JPS5941556Y2 - Light receiving circuit - Google Patents

Description

[Detailed description of the invention] This invention requires high sensitivity for a light receiving circuit that is ideal for the receiver of a photoelectric switch that uses a light emitter with a high modulation frequency, and is also at risk of being extremely affected by ambient light, electrical noise, etc. In some photoelectric switches, such as long-distance infrared security alarm systems, the modulation frequency of the projector may be increased by an order of magnitude or more to meet the requirements.

In other words, in this type of light source switch, the conventional modulation frequency was several KHz, but this can be modulated to several tens of KHz, or the modulation frequency of several tens of KHz can be further increased to several hundred HHz.
A method of double modulation at a low frequency of z is being put into practical use.

In that case, the light-emitting element used in the emitter has good response, such as a light-emitting diode or laser, and can follow a sufficiently high modulation frequency, but the light-receiving element in the receiver cannot follow it and does not exhibit its full performance. In many cases, this is not possible.

In other words, phototransistors, photodiodes, silicon solar cells, etc. that are currently widely used as light-receiving elements are several KH.
It is possible to respond to modulated light of up to about 100 kHz, as shown in Figure 1A, but when it comes to modulated light of several 10 KHz, the response is poor, and as shown in Figure 1B, the modulation degree is 100%, that is, intermittent. Even if light is applied, the electrical signal after photoelectric conversion has a rise delay and a fall delay of the light receiving element, so the desired alternating current component C is reduced and the sensitivity is considerably lowered.

Moreover, since it is highly sensitive to low modulation frequencies, it is subject to additional effects from low-frequency disturbance light and electrical noise, making it impossible to achieve the intended purpose.

In view of the above-mentioned points, an object of the present invention is to provide a light-receiving circuit that uses a normal light-receiving element and has a high response frequency and is less susceptible to disturbance light and electrical noise.

This will be explained below with reference to the drawings.

In FIG. 2, 1 is a phototransistor, 2 is an amplifier, and 3 is a negative feedback circuit, in which the emitter of phototransistor 1 and the input terminal of amplifier 2 are connected, and the electricity photoelectrically converted by phototransistor 1 is connected. The signal is amplified by an amplifier 2, and by interposing a negative feedback circuit 3 between the output terminal of the amplifier 2 and the base of the phototransistor 1, a part of the output of the amplifier is sent through the negative feedback circuit 3 to the base of the phototransistor 1. It is configured to allow negative feedback.

The frequency characteristics in this interception are as shown in Figure 3, when the vertical axis is the gain and the modulation frequency of the human-powered light is taken as the gain, and the ratio between the intensity of the human-powered light to the phototransistor 1 and the output voltage of the amplifier 2 is taken as the gain. It becomes like this.

In Fig. 3, the second curve is the characteristic when there is no negative feedback circuit 3, and the modulation frequency is almost constant from O to fl, and beyond that, the gain increases as the modulation frequency increases by 1. Decrease linearly.

The 0 curve is a characteristic when a negative feedback circuit 3 without frequency selectivity is used, and the maximum gain is lower than that of the 2 curves, but the frequency f3 at which the gain begins to decrease is equal to that of the 2 curves (fl ) becomes higher from O to its frequency f
The gain up to 3 is almost constant.

The R curve is a characteristic when a negative feedback circuit 3 having the function of a low-pass filter is used, and the gain is maximum at a certain modulation frequency.

When the modulation frequency of all emitters is set to f2 between fl and f3, the gain is Hl in the two curves without negative feedback circuit.
to H2, and the sensitivity decreases, and the gain remains high for modulation frequencies below f2, so if disturbance light or electrical noise with a modulation frequency in this range is mixed, the sensitivity to these noises will be lower than the target frequency f2. It will be highly influenced by it.

In the zero curve where the negative feedback circuit 3 has no frequency selectivity,
Although the overall gain decreases, the gain for the used frequency f2 and the gain for the modulation frequency lower than this become comparable, and are less affected by disturbance light and electrical noise.

Furthermore, in the R curve where the negative feedback circuit 3 has the function of a low-pass filter, the gain is maximum near the operating frequency f2, and the gain becomes low for noise in a frequency range that deviates from this, reducing the influence of noise etc. becomes very difficult to receive.

In other words, using a negative feedback circuit 3 that functions as a low-pass filter means using a circuit that increases the amount of negative feedback at modulation frequencies below f2.

By using such a negative feedback circuit, for modulation frequencies lower than f2, the amount of negative feedback increases in the low frequency region, that is, the gain decreases, and the influence of noise in this band is reduced.

In addition, in Figure 3, the higher frequency of the Q and R curves is 2.
The parallel shift higher than the curve is because the phototransistor is lower with the negative feedback circuit than without it.

As shown in the equivalent circuit of the phototransistor (not shown in FIG. 5), this is caused by the current 11 flowing through the feedback circuit, that is, the second
This phenomenon is caused by the existence of a current flowing into the negative feedback circuit through the base of the phototransistor 1 shown in the figure and a current flowing through the transistor 1 in the equivalent circuit.

The response characteristics of a phototransistor vary depending on the load, and the smaller the load, the better the characteristics in the high frequency range.

Therefore, according to the present invention, the high-frequency characteristics are improved to the extent that the impedance seen from the phototransistor is reduced, resulting in the characteristics shown in FIG. 3.

This is a more advantageous effect in light of the purpose of the present invention.

On the upper side, the output of the phototransistor is taken out from the emitter, but it is clear that the present invention can be applied as long as it is noted that the phase changes even when taken out from the collector. Although we have explained the case where the phototransistor 1 is used as a light receiving element,
The present invention can be practiced even if this light receiving element is replaced with another light receiving element.

That is, FIG. 4 shows an embodiment using a light receiving element 7 which is a combination of a silicon solar cell 5 and a transistor 6, and negative feedback is applied from a negative feedback circuit 3 to the connection point between the silicon solar cell 5 and the transistor 6. It is something.

It goes without saying that the operation in this case is exactly the same as that in the upper case.

- As explained above, according to the present invention, by feeding a part of the output of the amplifier 2 back to the light receiving element through the negative feedback circuit 3, it is possible to use up to a high modulation frequency, and it is possible to reduce disturbance light and electrical noise. A light-receiving circuit that is hardly affected can be obtained.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram of the response of the light receiving element, Fig. 2 is an example of the light receiving circuit according to the present invention, Fig. 3 is a modulation frequency-gain characteristic diagram of the light receiving circuit shown in Fig. 2, and Fig. 4 is the present invention. This is another example. FIG. 5 shows an equivalent circuit of the phototransistor 1. 1...Photo I transistor, 2...Amplifier, 3...Negative feedback circuit, 4...Load resistance of phototransistor 1, 5... ...Silicon solar cell.

Claims (1)

[Claim for Utility Model Registration] In a light receiver used opposite to a projector that outputs high-frequency modulated light, the output terminal of an amplifier 2 is connected to a negative feedback circuit 3 whose feedback coefficient changes using a phototransistor. A light receiving circuit characterized by: