JPH01161909A - Signal transmission circuit using photocoupler - Google Patents

Abstract

PURPOSE:To obtain an output signal with a large S/N by increasing the stimulated quantity of a light emitting diode and extracting an output signal of a photodetection transistor(TR) at the same level as an input signal via a voltage division resistor. CONSTITUTION:A variable resistor 17, in the input of a signal voltage to a TR 7, regulates a current IIN flowing to a light emitting diode 9 to set the operating level of the photodetection TR 10 as large as possible to the extent that the TR is not saturated. Thus, an output voltage appearing at the emitter of the photodetection TR 10 is increased. Voltage division resistors 18, 19 divide the output voltage and the output voltage with the same amplitude as a signal voltage outputted from a signal voltage source 2 is outputted at an output terminal 20. Thus, the noise caused in the photodetection TR 10 is reduced by a resistance ratio of the voltage division resistors 18, 19, an output signal with a large S/N is obtained.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a signal transmission circuit using a photocoupler,
More specifically, the present invention relates to a signal transmission circuit that can obtain an output signal with a good S/N ratio.

[Prior Art] FIG. 2 shows a commonly used signal transmission circuit using a photocoupler.

Figure 2 is a circuit diagram written in the September 1986 Special Issue of Television Technology, where (1) is the power supply, (2) is the signal voltage source,
(3) is the capacitor, (4) and (5) are the bias resistances of the transistor (7), and (6) is the emitter resistance.
Configure the input circuit. (8) is a photocoupler, (9)
and (10) are a light emitting diode and a light receiving transistor inside the photocoupler (8). (11).

(12) is the bias resistance of the light receiving transistor (lO),
(13) is a transistor, (14) is an emitter resistance, (
15) is a power supply, (1B) is a capacitor forming an output circuit, and (20) is an output terminal.

Next, the operation will be explained.

The AC voltage generated by the signal voltage source (2) is transferred to the capacitor (
After the DC component is removed in step 3), bias resistor (4)
, (5), an appropriate DC voltage (bias) is applied to the base of the transistor (7). In the transistor (7), a current IIN flows from the collector (collector current) in an amount proportional to the current flowing into the base (base current) due to the potential difference between the base and emitter. hand,
Change the brightness of the emitted light. Therefore, the amount of light generated by the light emitting diode (9) is proportional to the alternating current component of the signal generated by the signal voltage source (2).

Next, the light-receiving transistor (10) causes a current 1out proportional to the amount of light from the light-emitting diode (9) to flow through the collector, so that the signal generated at the base of the transistor (15) is the signal voltage source (2). A signal with the same amplitude as the generated signal is obtained. Then, after that, the bias resistance (1
1) An appropriate DC voltage is given in (12),
Transistors (13
) is inputted to the base of the signal voltage source (2) and outputted from the emitter as it is, and the same signal as that generated by the signal voltage source (2) is obtained at the output terminal (20).

[Problems to be Solved by the Invention] As an inherent characteristic of the element, the light-receiving transistor (10) generates noise with a constant amplitude in the output signal regardless of the amplitude of the input signal. Conventional signal transmission circuits have a circuit configuration in which a signal with the same amplitude as the signal generated by the signal voltage source (2) is emitted from the collector of the light receiving transistor (lO) as an output signal, so the S/N ratio can be increased. In order to do so, the noise level must be reduced, but this is considered difficult because the noise level generated in the light receiving transistor (10) cannot be fundamentally reduced.

The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to provide a signal transmission circuit that has a large S/N ratio and can obtain an output signal of a desired amplitude.

[Means for Solving the Problems] The signal transmission circuit of the present invention includes a means for increasing the amount of current flowing through the light emitting diode of a photocoupler within a range that does not saturate it, and an output signal of the photocoupler with the same amplitude as an input signal. It is characterized by comprising a partial pressure means for taking out the pressure.

[Function] In the variable resistor of the present invention, the zero voltage dividing means that adjusts the current flowing through the light emitting diode of the photocoupler so as to increase the amplitude of the output signal of the photocoupler within a range that does not saturate the photocoupler. The output signal is voltage-divided to the same amplitude as the input signal and extracted. Therefore, the level of the noise component output from the light receiving diode is reduced, and an output signal with a high S/N ratio is obtained.

[Embodiments of the invention] An embodiment of this invention will be described below.

Figure 1 is a circuit diagram of this embodiment, where the same symbols as in Figure 2 indicate the same components, and (17) is a variable resistor connected to the emitter of the transistor (7). Current flowing through the light emitting diode (3) I IH
The size of! IifM. (1B) and (18) are voltage dividing resistors connected between the emitter of the light-receiving transistor (lO) and GND, and the output signal is output from the connection point of the resistors (18) and (18) via the capacitor (1B). configured to be retrieved.

Next, the operation of this embodiment will be explained.

The variable resistor (17) adjusts the current IIN flowing through one light emitting diode (8) when a signal voltage is input to the transistor (7), and adjusts the current IIN flowing to one light emitting diode (8) to a value as large as possible without saturating the light receiving transistor (10). Set it so that Therefore, the output voltage appearing at the emitter of the light-receiving transistor (10) is increased compared to the conventional device. 2) Outputs an output voltage with the same amplitude as the signal voltage output from.

In this way, the noise generated in the light receiving transistor (lO) is reduced by the resistance ratio of the dividing resistors (18) and (19), so that an output signal with a high S/N ratio can be obtained.

In the above embodiment, the emitter resistor of the transistor (7) is formed of a variable resistor, but it may be a fixed resistor of an appropriate value.

Further, the means for increasing the amount of current flowing through the light emitting diode is not limited to the means for adjusting the emitter resistance of the transistor (7).

Further, it is desirable to increase the amount of light emitted by the light emitting diode (9) so that the division ratio by the 1-divider resistors (18) and (19) is 1/4 or less.

[Effects of the Invention] As described above, according to the present invention, the light emitting amount of the light emitting diode is increased and the output signal of the light receiving transistor is taken out at the same level as the input signal via the voltage dividing resistor. Therefore, a signal transmission circuit that can extract an output signal with a large S/N ratio can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of an embodiment of the present invention, and FIG. 2 is a diagram showing a signal transmission circuit using a conventional photocoupler. (2)...signal voltage source, (7)...transistor,
(8)...Photocoupler, (9)...Light emitting diode, (10)...Light receiving transistor, (17)...
Variable resistor. (18), (+9)...Voltage dividing resistor. In each figure, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] (1) A means for increasing the signal current flowing through the light-emitting diode of the photocoupler within a range that does not saturate it, and a voltage dividing means for dividing the output signal of the light-receiving transistor of the photocoupler to obtain an output signal at the same level as the input signal. A signal transmission circuit using a photocoupler.