JPH0983452A - Light receiving circuit - Google Patents

Abstract

(57) [Problem] To provide a light receiving circuit provided with a MOS transistor for bypassing a direct current component caused by sunlight or the like, and the sensitivity to a signal component does not decrease even under sunlight. A drain electrode and a source electrode of a MOS transistor 4 are connected in parallel to a load resistor 2 connected in series to a light receiving element 1. To the gate electrode of the MOS transistor 4, the voltage generated at both ends of the load resistor 2 is passed through the low-pass filter 5, and a voltage corresponding to only the direct current component is applied. Since only a direct current component flows through the MOS transistor 4, the direct current component due to sunlight flows through the MOS transistor 4 and the signal component flows through the load resistor 2 even under a light receiving condition such as in sunlight, so that the sensitivity is less deteriorated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light receiving circuit. More specifically, it relates to a semiconductor integrated circuit for a circuit that receives an optical signal.

[0002]

2. Description of the Related Art First, a conventional light receiving circuit will be described to clarify the background of the present invention. In a light receiving circuit such as an infrared remote controller (for example, Japanese Patent Laid-Open No. 3-113924), as shown in FIG. Thus, the potential A does not reach the power supply voltage.

[0003]

SUMMARY OF THE INVENTION In a conventional light receiving circuit,
When sunlight other than the signal is incident and the potential A becomes higher than the level at which the diode is turned on, there is a problem or a problem that the gain for the signal is significantly reduced. This is because the load resistance of the light receiving element drops significantly from the value when there is no sunlight.

[0004]

In order to solve the problems of the prior art, in the present invention, a MOS transistor for bypassing only the direct current of incident light is provided in parallel with the load resistor. The MOS transistor lowers the load resistance value for a direct current and maintains the load resistance value for an alternating current such as an input signal.

To the gate electrode of the MOS transistor connected in parallel with the load resistance, a DC voltage, which is an AC component attenuated by a low-pass filter, is applied from the voltage across the load resistance. 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.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described in detail below with reference to the drawings. FIG. 1 shows a specific circuit configuration of the present invention. In FIG. 1, there are various ways to realize the low-pass filter 5, but among them, the one that is easy to configure and is suitable for a CMOS integrated circuit is shown in FIG.
5, 6, and 7 are exemplified. The low-pass filter 5 will be described in detail after the description of FIG.

First, the circuit configuration will be described with reference to FIG. The light receiving element 1 is connected to the power supply 3 in series. Furthermore, one end of a load resistor 2 is connected to the light receiving element 1. The connection point A between the light receiving element 1 and the load resistor 2 is connected to the input of the low pass filter 5. The drain electrodes of the MOS transistors 4 are also commonly connected to the connection point A. The source electrode of the MOS transistor 4 is connected to the ground together with the other end of the load resistor 2, and the gate electrode of the MOS transistor 4 is connected to the output of the low pass filter 5.

The circuit operation will be described. First, a case will be described in which background light such as sunlight is not incident and only signal light is received. In this case, if the current obtained by photoelectrically converting the incident light of the signal light by the light receiving element is i1 and the resistance value of the load resistor 2 is RL, i1 × RL is provided at both ends of the load resistor 2.
Voltage is generated.

Next, the case where the incident intensity of the background light is increased will be described. As the incident intensity of background light increases, the direct current that photoelectrically converts the background light also increases. Therefore MOS
The gate voltage of the transistor 4 also increases. Further, as the intensity of the background light increases, the direct current also increases, and the MOS transistor 4
Gate voltage further increases and exceeds the threshold voltage (hereinafter Vth), the MOS transistor 4 is turned on. When the MOS transistor 4 is turned on, the direct current due to the background light is
Not only the load resistor 2 but also the drain electrode and the source electrode of the MOS transistor 4 will flow. 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 4 and the potential A does not increase much more than Vth of the MOS transistor 4. The change in the potential A is shown in FIG. 3 with the horizontal axis representing photocurrent due to background light and the vertical axis representing potential A.

