JPH10290024A - Light emitting detecting circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a photodetecting circuit whose sensitivity so signal components is not deteriorated even if under solar beams, by providing a MOS transistor for bypassing the DC components by solar beams, etc. SOLUTION: A drain electrode and a source electrode of a MOS transistor 9 are parallel-connected to a load resistor 4 series-connected to a photodetecting element 2. A gate electrode of the MOS transistor 9 is supplied with a DC component out of the voltage generated on both ends of the load resistor 4. In such a constitution, the MOS transistor 9 is supplied with the DC component only, so that the decline in the sensitivity may be mitigated since the DC component generated by solar beams runs into the MOS transistor 9 even if under the photodetecting conditions such as under solar beams while the signal component runs into the load resistor 4. Furthermore, since MOS transistor 9 is commonly used with a driving transistor for a photoemitting element 3, a new transistor need not be added.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light emitting / receiving circuit in an electronic circuit. More specifically, the present invention relates to a circuit for receiving and transmitting an optical signal.

[0002]

2. Description of the Related Art In a conventional light receiving circuit such as an infrared remote controller (for example, JP-A-3-113924), as shown in FIG.
Excessive DC current generated in the light receiving element 2 due to DC light input such as sunlight in the background of the signal is converted into a diode 10
To prevent the potential A from reaching the power supply voltage.

[0003]

SUMMARY OF THE INVENTION In a conventional light receiving circuit,
When sunlight other than a signal enters and the potential A becomes higher than the level at which the diode 10 turns on, and a current flows through the diode 10, the load resistance for the signal component becomes
The resistor 4 and the resistor 11 are connected in parallel, and the resistance value decreases. Therefore, when the diode 10 is turned on, there has been a problem or problem that the gain for the signal is significantly reduced.

Further, at the time of transmission, it is necessary to drive the light emitting element using a separately provided circuit or transistor.

[0005]

In order to solve the above problems, in the present invention, a MOS transistor for bypassing only direct current of incident light is provided in parallel with a load resistor of the light receiving element, and a MOS transistor is further provided between the light receiving element and the light emitting element. The elements were connected in parallel, and the light emitting element was driven by the MOS transistor during transmission.

At the time of reception, M connected in parallel with a load resistor
A DC voltage whose AC component is attenuated by a low-pass filter from the voltage across the load resistor is applied to the gate electrode of the OS transistor. DC is almost applied to the gate electrode, and AC component is small. Therefore, the current flowing through the MOS transistor is almost DC only, so that only the DC component can be bypassed.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention relates to an M / M for bypassing only a DC component of incident light in parallel with a load resistance of a light receiving element.
An OS transistor is provided. Further, a light emitting element is connected in parallel to the light receiving element, and the light emitting element is driven by a MOS transistor during transmission. The gate voltage of the MOS transistor is applied with only an unnecessary DC component after removing an AC component used as a signal by a low-pass filter from a current obtained by photoelectrically converting incident light with a light receiving element. Since only a DC current flows through the MOS transistor, the DC component of the incident light can be bypassed.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 shows a specific circuit configuration of an embodiment of the present invention. In FIG. 1, there are various methods of realizing the low-pass filter 8, and FIG. 4 shows an example of the simplest configuration among them.

First, the circuit configuration will be described with reference to FIG. The light receiving element 2 is connected to the power supply 1 in series. Light receiving element 2
Are connected to the light emitting elements 3 in parallel. Further, one end of a load resistor 4 is connected to the light receiving element 2. The connection point A between the light receiving element 1 and the load resistor 2, the reception output terminal 5, and the drain electrode of the MOS transistor 5 are commonly connected to the input of the low-pass filter 8. The source electrode of the MOS transistor 5 is connected to the ground in common with the other end of the load resistor 4, and the gate electrode of the MOS transistor 5 is connected to the output of the switch 7. The switch 7 selects and outputs the output of the low-pass filter 8 and the transmission input 6. FIG.
Then, the switch 7 selects the transmission input 6 and the circuit is in the transmission state. When the switch 7 selects the output of the low-pass filter 8, the circuit enters a receiving state.

First, the circuit operation in the transmission state will be described. The switch 7 selects the transmission input 6. The transmission data input from the transmission input 6 drives the gate of the MOS transistor 9. When a voltage is applied to the gate electrode, the MOS transistor 9 conducts between the drain electrode and the source electrode and enters a state in which current flows. When a current flows between the drain electrode and the source electrode, a current flows from the power supply 1 through the light emitting element 3, and light is emitted from the light emitting element 3.

Next, the operation of the circuit in the receiving state will be described. The switch 7 selects the output of the low-pass filter 8. When only the signal light is received without the incidence of background light such as sunlight, the current obtained by photoelectrically converting the incident light of the signal light by the light receiving element 2 is represented by i1, and the resistance value of the load resistor 4 is represented by RL. , A voltage of i1 × RL is generated at both ends of the load resistor 4, that is, at the reception output 5.

When background light such as sunlight is incident, the DC current obtained by photoelectrically converting the background light by the light receiving element 2 increases and the voltage of the potential A increases with the increase in the incident intensity of the background light.
The increase in the potential A also increases the gate voltage of the MOS transistor 9 through the low-pass filter 8 and the switch 7. Further, as the intensity of the background light increases, the DC current further increases, and the gate voltage of the MOS transistor 9 further increases.
When the voltage exceeds a threshold voltage (hereinafter, Vth), the MOS transistor 9 is turned on. When the MOS transistor 9 turns on,
The DC current caused by the background light is not only the load resistance 4 but also the MOS
The current flows between the drain electrode and the source electrode of the transistor 9. As a result, even if the DC current due to the background light increases more than ever, the increased current flows through the MOS transistor 9 and the potential A becomes the V of the MOS transistor 9.
It does not increase much more than th.

Consider a case where the signal light is input while being superimposed on the background light while the MOS transistor 9 is turned on by the background light. Since the signal light is alternating current, the output of the low-pass filter 8 does not change. Therefore, the current obtained by photoelectrically converting the signal light flows only through the load resistor 4 without flowing through the MOS transistor 9. Assuming that a current obtained by photoelectrically converting the incident light of the signal light by the light receiving element is i1, a voltage of i1 × RL is generated at both ends of the load resistor 4, that is, at the reception output 5. More specifically, the voltage amplitude of the reception output 5 is equal to the input amplitude of the low-pass filter 8, and the characteristics of the low-pass filter 8 are set so that the frequency of the signal light hardly passes. Even if there is an input amplitude, the output of the low-pass filter 8 hardly swings at a constant voltage. Switch 7 from output of low pass filter 8
The gate electrode of the MOS transistor 9 connected through the MOS transistor 9 also hardly swings at a constant voltage, and almost only a direct current flows through the MOS transistor 9. Since the AC current i1 due to the signal light almost flows through the load resistor 4, the gain does not decrease so much even if the photocurrent increases due to the background light.

In FIG. 3, the horizontal axis represents the photocurrent due to the background light, and the vertical axis represents the voltage of the potential A and the value of i1 × RL as the gain. When the photocurrent due to the background light is small,
Since both the photocurrent due to the background light and the photocurrent due to the received signal flow through the load resistor 4, the potential A increases in proportion to the photocurrent. When the photocurrent due to the background light increases and the potential A exceeds the Vth of the MOS transistor 9, the photocurrent due to the background light becomes
Although the potential A does not increase because the MOS transistor 9 flows, the gain does not decrease so much because the photocurrent due to the received signal flows through the load resistor 4.

The characteristics required of the low-pass filter 8 are as follows:
First, it is necessary to pass a direct current or a low frequency component such as sunlight. Next, it is necessary to sufficiently attenuate AC components such as signal light. The output of the low-pass filter 8 is amplified by the MOS transistor 9 and the load resistor 4 and fed back to the input of the low-pass filter 8. That is, the phase is 1 except for the low-pass filter 8.
A negative feedback loop that rotates by 80 degrees is configured. In order to stabilize the negative feedback loop, it is necessary to make the phase rotation in the low-pass filter 8 180 degrees or less.
As the order of the low-pass filter 8, a first-order filter whose phase is turned only by 90 degrees or a filter that can be approximated to the first order is optimal. The simplest filter can be composed of one resistor and one capacitor as shown in FIG.

Hereinafter, the characteristics of the circuit shown in FIG. 1 will be described quantitatively.
Here, the low-pass filter 8 will be described using the simplest filter composed of the resistor 12 and the capacitor 13 shown as an example in FIG. The current generated when the light receiving element senses light
In the current source i1, the MOS transistor is represented by a voltage-controlled current source having a value of conductance gm with respect to the input amplitude v1 of the gate electrode, and the values of the resistor 12 and the capacitor 13 are Rf and Cf, respectively. The AC equivalent circuit of the above circuit is as shown in FIG. Input / output transfer function (Vout /
(I1 · RL)) is as follows: Vout / i1 · RL = (1 + S · Cf · Rf) / (gm · RL + 1 + S · Cf · Rf) (Equation 1) FIG. 8 shows the absolute value (gain) of Equation 1 as an angular frequency on the horizontal axis. Therefore (gm
The values of Rf and Cf may be set so that the value of (RL + 1) / (Cf · Rf) is sufficiently lower than the signal frequency to be used. Then, a low frequency component such as sunlight can lower the gain (Vout / (i1.RL)), and can increase the gain at the signal frequency.

