JP2008153930A - Light receiving device for spatial optical communication - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light receiving device for spatial optical communication which can easily take out even a small photocurrent and is suitable for being made into an IC. <P>SOLUTION: The light receiving device for the spatial optical communication comprises: a photodiode D1 for receiving light from a light transmitting device and converting a light quantity to a current; a current/voltage conversion part 10 for converting the change of the current I1 converted in the photodiode D1 to a voltage (voltage at a terminal Vo); and an amplification part 11 for fetching and amplifying the voltage signal v1 of the connection point (a) of the photodiode D1 and the current/voltage conversion part 10 and feeding the amplified voltage signal v2 back to the connection point (a). In the light receiving device for the spatial optical communication, a FET Q5 and FET Q6 for controlling the operating current I3 of the amplification part 11 corresponding to the current I1 flowing to the photodiode D1 are provided. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a light receiving device for spatial light communication that receives an optical signal transmitted from a light transmitting device.

  As a light receiving device for spatial light communication that receives an optical signal transmitted from a light transmitting device, a plurality of light receiving portions each including one light receiving element and one preamplifier are provided, and a lens is provided in front of each light receiving portion. An arrangement has been proposed (see, for example, Patent Document 1).

  In the spatial light communication light-receiving device, a comparison circuit and a selection circuit are connected to each light-receiving unit, and a signal from each light-receiving unit is input to the comparison circuit and the selection circuit. The comparison circuit compares the signal levels of the signals input from the light receiving units to detect the maximum signal level, while the selection circuit has an external output terminal connected to the light receiving unit with the maximum signal level. The line is switched so that the signal from the light receiving unit receiving the strongest light is output to the outside.

  However, in the above light receiving device for spatial light communication, a drive current is always supplied to all of the light receiving parts in order to operate the preamplifier, so that the power consumption is very large and when the selection circuit switches the line, There is a problem that a signal transmission error is likely to occur because the signal output to the terminal is momentarily interrupted.

  Further, wiring is necessary between the preamplifier and the comparison circuit of each light receiving unit, between the preamplifier and the selection circuit of each light receiving unit, and between the comparison circuit and the selection circuit, and the wiring becomes very complicated.

Therefore, instead of putting all of the plurality of light receiving units arranged in a matrix into an operating state, the light amount of the photocurrent of each light receiving unit is equal to or greater than a predetermined value by using a current-voltage conversion resistor and a comparator. By turning on / off the operation of the amplifier operating in a pair with the photodiode in the same light receiving unit, the operation of only the light receiving device receiving the light is achieved. The same applicant has applied for a spatial light communication light-receiving device capable of suppressing the operating current (Japanese Patent Application No. 2005-308106).
JP-A-11-150514

  However, in the light receiving device for spatial light communication described in Japanese Patent Application No. 2005-308106, in order to reduce current consumption, it is determined by a comparator whether the photocurrent flowing through the light receiving element is equal to or greater than a predetermined value. Since the operation of the amplifier is controlled based on the above, the photocurrent cannot be extracted when the photocurrent is smaller than a predetermined value.

  In the above-described light receiving device for spatial light communication, a comparator for detecting a direct current component of a photocurrent and determining whether or not a predetermined value is exceeded, and a resistor having a relatively large time constant so that the comparison output does not fluctuate. And a capacitor are required, and it is not suitable for IC.

  The present invention has been made in view of these points, and an object of the present invention is to provide a light receiving device for spatial light communication that can easily take out a small photocurrent and is suitable for integration into an IC.

  In order to solve the above-mentioned problem, the invention described in claim 1 is a light-receiving element that receives light from a light-transmitting device and converts a light amount into a current, and converts a change in current converted by the light-receiving element into a voltage. A spatial light comprising: a current-voltage conversion unit configured to perform the operation; and amplifying the voltage signal received at the connection point between the light receiving element and the current-voltage conversion unit and feeding back the amplified voltage signal to the connection point. The communication light receiving device is characterized in that a control means for controlling an operating current of the amplifying unit according to a current flowing through the light receiving element is provided.

  According to a second aspect of the present invention, in the first aspect, a plurality of light receiving units each including the light receiving element, the current-voltage converting unit, and the amplifying unit are provided, and the light receiving units are arranged in a matrix, The voltage output from each of the light receiving units is added.

  According to the present invention, it is possible to easily extract a relatively small photocurrent, and to realize a spatial light communication light-receiving device suitable for an IC.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a configuration diagram of a light receiving device for spatial light communication according to the present invention. In this spatial light communication light receiving device 1, as shown in the figure, basic cells 2 as light receiving portions are arranged in a matrix. In this embodiment, a total of 36 basic cells 2 are arranged in 6 rows and 6 columns. The number of basic cells 2 is not limited to 36, and a plurality of basic cells 2 may be provided. For example, the total number may be 25 in 5 rows and 5 columns, 49 in total in 7 rows and 7 columns, and the number in the row direction and the number in the column direction may be 5 rows and 6 columns or 7 rows and 5 columns. May be different.

