed by the operational amplifier 5. Assuming that the center position of the variable resistor 6 is a, the output voltage V _OUT is given by the following equation.

[0027]

[Formula 5] V _OUT = −R ₁ I · R ₃ / R ₂ [1+ (1−a) / a + (1−a) · r / R ₃ ] From this equation, the central position a of the variable resistor 6 It can be seen that a non-linear output voltage V _OUT can be obtained by changing
Particularly, when a≈0, it is possible to obtain a very large output value close to infinity, and when a = 1, the output value R ₁
I and a wide range of output voltage values can be obtained.

Further, since the large change in the resistance value r is less than that in the conventional example, it is less affected by the change in the offset voltage and the change in the input impedance, and only one variable resistor is required, so that the price is low. Downsizing and downsizing can be achieved.

[0029]

As described above, according to the present invention,
Since the monitor output amplifier circuit is equipped with an amplifier circuit that changes the gain non-linearly with respect to the change in the value of the variable resistor, it is possible to change the gain in a wide range and there are two variable resistors for fine adjustment and coarse adjustment. The effect that it is not necessary is obtained. Further, this makes it possible to achieve cost reduction and miniaturization, and particularly when the constant current circuit, the switching circuit, the monitor output amplifier and the like are configured by one custom IC (integrated circuit), a remarkable effect is obtained.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration example of a conventional laser control circuit that performs APC.

FIG. 2 is a circuit diagram showing a configuration example of a conventional monitor output amplifier circuit.

FIG. 3 is a circuit diagram showing another configuration example of a conventional monitor output amplifier circuit.

FIG. 4 is a graph showing a gain characteristic obtained by the conventional circuit of FIG.

5 is a graph showing a gain characteristic obtained by the conventional circuit of FIG.

FIG. 6 is a circuit diagram showing still another configuration example of a conventional monitor output amplifier circuit.

FIG. 7 is a circuit diagram showing still another configuration example of a conventional monitor output amplifier circuit.

FIG. 8 is a block diagram showing an overall basic configuration of an embodiment of the present invention.

9 is a circuit diagram showing a basic configuration of a monitor output amplifier circuit according to the present invention which constitutes the laser light detecting means of FIG.

10 is a graph showing a gain characteristic obtained by the circuit of FIG.

FIG. 11 is a circuit diagram showing a configuration of a monitor output amplifier circuit according to the first embodiment of the present invention.

FIG. 12 is a circuit diagram showing a configuration of a monitor output amplifier circuit according to a second embodiment of the present invention.

FIG. 13 is a circuit diagram showing a configuration of a monitor output amplifier circuit according to a third embodiment of the present invention.

[Explanation of symbols]

 1 Photodiode 2, 3, 4 Resistance 5 Operational Amplifier 6 Variable Resistance

Claims (1)

[Claims] 1. A light quantity control device for an image forming apparatus, which irradiates a light beam emitted from a light emitting element onto an image carrier to form an image by an electrophotographic process. And a feedback circuit for applying negative feedback to the operational amplifier via a variable resistor incorporated between the output terminal of the operational amplifier and the ground. A light quantity control device for an image forming apparatus, characterized in that