Consider a case where the signal light is input while being superimposed on the background light while the MOS transistor 4 is turned on by the background light. Also in this case, assuming that the 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 2, that is, the output end 6.
FIG. 3 simultaneously shows the value of i1 × RL as the gain on the vertical axis and the photocurrent by the background light on the horizontal axis. The voltage amplitude of the output terminal 6 is equal to the input amplitude of the low-pass filter 5, and the characteristics of the low-pass filter 5 are set so that the frequency of the signal light is hardly passed. Also low pass filter 5
The output of is almost constant with a constant voltage. MOS transistor 4 connected to the output of low-pass filter 5
The gate electrode of the MOS does not fluctuate at a constant voltage, and the MOS
Almost only direct current flows through the transistor 4.
Since the AC current i1 due to the signal light almost flows through the load resistor 2, the gain does not decrease so much even if the photocurrent increases due to the background light.

The characteristics required for the low-pass filter 5 are as follows:
First, it is necessary to let through direct current or low frequency components such as sunlight. Next, it is necessary to sufficiently attenuate AC components such as signal light. The output of the low-pass filter 5 is amplified by the MOS transistor 4 and the load resistor 2 and fed back to the input of the low-pass filter 5. That is, the phase is 1 except for the low-pass filter 5.
A negative feedback loop that rotates by 80 degrees is configured. In order to stabilize the negative feedback loop, the phase rotation in the low-pass filter 5 must be 180 degrees or less, and for that purpose,
The order of the low-pass filter 5 is optimally a first-order filter whose phase rotates only 90 degrees or a filter which can be approximated to the first-order. The simplest filter can be composed of one resistor and one capacitor as shown in FIG.

The characteristics of the circuit shown in FIG. 1 will be quantitatively described below with reference to FIG. Here, the low-pass filter 5 will be described using the simplest one composed of the resistor 9 and the capacitor 10 shown in FIG. As described above, the filter is preferably a first-order filter, and the filters in FIGS. 5, 6, and 7 are equivalently first-order low-pass filters consisting of resistors and capacitors.
It goes without saying that it can be used in the present invention.

The current generated when the light receiving element senses light is
A MOS transistor is represented by a voltage control 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 9 and the capacitor 10 are respectively R
f and Cf, the AC equivalent circuit of the circuit of FIG.
It becomes as shown in. Input / output transfer function (Vout /
(I1 · RL)) is

[0014]

[Equation 1]

[0015] FIG. 9 shows the absolute value (gain) of Expression 1 with the horizontal axis as the angular frequency. Therefore, the values of Rf and Cf may be set so that the value of (gm · RL + 1) / (Cf · Rf) is sufficiently lower than the signal frequency used. Then, if the frequency component such as sunlight is low, the gain (Vout / (i1 · RL))
Can be made low and the gain can be made high at the signal frequency.

FIGS. 4, 5, 6, and 7 show examples of implementation of the low-pass filter 5 applicable to the present invention. FIG.
Is a filter consisting of the simplest resistor 9 and capacitor 10. FIG. 5 shows a configuration in which a resistor 11 of FIG. 4 is connected in parallel with a capacitor 11 to limit the band of thermal noise generated from the resistor to reduce noise. FIG. 6 shows an N-channel MOS transistor 14 and a P-channel MOS transistor 12 instead of resistors, which are turned on and connected in parallel to reduce the chip size when integrated into an N-channel MOS transistor. The gate electrode of the MOS transistor 14 is connected to the power supply 13, and the gate electrode of the P-channel MOS transistor 12 is connected to the ground to turn on each transistor.

FIG. 7 shows a further reduction in chip size by using an N-channel MOS transistor 14 in which a voltage of almost twice Vth is applied to the gate electrode instead of the resistor. N-channel MOS in which a current source 16 is connected to a power source 13 and a gate electrode and a drain electrode are respectively connected
Transistors 15 and 17 are connected in series, and a voltage of approximately twice Vth is applied from the gate electrode of MOS transistor 15 to the gate electrode of N-channel MOS transistor 14. In the filter of FIG. 7 as well, when the intensity of the background light is high, the voltage at the input terminal of the filter is approximately Vth. On the other hand, since the gate electrode of the MOS transistor 14 is always applied with a voltage of almost double Vth, the MO
The gate-source voltage of the S transistor 14 is almost V
The voltage becomes th. The resistance value of the MOS transistor 14 depends only on the voltage between the gate and the source.
In the circuit of Fig.6, since the fluctuation of the voltage between the gate and the source is small, it is possible to obtain a relatively stable resistance value compared to the case where the power supply voltage is applied as the gate voltage as shown in Fig.6. Since the absolute value of the voltage of is also small,
A resistance value equivalent to that in FIG. 4 can be obtained with a small transistor size.