  Each basic cell 2 has a rectangular shape, and a signal line 3 is attached to one side (the upper side in FIG. 1). The signal lines 3 of each basic cell 2 are connected to signal extraction lines 4 arranged in the row direction. In this embodiment, the basic cells 2 are arranged in six lines, so that there are six signal extraction lines 4. It has been routed.

  The six signal extraction lines 4 are combined into one signal main extraction line 5 via the adder S, and the signal main extraction line 5 is connected to the output terminal 6. A wiring 7 is connected in the middle of the signal main lead-out line 5, and this wiring 7 is grounded via a resistor R.

  FIG. 2 shows a detailed configuration of the basic cell 2. The basic cell 2 is the same as the light receiving unit. As shown in FIG. 2, the basic cell 2 has a rectangular outer shape (in this embodiment, a square), and an amplifier unit 8 is provided at the corner of the rectangle. A light receiving element 9 is provided at a portion other than the amplifier unit 8, and the light receiving element 9 is electrically connected to the amplifier unit 8. The shape of the basic cells 2 may be a regular hexagon. In this case, the plurality of basic cells 2 are arranged in a honeycomb structure.

  FIG. 3 shows a circuit configuration of the amplifier unit 8. As shown in FIG. 3, the amplifier unit 8 includes a current-voltage conversion unit 10 and an amplification unit 11. The current-voltage converter 10 is provided with MOS FETs (Metal Oxide Semiconductor Field Effect Transistors: hereinafter referred to as FETs) Q1, Q2, Q3, Q4, and Q5. The cathode side of the photodiode D1, which is the light receiving element 9, is connected to the source of the FET Q1. The anode side of the photodiode D1 is grounded.

  The drain of the FET Q1 is connected to the drain of the FET Q2, and the source of the FET Q2 is connected to the power source Vdd. The source of the FET Q3 is connected to the power source Vdd, and the drain of the FET Q3 is connected to the terminal Vo. The drain of the FET Q3 is grounded through the load resistor R1. The terminal Vo is connected to the signal line 3 (see FIGS. 1 and 2).

  The power source Vdd is connected to the source of the FET Q4, and the drain of the FET Q4 is connected to the drain of the FET Q5. The source of the FET Q5 is grounded, and the FET Q5 is so-called diode-connected in which its gate and drain are connected.

  The FET Q2 has a diode connection in which its gate and drain are connected, and the drain of the FET Q2 and the gate of the FET Q3 are connected. The gate of the FET Q2 and the gate of the FET Q4 are connected to each other via a resistor R2. An intermediate point between the resistor R2 and the gate of the FET Q4 is connected to the power source Vdd via the capacitor C2.

  Here, FET Q2 and FET Q3, and FET Q2 and FET Q4 each constitute a current mirror circuit.

  The amplification unit 11 is provided with FETs Q6, Q7, Q8, Q9, and Q10. The source of the FET Q6 is grounded, the gate of the FET Q6 is connected to the gate of the FET Q5 of the conversion unit 10, and the FET Q5 and the FET Q6 constitute a current mirror circuit.

  The drain of the FET Q6 is connected to the source of the FET Q7 and the source of the FET Q8, respectively, and the reference voltage Vr is applied to the gate of the FET Q8. The drain of the FET Q7 is connected to the drain of the FET Q9, and the drain of the FET Q8 is connected to the drain of the FET Q10. Here, the FET Q7 and the FET Q8 constitute a current mirror circuit.

  The gate of FET Q9 and the gate of FET Q10 are connected to each other, and FET Q10 has a diode connection with the gate connected to the drain. Further, the source of the FET Q9 and the source of the FET Q10 are connected to the power source Vdd. Here, the FET Q9 and the FET Q10 constitute a current mirror circuit.

  In this embodiment, a connection point between the photodiode D1 and the current-voltage conversion unit 10, that is, a connection point a between the photodiode D1 and the source of the FET Q1 is connected to the gate of the FET Q7 through the wiring 12, and further, the FET A connection point b between the drain of Q7 and the drain of the FET Q9 is connected to the gate of the FET Q1 through the wiring 13.

  Next, the operation of this embodiment will be described.

