JPH0637556A - Differential photodiode output device - Google Patents

Abstract

PURPOSE:To obtain a photodiode output device improved in S/N. CONSTITUTION:A differential photodiode output device is comprised so as to obtain signal currects with different polarity from the cathode and anode of an inversely biased photodiode. Concretely, it is comprised of the photodiode to which inverse bias voltage is applied via load resistors 4-1, 4-2 connected to the cathode and the anode, respectively, and two current amplifiers 5-1, 5-2 connected to the connecting point of the cathode and anode of the photodiode 1 and the load resistors, respectively, and input currents -iP and +iP in directions opposite to each other are supplied to the two current amplifiers 5-1, 5-2, and detection output can be obtained from the output terminals of both amplifiers 5-1 and 5-2. The differential photodiode output device can be used in an optical/electrical conveter to detect a weak optical signal in a wide range.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a differential photodiode output device, and in particular to an optical pickup in an optical information recording / reproducing apparatus for detecting an optical signal of weak energy and an optical cable transmitting device for transmitting an optical signal. The present invention relates to a photodiode output device suitable for a photoelectric conversion device such as a container.

[0002]

2. Description of the Related Art In reproducing equipment for optical discs such as compact discs (CD) and laser discs (LD),
For example, a minute laser light spot from a semiconductor laser is irradiated onto an optical disk memory, and reflected light (or transmitted light) is received by a photodetector (optical pickup) including a photodiode to detect information recorded on the disk. ,
Alternatively, it detects a signal for focusing control or tracking control of the laser spot.

In this case, for example, in a conventional ROM disk such as a non-writable CD, a metal reflecting surface such as an aluminum vapor deposition film is provided on the disk surface, so that a high reflectance with respect to a laser beam to be irradiated is provided. It has a high S value because it can withstand high energy irradiation.
It is possible to obtain the detection output of the / N ratio, but recently writable write-once type optical disks (write once or WO disk), and further magneto-optical recording materials (MO) and phase change type recording materials, etc. In a rewritable optical disc (RW disc) capable of recording / reproducing using, the reflectance is small, and the irradiation energy for reproducing cannot be increased. In particular, so-called phase change recording which is expected to be developed in the future. As for the material, the rate of change in reflectance is small, so the signal level obtained is 1 for CD.
It will be reduced to / 20. Because of this,
As one of the improvement measures, a photodiode output device having a high S / N ratio and high output is desired.

FIG. 1 is an equivalent circuit of a photo-detecting element composed of a photodiode. It is assumed that a current iP flows across the ideal diode 1 in proportion to the amount of incident laser light emitted from a semiconductor laser LD, for example. For example, the circuit configuration is such that the current source 2 that generates the current iP and the interelectrode capacitance 3 are connected. In deriving the detection output from the photodetector, a reverse bias Vcc is conventionally added to increase the response speed and linearity range of the photodiode (PD) 1, as shown in FIG. The interelectrode capacitance is reduced, and RL-1 (4-1) or RL-2 (4-2) as the load resistance 4 is connected to the anode or the cathode. Then, the movement of the electron-hole pair is induced in proportion to the amount of incident light, and the current iP flows, whereby the detection output voltage of either polarity is derived from both ends of the load resistor 4 (see the same figure). a or b). In addition, such a photodiode contains intrinsic noise (thermal noise and shot noise) generated from its physical structure, and the amount of light incident on the photodiode must sufficiently exceed its noise equivalent power.

Further, in a normal photodetector, as shown in FIG. 3, a preamplifier 5 is connected to such a photodetection element to be integrally formed. FIG. 6A shows an example in which voltage amplification is performed by a voltage amplifier having a high input impedance, and FIG. 6B shows an example in which current amplification is performed by a negative feedback type current amplifier. The signal detected from the characteristics of the photodiode as a current source can be considered as a current source, and a large voltage can be obtained if it is received by a high resistance as shown in FIG. The voltage amplification is impractical because the frequency band is reduced due to the distributed capacity of the. In general,
As shown in FIG. 8B, current amplification is used because the negative feedback amplifier can reduce the influence of various capacitances and can broaden the band.