FIG. 5 shows another embodiment of the present invention. Except that the switch 14 is connected in series to the light receiving element 2,
This is the same as the embodiment of FIG. The switch 14 is opened in a transmission state, that is, when light is emitted from the light emitting element 3, and is closed in a reception state, that is, when the light receiving element 2 receives a signal. When the optical distance between the light receiving element 2 and the light emitting element 3 is not sufficient, the light emitted from the light emitting element 3 is received by the light receiving element 2 to prevent unnecessary current from flowing. Switch 1
Except for the operation of FIG. 4, the circuit of FIG. 4 performs exactly the same operation as the circuit of FIG.

FIG. 6 shows a light emitting / receiving circuit 15 according to the present invention.
An embodiment applied to an infrared transceiver 16 is shown. In the infrared transceiver 16, 1 or 0 transmission data input from the transmission data input terminal 19 is encoded by the encoding circuit 17. The encoding circuit determines the light emission time, the light emission frequency, and the like for the transmission data of 1 or 0 according to the application. The output of the encoding circuit 17 is the light emitting / receiving circuit 15 of the present invention.
To the transmission input terminal 6 for transmission.

The signal received by the light receiving / emitting circuit 15 is decoded from the reception output terminal 5 by a decoding circuit 18 having a function opposite to that of the encoding circuit 17 and output from a reception data output terminal 20. Since the infrared transceiver 16 uses the light receiving circuit 15 of the present invention, the sensitivity is less reduced even in an environment with DC background light such as under sunlight as described above. Also,
Transmission can be performed without adding a new transistor for driving a light emitting element.

[0020]

According to the present invention, by providing a MOS transistor that bypasses only a DC component from incident light, DC flows through the MOS transistor and AC flows through the original load resistance. Therefore, it is possible to provide a light receiving circuit with less decrease in sensitivity even in an environment having a DC background light such as under sunlight.

The DC current value obtained by photoelectrically converting sunlight is much larger than the current value obtained by photoelectrically converting a signal. Therefore, it is necessary to use a large-sized MOS transistor 9 for bypassing the DC current obtained by photoelectrically converting sunlight. Further, in order to drive the light emitting element, a large driving current is required, so that it is necessary to drive the light emitting element with a large size MOS transistor. According to the present invention, since a large-sized MOS transistor can be shared for reception and transmission, the mounting area can be reduced.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing one embodiment of the present invention.

FIG. 2 is a circuit diagram illustrating an example of a conventional light emitting / receiving circuit.

FIG. 3 is a graph showing characteristics of the light emitting and receiving circuit of the present invention.

FIG. 4 is a circuit diagram showing an example of a low-pass filter used in one embodiment of the present invention.

FIG. 5 is a circuit diagram showing another embodiment of the present invention.

FIG. 6 is a block diagram of an infrared transceiver to which the light emitting and receiving circuit according to the present invention is applied.

FIG. 7 is an AC equivalent circuit of a light receiving circuit according to the present invention.

FIG. 8 is a graph showing frequency characteristics of the light receiving circuit according to the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 power supply 2 light receiving element 3 light emitting element 4, 11, 12 resistance 5 reception output terminal 6 transmission input terminal 7, 14 switch 8 low-pass filter 9 N-channel MOS transistor 10 diode 13 capacitance 15 light receiving / emitting circuit 16 infrared transceiver 17 encoding circuit 18 Decoding circuit 19 Transmit data input terminal 20 Receive data output terminal

──────────────────────────────────────────────────の Continued on front page (51) Int.Cl. ⁶ Identification code FI H04B 10/04 10/06

Claims (2)

[Claims] 1. A light receiving element for converting the intensity of incident light into a current level, a light emitting element connected in parallel to the light receiving element for converting the current level into light level, and a light emitting element connected in series to the light receiving element. A resistor, a low-pass filter that receives the voltage across the resistor as input, a switch that selects and outputs an output of the low-pass filter and an external transmission signal, one end of the resistor is a drain electrode, and A light emitting / receiving circuit comprising a source electrode at the other end, and MOS transistors each having an output of the switch connected to a gate electrode. 2. The light emitting and receiving circuit according to claim 1, wherein a switch is connected to the light receiving element in series.