1. Field of the Invention
The present invention relates to a photoelectric current and voltage converting circuit which converts a photoelectric current generated from a light-receiving element into voltage and outputs the voltage as a binary signal.
2. Description of the Related Art
A photoelectric current and voltage converting circuit, which converts a photoelectric current generated from a light receiving element into voltage, such as a photodiode, and outputs the voltage as a binary signal, is applied in various technical fields.
For example, the photoelectric current and voltage converting circuit is used in a light reception circuit of a photo-coupler in order to isolate an input and output electrically as Factory Automation.
The photo-coupler supplies a light emitting element (e.g. a light-emitting diode) on input side with an electric signal to transmit a light signal from the light emitting element to a light receiving element on output side, and outputs an electric signal from the light receiving element.
This kind of photoelectric current and voltage converting circuit is made into an IC and is used as a light receiving IC.
Hereinafter, a photoelectric current and voltage converting circuit 200 is explained as a conventional technique with reference to FIG. 1.
The photoelectric current and voltage converting circuit 200 includes a photodiode 1, an amplifier 12, a reference voltage circuit 16, a voltage divider circuit 20, and a comparator 30. Here, an anode is grounded to the photodiode 1.
The amplifier 12 includes an inverting amplifier 13, a non inverting input terminal of which inputs a voltage source 15 (voltage is described as “Vo”) based on voltage of the ground. A cathode of the photodiode 1 is connected to an inverting input, and a feedback resistor 14 is connected between the inverting input terminal and the feedback resistor 14.
The reference voltage circuit 16 includes a non inverting amplifier 17 as an operational amplifier. An offset resistor 18 is connected between an inverting input and an output. The inverting input is grounded through a constant current source 19, and the non inverting input terminal is connected to the inverting input of the inverting amplifier 13.
As for the voltage divider circuit 20, a resistor 21 and a resistor 22 are connected in series with the outputs of the amplifier 12 and the reference voltage circuit 16. The series connection point is grounded through a condenser 23.
Further, as for the comparator 30, the non inverting input of which is connected to the output of the amplifier 12. The inverting input of the comparator 30 is connected to the connection points of the first resistor 21 and the second resistor 22.
An operation of the photoelectric current and voltage converting circuit 200 having such constitution will be described bellow.
A photoelectric current Ipd is never generated without light-input to the photodiode 1.
Therefore, the photoelectric current Ipd does not flow through the feedback resistor 14 in the amplifier 12. As a result, the voltages of the output and the inverting input in the amplifier 12 becomes equal, and also becomes equal to the voltage Vo of the non inverting input in the amplifier 12 by a virtual short.
On the other hand, on the reference voltage circuit 16, an offset voltage Vos is generated by the offset resistor 18 and the constant current source 19.
A higher voltage is outputted as a reference voltage Vref (=Vo+Vos) than the voltage of the inverting input in the inverting amplifier 13.
The higher voltage to be outputted is offset according to the difference of the offset voltage Vos.
The voltage between the outputs of the amplifier 12 and the reference voltage circuit 16 is divided by the first and second resistors 21 and 22 in the voltage divider circuit 20 to develop a threshold voltage Vth.
Therefore, when the voltage Va=Vo on the output in the amplifier 12 is outputted, the voltage is compared to the threshold voltage Vth by comparator 30.
Here, the output voltage Vo of the amplifier 12 is lower than the threshold voltage Vth.
As a result, the binary signal in the Low level is outputted as an output signal Vout from the comparator 30 in the photoelectric current and voltage converting circuit 200 shown in FIG. 1.
On the contrary, the photoelectric current Ipd is generated by the light-input to the photodiode 1 in response to the quantity of light.
The photoelectric current Ipd flows through the feedback resistor 14 in the direction of the inverting input from the output of the inverting amplifier 13.
As a result, the voltage of the photoelectric current Ipd is converted into a voltage Vr=Ipd×Rf (the Rf means resistance value of the feedback resistor 14) between the two edges of the feedback resistor 14, and then the voltage Va on the output develops Va=Vo+Vr. When this voltage Va is outputted to the comparator 30 from the output of the amplifier 12, the voltage is compared with the threshold Vth from the voltage divider circuit 20.
When the intensity of the light-input to the photodiode 1 is lager than a certain level, the voltage Va develops larger than the threshold Vth, then, judged as being input a signal.
As a result, on the contrary of the above mentioned case of no light-input, a binary signal in the high level is outputted.
On the other hand, the quantity of the light input to the photodiode 1 is less than the certain level, the voltage Va becomes less than the threshold Vth, and then, a binary signal in the same level as the case without the light-input is outputted. (Refer to Japanese Patent JP B 3121339)
The operation of the photoelectric current and voltage converting circuit is explained bellow referring to the FIG. 2.
The output voltage Va of the amplifier 12 varies from Vo to Vo+Vr as shown in FIG. 2A. On the other hand, the reference voltage Vref has no relation to Ipd and is equal to V0+Vos. This reference voltage Vref and output voltage Va is divided by the first resistor 21 and the second resistor 22 and delayed by the condenser 23 and develops the threshold voltage Vth as shown in FIG. 2A.
FIG. 2A shows the timing chart under the condition that the ratio of the first resistor and the second resistor is about 1 to 2 as described in the Japanese Patent JP B 3121339. The output of the comparator 30 changes after rising and trailing of the output Va of the amplifier 12. Then the output signal Vout of the comparator 30 becomes the form as shown in FIG. 2B.
In the case of the conventional circuit 200, the threshold voltage Vth is generated using the reference voltage circuit 16 including the non inverting amplifier and the voltage divider circuit, the size of a chip is large so that the cost for making such circuit becomes high.