[0006]

Therefore, considering a photodiode as a photodetector in a high-density recording / reproducing device such as WO or MO as described above, a sufficient amount of incident light cannot be given. Amplifier noise is becoming a problem. That is, since the intrinsic noise due to the photodiode is physically determined even though the amount of incident light cannot be made sufficiently large, in order to obtain a high output and a high S / N ratio detection output, At the same time as operating the diode under the optimum operating condition, it is necessary to increase the signal gain in the preamplifier and not increase the noise.

[0007] For example, at the present time, in reproducing a WO disc, the disc surface power is set to 1.5 mW from the viewpoint of S / N. However, there is a difference between amplifier noise and disc noise caused by the disc structure or recording structure. The difference is about 10 dB,
The CN ratio (Carrier to Noise ratio) is reduced by about 0.4 dB. Moreover, if the surface power of the disk is 1.5 mW, the disk may be erased and there is a problem in life. Therefore, it is necessary to further reduce the amount of incident light, and therefore, further improvement of the amplifier S / N is desired.

However, even if the signal amplifier in the above-described current amplifier of FIG. 3b is merely considered to be increased, if the feedback resistance R is increased in order to increase the gain, the S / N ratio is rather decreased, which is a good improvement measure. S / N
There is a need for a preamplifier configuration that can increase the signal gain without lowering the signal gain. In order to improve the amplifier S / N, there are a method of installing two amplifiers in parallel, a method of performing an optimized design with a discrete configuration, etc. All of these methods have a problem that they are not very feasible. Therefore, the present invention is used in an optical-electrical conversion device for detecting a weak optical signal such as an optical pickup for recording / reproducing an optical disk including the above-mentioned WO disk or a light receiver in an optical cable transmission device for transmitting an optical signal. It is an object of the present invention to obtain a photodiode output device having an improved S / N ratio which can be achieved.

[0009]

According to the present invention, a differential photodiode output device is constructed so as to obtain signal currents having different polarities from both the cathode and the anode of a reverse biased photodiode. . Specifically, a photodiode to which a reverse bias voltage is applied via load resistors respectively connected to the cathode and the anode, and two photodiodes respectively connected to connection points of the cathode and anode of the photodiode and the load resistor are provided. The two current amplifiers are supplied with input currents in opposite directions, and a detection output is obtained from between the output terminals of both amplifiers. The differential photodiode output device can be used as an opto-electric conversion device for detecting a wide and weak optical signal.

[0010]

According to such a configuration, the differential output current is taken out from the photodiode which is the current source, and the differential current amplifier is configured, so that the signal output is doubled without increasing the amplifier noise. You can

[0011]

FIG. 4 is a circuit diagram for explaining the principle of a differential photodiode output device according to the present invention, in which the load resistance 4-1 (resistance value RL is provided on both the cathode side and the anode side of the photodiode 1). -1) and 4-2 (resistance value RL-2) are connected, and two output terminals are taken out from the connection point with those photodiodes. Here, RL-1 = RL-2 = RL. This circuit has a configuration in which the conventional circuits shown in FIGS. 2A and 2B are stacked, and the two output terminals have a signal voltage −iPRL, respectively.
And iPRL are generated, and the phases are opposite to each other. Therefore, if each of these signal voltages is received by a current amplifier, the signal current can be doubled.