  In the present embodiment, the FET Q1 of the conversion unit 10 sends the signal voltage existing on the source side to the amplification unit 11 via the wiring 12, amplifies the amplification unit 11, and the amplified signal voltage is wired. By feeding back to the gate of the FET Q1 via 13, the input impedance viewed from the source can be equivalently reduced. When the photocurrent I1 flows through the FET Q2, the FET Q1, and the photodiode D1, the photocurrent I4 equivalent to the photocurrent I1 flows through the FET Q3 by the current mirror of the FET Q2 and the FET Q3. A voltage (I4 * R1, that is, I1 * R1) can be taken out from the terminal Vo by the load resistor R1.

  Further, due to the current mirror of FET Q2 and FET Q4, a photocurrent I2 equivalent to the photocurrent I1 flows through the FET Q4 (this photocurrent I2 is also equivalent to the photocurrent I4). As a result, the FET Q5 A photocurrent I2 flows. The resistor R2 and the capacitor C2 are LPFs (Low Pass Filters) for extracting the direct current component of the photocurrent I1 to the photocurrent I2.

  When the photocurrent I2 flows, the photocurrent I3 flows in the FET Q6 by the current mirror of the FET Q5 and the FET Q6. That is, here, the FET Q5 and the FET Q6 constitute a control means for controlling the operating current of the amplifier 11.

  FET Q2 and FET Q3, FET Q2 and FET Q4 have a current ratio of 1, and if FET Q2 and FET Q4 have a current ratio of 2, I3 = 2 * 1, and the current flowing through FET Q1 to FET Q5 is amplified. The currents flowing in the FET Q7 and FET Q8, which are the working pairs of the unit 11, can be made the same.

  The amount of light striking the photodiode D1 becomes a photocurrent I1, which is output from the terminal Vo, and at the same time, the drain currents of the operational amplifier FETs Q7 and Q8 of the amplifier 11 are also determined. If light does not strike the photodiode D1 and there is no light, the photocurrents I1 to I4 do not flow.

  The photocurrent I1 causes a constant photocurrent to flow even at the lowest value of amplitude modulation due to the extinction ratio on the light emission (light transmission) side. That is, no deep modulation is performed until the light is completely extinguished. The adjustment of the minimum light quantity and the maximum light quantity is performed on the light emitting side, but in the basic cell (light receiving part) 2 of the present embodiment, the operation of the amplifying part 11 is good even at a small current by using this. It is supposed to be.

  4A shows a case where there is a certain minimum amount of light, and FIG. 4B shows a case where the minimum amount of light is 0. The MOS used for the current-voltage conversion unit 10 and the amplification unit 11 on the L side of data communication. No bias current is applied to the transistor, and its operation is non-linear.

  In a light receiving device in which such basic cells 2 are arranged in a matrix in the vertical and horizontal directions as shown in FIG. 1, the basic cell 2 in the portion where the light is applied has a current-voltage converter 10 and an amplifier according to the amount of light. 11 is caused to flow, so that the voltage V (the voltage at the output terminal in FIG. 1) obtained by adding the current output from each basic cell 2 shown in FIG. ) Is an added output corresponding to each photocurrent.

  The amplifying unit 11 in the basic cell 2 is for reducing the influence of the capacitance between the terminals of the photodiode D1 on the frequency characteristics, and normally suppresses the input impedance viewed from the source of the FET Q1 to several tens of Ω or less.

  The decrease in the photocurrent I3 due to the decrease in the amount of light gradually increases the input impedance at a high frequency, but the influence after the addition is small because the current obtained from the basic cell 2 having a large amount of light is dominant.

  Next, the reason why the input impedance viewed from the source of the FET Q1 can be lowered will be described in detail.

First, as shown in FIG. 3, the voltage in the wiring 12 is set to v1, and the voltage in the wiring 13 is set to v2. When the input impedance at the connection point a between the cathode side of the photodiode D1 and the source of the FET Q1 is Zi, Zi≈1 / gm1 (where gm1 is the FET) In FIG. 3, considering that the source of the FET Q7 and the source of the FET Q8 are in virtual ground,
v2 = −v1 * gm2 * R2 (1)
i1 = (v2-v1) gm1 (2)
It is. Here, gm2 is the mutual conductance of the FET Q7. I1 is the same as I1.

From the above formulas (1) and (2),
Zi = −v1 / i1 = 1 / gm1 (1 + R2 * gm2) (3)
Thus, the input impedance Zi can be made much smaller than the input value of the gate ground.

On the other hand, when a signal current i2 (= I3) flows between the connection point of the source of FET Q7 and FET Q8 and GND,
v2 = −k * i2 * R2 (4)
i1 = (v2-v1) gm1 (5)
It is. Here, k is a diversion ratio of i2.

Since v1 and v2 are expressed as the source follower operation of the FET Q1, from the above equations (4) and (5),
v1 / v2 = gm1 * Rs / (1 + gm1 * Rs) (6)
It becomes. Here, Rs is a resistance between the sources of the FETs Q7 and Q8 and GND.