FIG. 10 shows a light receiving circuit 18 according to the present invention,
An embodiment applied to the remote control receiving circuit 26 will be shown. Number 10
Infrared light having a light emission cycle of kHz is converted into an electric signal by the light receiving element 1 such as a photodiode that performs photoelectric conversion, and is input to the remote control receiving circuit 26 through the input terminal 19. In the remote control receiving circuit 26, the number of light emission cycles is 10k
Detects whether the infrared light of Hz is incident or not.

In general, the input signal level of the input terminal 19 is as weak as 50 μV. Remote control receiver circuit 2
In the inside of 6, the low noise amplifier 20
, The amplitude is then limited by a limiter 21 to a certain value or less, only the signal component is extracted by the bandpass filter 22 tuned to the light emission period, and the detection is performed by the detection circuit 23, and the detected DC level Through a comparison circuit 24 that outputs a high or low level by comparing
Output to

The remote control receiving circuit 26 is also used outdoors under sunlight. Direct-current light such as sunlight becomes the background light of the input signal having a light emission period of 10 kHz. When the light receiving circuit 18 of the present invention is inserted between the input terminal 19 and the low noise amplifier 20 as shown in FIG. 10, since the DC component flows through the light receiving circuit 18 as described above, the voltage at the input terminal 19 is the power source. There is no saturation in the voltage, and the decrease in gain with respect to the AC component of the input signal is small. That is, the remote control receiving circuit 26 using the light receiving circuit 18 according to the present invention exhibits good characteristics even in sunlight. Further, since the light receiving circuit according to the present invention uses all the elements that can be realized by the CMOS process, it is suitable for forming a CMOS integrated circuit together with the remote control receiving circuit.

[0021]

According to the present invention, since the MOS transistor for bypassing only the direct current component due to the incident light is provided, the direct current flows through the MOS transistor and the alternating current flows through the original load resistance. Therefore, it is possible to provide a light receiving circuit in which the sensitivity is not significantly deteriorated even in an environment with direct current background light such as sunlight.

[Brief description of drawings]

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

FIG. 2 is a circuit diagram showing an example of a conventional technique.

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

FIG. 4 is a circuit diagram of a low-pass filter as an embodiment of the present invention.

FIG. 5 is a circuit diagram of a low pass filter as another embodiment of the present invention.

FIG. 6 is a circuit diagram of a low-pass filter as another embodiment of the present invention.

FIG. 7 is a circuit diagram of a low-pass filter as another embodiment of the present invention.

FIG. 8 is an AC equivalent circuit of the light receiving circuit according to the present invention.

FIG. 9 is a graph showing frequency characteristics of the light receiving circuit according to the present invention.

FIG. 10 is a remote control receiving circuit to which the light receiving circuit according to the present invention is applied.

[Explanation of symbols]

1 light receiving element 2,8,9 resistance 3,13 power supply 4,14,15,17 N channel MOS transistor 5 low pass filter 6 output terminal of light receiving circuit 7 diode 10,11 capacitance 12 P channel MOS transistor 16 current source 18 light receiving circuit 19 Remote Control Reception Circuit Input Terminal 20 Low Noise Amplifier 21 Limiter 22 Bandpass Filter 23 Detection Circuit 24 Comparison Circuit 25 Remote Control Reception Circuit Output Terminal 26 Remote Control Reception Circuit

Claims (1)

[Claims] 1. A light receiving element for converting the intensity of incident light into an intensity of current, a resistor connected in series to the light receiving element, a low-pass filter for inputting the voltage across the resistor, and one end of the resistor. Is a source electrode, the other end is a drain electrode, and the output of the low-pass filter is connected to a gate electrode.