JPS596019Y2 - Optical signal receiver - Google Patents

Description

[Detailed Description of the Invention] This invention relates to an optical signal receiving device that receives intensity-modulated light and converts the optical signal into an electrical signal.

This type of optical signal receiving device is used in optical communication devices that use optical fibers or space as a transmission medium, and most devices use photodiodes or avalanche photodiodes as photoelectric conversion elements.

FIG. 1 shows an example of such an optical signal receiving device, and as shown in the figure, a signal light 1 is incident on a photodiode 2 and converted into an electric current.

A signal voltage generated across the load resistor 3 by the signal current is applied to the amplifier 5 through the supply capacitor 4.

Note that a transmission voltage of 10 B is applied to the photodiode 2.

Therefore, the bypass capacitor 6 is connected to the photodiode 2.
is connected to the cathode side of the

Now, it is known that in the optical receiving device as described above, the signal voltage Vs generated across the load resistor RL3 is calculated as follows.

Ps is the optical power input to photodiode 2, C
O is the barrier capacitance of the photodiode 2, ω is the modulation angular frequency of the optical signal, and S is the sensitivity of the photodiode 2.

RA is a parallel combination of the load resistance RL and the input impedance of the amplifier, and is expressed as follows.

As is clear from the above equation (1), as long as the signal frequency ω is not so high as to satisfy the condition, the signal voltage Vsl increases as RA increases.

Therefore, in order to increase lVsl, increase charging conversion efficiency, and increase signal-to-noise ratio, it is necessary to increase RA as much as possible.

However, the input impedance of amplifiers, integrated circuits, etc. made of ordinary transistors is only a few K at most.
ρ, and no higher RA can be obtained.

FIG. 2 shows another embodiment of the optical receiving device manufactured by Jumei.

In this example, the amplifier is constructed using a field effect transistor FET, and a high RA can be obtained.
It is possible to increase Vsl.

In addition, in Fig. 2, the symbol D is the drain, G is the gate,
S indicates the source.

The applicant conducted an experiment and found that the load resistance Rt. It was found that the noise of the amplifier output increases approximately in proportion to.

Therefore, increasing the IVSI does not improve the signal-to-noise ratio in any way.

As described above, in the conventional optical signal receiving apparatus, it is difficult to obtain a good signal-to-noise ratio when PS is small.

This invention was made in order to improve the drawbacks of the conventional devices as described above.

FIG. 3 is a diagram showing an embodiment of this invention.

The feature of this invention is that the signal voltage appearing on the load resistor RL3 of the photodiode 2 is applied to the next stage amplifier 5 via the FET 7 as shown in FIG.

In other words, as shown in FIG. 3, the circuit including the FET 7 is a source follower circuit and has no amplification effect, so that the signal applied to the input resistor 12 appears almost at the output resistor 13. It has become.

Note that the input resistance of the FET 7 is extremely high, several M47 or more, so RA in equation (1) can be made as high as necessary by increasing the load resistance 3 of the photodiode 2 and the input resistance 12. .

By the way, regarding this idea, according to the applicant's experiments, R
It was confirmed that even if A was increased to about 100 K.2, the noise of the amplifier 5 hardly increased.

Therefore, the optical receiving device of this invention can obtain a larger signal voltage than the conventional device, and also has no increase in noise, so that the signal-to-noise ratio can be significantly improved.

Note that although all of the above explanations have been made regarding the case where photodiodes are used, the effect remains the same even when an avalanche photodiode is used instead of the photodiode.

[Brief explanation of the drawing]

1 and 2 are diagrams showing an embodiment of a conventional optical receiving device, respectively, and FIG. 3 is a diagram showing an embodiment of an optical receiving device according to this invention. Photodiode, 3 is load resistance, 4 is capacitor, 5 is amplifier, 6 is capacitor, 7 is FET, 8, 9, 10 is resistor, 11 is capacitor, 12 is input resistor, 13 is resistor, 14 is capacitor It is. Note that the same or corresponding parts in the figures are indicated by the same reference numerals.

Claims (1)

[Scope of utility model registration request] An optical signal receiving device having a photodiode or an avalanche photodiode and an amplifier connected to the diode for receiving intensity-modulated light and converting the optical signal into an electrical signal, wherein a field effect transistor is provided at a stage before the amplifier. An optical signal receiving device characterized by having a source follower circuit using an FET.