FIG. 5 shows a circuit configuration therefor, and is a circuit diagram showing a basic configuration of a differential photodiode output device according to the present invention. Using the photodiode 1 as a current source, a current amplifier 5 is provided at each output terminal so that signal currents -iP and iP in opposite directions are used as input currents.
-1 and 5-2 are connected to form a differential input current amplifier. In this way, as shown in the drawing, the ring current iP flows from the current amplifier 5-1 to the current amplifier 5-2 via the photodiode 1 which is the current source, and _Vout -V between the amplifier output terminals is reached. it is possible to obtain a signal output of the double-level to _out. That is, the conventional current amplifier (see FIG.
In b), only the induced current iP of the photodiode is current-voltage converted by Rf. However, in the circuit of the present invention, iP becomes a ring current and drives the differential amplifier.
iPRf + iPRf is converted to current-voltage,
Since the signal current is doubled, a signal output having a doubled level can be obtained. As a result, the amplifier S / N ratio is 6 dB
You can upload it.

FIG. 6 shows an example in which the differential photodiode output device according to the present invention is applied to an optical pickup of an optical disc recording / reproducing device such as the WO described above, and FIG.
And b are a conventional photodiode output circuit (single input amplifier) and a differential photodiode output circuit according to the present invention (using a differential configuration amplifier IC (CXA-1077) commonly used as a preamplifier, respectively. This is an actual circuit that constitutes a differential input amplifier). These circuits were compared and tested by injecting the light emitted from a semiconductor laser modulated by a test signal of 0 to 50 MHz and measuring the obtained output level. As a result, the differential photodiode according to the present invention was used over almost the entire area. It was confirmed that the S / N ratio of the output circuit was increased by 6 dB.

As a photodiode for an actual optical pickup in the above-mentioned optical disk recording / reproducing apparatus, a photodiode formed by integrating photodiodes divided into two or four on a substrate is used. It is needless to say that the cathode of each photodiode must be formed separately in applying the above method.

The differential photodiode output device according to the present invention detects a wide range of weak optical signals such as the optical pickup of the above-mentioned optical disk recording / reproducing device and a light receiver in an optical cable transmitting device for transmitting an optical signal. It can be applied to a photoelectric conversion device for

[0016]

According to the above-mentioned differential photodiode output device of the present invention, the signal level can be doubled without raising the noise level. S / N can be improved, and if it is used in a device or system that requires CD or LD reproduction or laser light signal detection such as a photodetector in an optical cable transmission device, the required laser power can be obtained. High S / N can be maintained while saving.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an equivalent circuit of a photo-detecting element including a photodiode.

FIG. 2 is a circuit diagram for explaining the principle of a conventional photodiode output device.

FIG. 3 is a circuit diagram showing a basic configuration of a conventional photodiode output device.

FIG. 4 is a circuit diagram for explaining the principle of the differential photodiode output device according to the present invention.

FIG. 5 is a circuit diagram showing a basic configuration of a differential photodiode output device according to the present invention.

FIG. 6 is a circuit diagram showing an example of an actual circuit of an optical pickup according to the related art and the present invention.

[Explanation of symbols]

 1 Photodiode 2 Current source 3 Inter-electrode capacitance 4 Load resistance 5 Preamplifier 6 Amplifier IC

Claims (3)

[Claims] 1. A differential photodiode output device, wherein signal currents having different polarities are obtained from both the cathode and anode of a reverse biased photodiode. 2. A photodiode to which a reverse bias voltage is applied via load resistors respectively connected to the cathode and the anode, and two photodiodes respectively connected to the connection points of the cathode and anode of the photodiode and the load resistor. A differential photodiode output device comprising a current amplifier, wherein input currents in opposite directions are supplied to the two current amplifiers, and a detection output is obtained from between output terminals of both amplifiers. 3. An opto-electric converter for receiving a laser beam modulated by an information signal containing control information and detecting the information signal, wherein a reverse bias voltage is applied via load resistors respectively connected to the cathode and the anode. An applied photodiode and two current amplifiers respectively connected to the connection points of the cathode and the anode of the photodiode and the load resistor are provided, and the two current amplifiers are supplied with input currents in opposite directions. A photoelectric conversion device, wherein a detection output is obtained between output terminals of both amplifiers.