JPS6224437A - Photodetecting circuit - Google Patents

Abstract

PURPOSE:To considerably improve the detection accuracy of the main frequency band of an optical signal by adding an inductance element and giving a resonance frequency characteristic to a frequency characteristic with the aid of the inner capacity of an element converting the optical signal into a current one. CONSTITUTION:An inductance 6 is inserted into a load resistance 3 in parallel. When the optical signal is made incident on a photodiode 2, said diode 2 flows a current in proportion to the luminous energy of the optical signal to the load resistance 3 and the inductance 6, and a parallel circuit made of the load resistance 3 and the inductance 6 converts the optical signal into a voltage one. An amplifier 4 amplifies the voltage signal which is taken out of an output terminal 5. The jointing capacity of the photodiode 2 and the resonance frequency characteristic by the inductance 6 are fitted to a signal frequency band, whereby a reproduction signal with less noises and an excellent S/N can be taken out of the output terminal 5.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a photodetection circuit used in optical recording/reproducing devices and the like.

[Conventional technology]

A conventional example of this type of photodetection circuit is shown in FIG.

In the figure, 1 is a bias voltage (VB) terminal, 2 is a photodiode, 3 is a load resistor, 4 is an amplifier, and 5 is a power terminal.

In this configuration, when an optical signal is incident on the photodiode 2, the photodiode 2 causes a current proportional to the amount of light of the optical signal to flow through the load resistor 3, and the load resistor 3 converts the optical signal into a voltage signal. . This voltage signal is amplified by an amplifier 4 and taken out from an output terminal 5.

FIG. 5 is an equivalent circuit of the photodetection circuit, in which the photodiode 2 is shown by a current source 1p, an internal resistance Rp (8), and an internal junction capacitance ICp (9). RL is the resistance value of the load resistor 3. Note that the internal resistance Rp (8) and the internal junction capacitance Cp (9) include resistance components and capacitance components due to the electrical wiring up to the input terminal of the amplifier 4.

[Problem that the invention seeks to solve]

As is clear from this equivalent capacitance, the input end circuit of the amplifier 4 has resistance components RL, Rp, and a capacitance component Cp. Therefore, if the amplifier 4 has a sufficiently wide band, its frequency characteristics will be as shown in Figure 6. As shown in , the characteristics are flat. fc is the cutoff frequency.

Therefore, even if the signal frequency band of the optical signal to be detected is limited or the frequency spectrum is concentrated in a specific frequency band, the optical detection circuit has a wide band, so the noise band is wide. There was a problem in that the S/N ratio was poor and it was difficult to reproduce with sufficient fidelity.

This invention was made to solve the above problem.
It is an object of the present invention to obtain a photodetection circuit that can significantly improve the detection accuracy of the main frequency band of an optical signal compared to the conventional one.

[Means to solve the problem]

In order to achieve the above object, the present invention adds an inductance element, and uses the element and the internal capacitance of the element that converts an optical signal into a current signal to give a resonant frequency characteristic to the frequency characteristics.

[Effect]

The frequency characteristics of the photodetection circuit according to the present invention are determined by the junction capacitance of the photodiode, the resonance characteristics of the newly added inductance component, and damping by the resistance component, and by selecting the value of the inductance component, a limited frequency band can be achieved. It has an excellent regeneration effect on optical signals with

〔Example〕

FIG. 1 shows an embodiment of the invention. In the figure, 6 is an inductance, which is inserted in parallel with the load resistor 3. Since the other configurations are the same as those in FIG. 4, they are indicated by the same reference numerals.

In this configuration, when an optical signal is incident on the photodiode 2, the photodiode 2 causes a current proportional to the amount of light of the optical signal to flow through the load resistor 3 and the inductance 6, and the parallel circuit of the load resistor 3 and the inductance 6 causes the above-mentioned The optical signal is converted into a voltage signal. This voltage signal is amplified by an amplifier 4.

and is taken out from the output terminal 5.

In this photodetection circuit, as shown in the equivalent circuit of FIG. 2, there is a capacitance component Cp and an inktance component LL. Therefore, assuming that the amplifier 4 has a wide band, its frequency characteristics are shown in FIG. As shown, it has a resonant frequency characteristic, and the resonant frequency fo is fo=1/2πL口]Tc-.

Therefore, when the frequency band of an optical signal has a limited frequency band characteristic, by matching the above-mentioned resonant frequency characteristic to this signal frequency band, a reproduced signal with less noise and a good S/N ratio can be transmitted to the output terminal 5. It can be taken out from

Note that in this example, the junction capacitance N of the photodiode 2 (including the capacitance generated in the electrical wiring) is used as the capacitive component, but with this alone, it is not possible to obtain the necessary resonance characteristics. If necessary, add other capacitive elements.

〔Effect of the invention〕

As explained above, this invention provides resonance frequency characteristics to the frequency characteristics of the circuit, so that optical signals in a limited frequency band can be reduced to noise band and S

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing an embodiment of the present invention, Fig. 2 is an equivalent circuit diagram of the above embodiment, Fig. 3 is a frequency characteristic diagram of the above embodiment, and Fig. 4 is a circuit diagram of a conventional photodetection circuit. , FIG. 5 is an equivalent circuit diagram of the conventional circuit, and FIG. 6 is a frequency characteristic diagram of the conventional circuit. In the figure, 2 - photodiode, 3 - load resistance, 4
Amplifier, 6-.-Inductance. In addition, in the figures, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] In a photodetection circuit having a first element that converts an optical signal into a current signal, a second element that converts the current signal into a voltage signal, and a third element that amplifies the voltage signal, the second element A photodetection circuit characterized in that a fourth inductance element is provided in parallel, and the frequency characteristic has a resonance frequency characteristic due to the internal capacitance of the first element and the fourth inductance element.