JPS5856095B2 - light detection circuit - Google Patents

Description

[Detailed description of the invention] The present invention relates to a photodetection circuit using a photodiode.

Since a photodiode has a junction capacitance due to a PN junction, its response changes depending on the magnitude of the load resistance, but on the other hand, there is a photodetection circuit using an accumulation method that utilizes this junction capacitance.

In this method, light is intermittent, but the output waveform of the photodetector circuit is a result of differentiating changes in light.

If the response speed is increased here, the differential waveform becomes very sharp and becomes a pulse waveform with a narrow width.

Therefore, when comparing the photodetection output with a reference voltage, if the responsiveness of the comparison circuit is low, the pulse width of the photoelectric output is too narrow, making it impossible for the comparison circuit to respond.

It is an object of the present invention to solve the above-mentioned problems in storage-type photodetection circuits using photodiodes.

First, a conventional example will be explained in order to clarify the meaning and purpose of the present invention.

FIG. 3 shows a conventional circuit.

LED is a light emitting diode, PD is a photodiode, and L
The light emitted by the ED is detected by the PD.

A sheet-like object to be inspected passes between the LED and the PD, and defects are detected by changes in the amount of transmitted light.

The light emission of the LED is chopped, and the transistor T4 in series with the LED is a switch for this chopping and is controlled by an external square wave pulse.

FIG. 4 1 shows the control rectangular wave pulse of the transistor T4, and the LED in the figure shows the light emission schedule of the light emitting diode LED.

T3 is a transistor connected in series with the photodiode, and a signal obtained by inverting the input to T4 is applied to the base, and the timing of the conduction cutoff of T4 is contradictory.

T3 is a switch that disconnects conduction between the photodiode PD and the power source.

FIG. 4I shows the schedule of voltages applied to photodiode PD.

A threshold voltage at which the light emitting diode LED does not emit light is applied to the PD, and the PD is disconnected from the power supply during the light emitting period.

A capacitor C1 indicated by a dotted line represents the junction capacitance of the photodiode PD.

While the light emitting diode LED is not emitting light, a voltage is applied to the photodiode PD, but no photocurrent is flowing through the PD, so the capacitor C1 is charged through the resistor r, and the voltage across the PD is the fourth As shown in the period tl in Figure PD, the voltage gradually rises and reaches the power supply voltage.

Then, since the PD is disconnected from the power supply during the period when the LED is emitting light, the charge on C1 is discharged through the PD as a photocurrent, and the voltage across the PD returns to O.

This results in a waveform as shown in FIG. 4 PD.

Since the charging current of the capacitor C1 flows through the resistor r, the voltage across r has a waveform as shown in FIG. 4 r.

The peak width of this r waveform is given by a time constant rXc1 determined by the capacitance of the capacitor C1 and the resistor r.

Since the discharge of C1 is performed as a photocurrent of the PD, the time constant during discharge varies depending on the intensity of the incident light on the PD, and becomes longer as the incident light is weaker.

Therefore, the charge of C1 is not fully discharged during the lighting period of the LED, and the residual charge increases as the incident light of the PD becomes weaker.
The height of the waveform D becomes lower, and the peak of r in the figure also becomes lower.

In this way, the height of the peak in FIG. During the conduction period, the voltage across the PD cannot reach the power supply voltage, which results in a decrease in the peak height in FIG. 4r.

This tendency becomes particularly strong when the incident light on the PD is strong and the height of the waveform of the PD in FIG. 4 becomes high, resulting in poor response.

Therefore, the value of resistor r is selected such that C1 is fully charged and saturated during the conduction period of T3.

Therefore, even if the width of the bottom of the peak in FIG.

When putting such a waveform into a comparison circuit and comparing its peak value with a reference value, if the response speed of the comparison circuit is slow, it will not be possible to compare the correct peak value with the reference value. You will have to adopt a faster one.

The present invention solves the above-mentioned drawbacks of the conventional circuit by obtaining an output waveform close to a rectangular wave as shown in Fig. 4A instead of the sharp peak-like output shown in Fig. 4R. This eliminates the need to take particular account of the response speed of the circuits after the photodetector circuit, which simplifies the comparator circuit and the like.

The structure of the present invention will be explained below with reference to the embodiment shown in FIG.

FIG. 2 is a time chart showing the operation of the circuit shown in FIG.

In FIG. 1, parts corresponding to those in FIG. 3 are given the same reference numerals.

The structural feature of the present invention is that the resistor r of the conventional example shown in FIG.
is divided into two parallel elements, namely a short-circuiting switch T1 and a resistor R2, and the junction capacitance C1 of the photodiode PD is
The charging current is divided in time so that it initially flows through R2, and at a later stage, it flows through switch T1.

Therefore, the value of the resistor R2 is much smaller than the resistor r of the conventional example shown in FIG.

Conventionally, the conduction and interruption of transistors T3 and T4 are contradictory, and the junction capacitance C1 of the photodiode PD is charged during the period when the light emitting diode LED is turned off, and when the LED is turned on, the charged charge of the above C1 is discharged as a photocurrent of the PD. Same as example.

In FIG. 2, I is the control signal for T4, and the control signal for T3 is an inversion of this.

This inverted signal is further delayed by a time t using a delay circuit to control the transistor T1 as a switch.

FIG. 2 T1 shows the conduction period of the transistor T1.

Also consider the case where T4 is cut off and T3 is conductive.

At this time, the LED is off and the junction capacitance of PD is charged, but T1 is still cut off during the initial time t, the charging current of C1 flows as the base current of transistor T2, and T2 becomes conductive. Then, a voltage appears across R2, which becomes the output.

Figure 2 PD is the voltage across the photodiode PD, and since the base current of T2 can only flow slightly due to the emitter follower resistor R2, the charging time constant of C1 is extremely slow, and therefore the voltage of PD rises slowly. There is.

After time t has elapsed, transistor T1 becomes conductive, so that the circuit from the base of T2 to ground via resistor R2 is short-circuited, C1 is rapidly charged, and the voltage across PD almost instantaneously reaches the power supply voltage.

After that, when T4 becomes conductive and T3 is cut off, the LED is turned on and the charge on C1 is discharged as a photocurrent, so that the voltage across the PD rapidly decreases.

On the other hand, considering the voltage across the resistor R2, the base current flows through T2 and T2 is conducting, and the upper end potential of R2 is T2.
Since it is approximately equal to the base of 2, it is equal to the power supply voltage minus the voltage across the PD, which is naturally the power supply voltage and shows a change that gradually decreases.

This condition is shown at period t in FIG. 2 R2.

After that, when T1 becomes conductive, T2 is cut off, so the voltage across R2 becomes O.

Furthermore, even if T4 is made conductive and T3 is cut off, the voltage across R2 remains at O, so that the voltage across R2 eventually deforms as shown in R2 in FIG. 2 and becomes a substantially rectangular waveform.

In order to make this waveform closer to a correct rectangular wave, the charging current to C1 during period t in Fig. 2 should be reduced as much as possible, and for that purpose, the base current of T2 should be reduced, so as T2, the current should be amplified. All you have to do is choose a value that is close to the ratio and increase the resistance R2.

Since C1 is charged in a short-circuit manner after the period t, there is no problem even if the time constant for the period t is arbitrarily large.

The photodetection circuit of the present invention has the above-mentioned configuration and is of an accumulation type using the junction capacitance of a photodiode, but since the charging time constant during the charging period is switched, an arbitrarily large time constant can be adopted. As a result, the detection output becomes close to a rectangular wave, and the response speed is not impaired, so there is no need to pay special consideration to the response speed of the circuit after the photodetector.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of an embodiment of the present invention, FIG. 2 is a time chart showing the above operation, FIG. 3 is a circuit diagram of a conventional example, and FIG. 4 is a time chart showing the operation. LED... Light emitting diode, PD...
Photodiode, C1... Capacity equivalent to the junction capacitance of the above PD.

Claims (1)

[Scope of Claims] 1. A storage-type circuit configuration using a photodiode and utilizing its junction capacitance, wherein the charging time constant during the charging period of the photodiode is switched during the same period, and the time constant is set to dog in the earlier period, The light detection circuit was made smaller in the later stage. 2 The resistances of the two circuits that are parallel to each other and both in series with the photodiode are different, and charging is performed through a dog circuit of resistance in the early period of the charging period of the photodiode, and through a circuit with a small resistance in the latter period. Claim 1, wherein the voltage across the resistor circuit is the output signal.
The photodetection circuit described in section. 3 Of the two circuits with different resistances in series with the photodiode, the dog resistor is a circuit that passes from the base of the transistor having an emitter-follower resistor through the emitter and through the follower resistor, and the output signal is applied to both ends of the emitter-follower resistor. The circuit with smaller resistance is the circuit that runs from the collector to the emitter of the transistor,
3. The photodetection circuit according to claim 1, wherein a control signal for switching between two circuits having different resistances is applied to the base of a transistor in this circuit.