  Now, the FET Q5 and the FET Q6 constitute a current mirror. When a current flows to the FET Q5 side, a current also flows to the FET Q6 side, and the FET Q6 can be regarded as a constant current source. Then, Rs = ∞ and v1≈v2, i1≈0. Therefore, i2 does not appear in the output Vo.

  3 receives a light, a current corresponding to the intensity of the light flows, which can be expressed as the sum of a direct current I and an alternating current i. In addition, the capacitance corresponding to the area is provided between the terminals, that is, Rs in the above equation is actually small at a high frequency, and v1≈v2 does not hold.

  Then, a current due to i2 corresponding to the reciprocal of the impedance of the capacitance also flows through the resistor R1.

  In the configuration of FIG. 3, an LPF including a resistor R2 and a capacitor C2 is provided to prevent the feedback of i2. The time constant of the LPF is not intended to remove the data signal itself including the low frequency component, but is intended to remove the high frequency component. Therefore, the time constant of the LPF is sufficiently smaller than that of the conventional example, and is suitable for IC implementation. .

  FIG. 5 shows a second embodiment. In this embodiment, the amplifying unit 11 includes an FET Q16, an FET Q17, an FET Q19, and an FET Q20. Compared to the first embodiment, the FET Q16 corresponds to the FET Q6, the FET Q17 corresponds to the FET Q7, the FET Q19 corresponds to the FET Q9, and the FET Q20 corresponds to the FET Q10. That is, in this embodiment, there is no equivalent to the FET Q8 in the first embodiment.

  The drain of the FET Q16 is connected to the drain of the FET Q20, and the source of the FET Q17 is grounded.

  Also in this embodiment, the connection point a between the photodiode D1 and the source of the FET Q1 is connected to the gate of the FET Q17 through the wiring 12, and further, the connection point b between the drain of the FET Q17 and the drain of the FET Q19. Is connected to the gate of the FET Q1 through the wiring 13, and the same effect as in the first embodiment can be obtained.

  FIG. 6 shows a third embodiment. In the present embodiment, the current-voltage conversion unit 10 is provided with FET Q1, FET Q2, and FET Q3. That is, the FET Q4 provided in the first embodiment is missing. The amplification unit 11 is provided with an FET Q26 and an FET Q27.

  The drain of the FET Q26 is connected to the drain of the FET Q27, and the source of the FET Q27 is grounded. The source of the FET Q26 is connected to the power supply Vdd, and the gate of the FET Q26 is connected to the gate of the FET Q2 through the resistor R2. Further, the gate of the FET Q26, the resistor R2, and the intermediate point (the intermediate point on the side of the current-voltage converter 10) are connected to the power source Vdd via the capacitor C2.

  Also in the present embodiment, the connection point a between the photodiode D1 and the source of the FET Q1 is connected to the gate of the FET Q27 through the wiring 12, and further, the connection point b between the drain of the FET Q26 and the drain of the FET Q27. Is connected to the gate of the FET Q1 through the wiring 13, and the same effect as in the first embodiment can be obtained.

It is a block diagram of the light receiving device for spatial light communication according to the present invention. It is a top view which shows the detailed structure of a basic cell. 4 is a circuit configuration diagram of an amplifier unit including a light receiving element according to Embodiment 1. FIG. The change of the light quantity which a light receiving element receives is shown, (a) is a figure in case there exists a fixed minimum light quantity, (b) is a figure in case the minimum light quantity is zero. 6 is a circuit configuration diagram of an amplifier unit including a light receiving element according to Embodiment 2. FIG. 6 is a circuit configuration diagram of an amplifier unit including a light receiving element according to Embodiment 3. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light receiving device for space optical communication 2 Basic cell 8 Amplifier part 9 Light receiving element 10 Current-voltage conversion part 11 Amplifying part a, b Connection point D1 Photodiode Q1-Q10 FET
Q16, Q17, Q19, Q20 FET
Q26, Q27 FET
S adder

Claims (2)

A light receiving element that receives light from the light transmitting device and converts the amount of light into a current;
A current-voltage converter that converts a change in current converted by the light receiving element into a voltage;
In the spatial light communication light receiving device including a voltage signal at a connection point between the light receiving element and the current-voltage conversion unit and amplifying the voltage signal, and an amplification unit that feeds back the amplified voltage signal to the connection point.
A light receiving device for spatial light communication, comprising control means for controlling an operating current of the amplifying unit in accordance with a current flowing through the light receiving element.   A plurality of light receiving units each including the light receiving element, the current-voltage converting unit, and the amplifying unit are provided. The light receiving units are arranged in a matrix and the voltages output from the respective light receiving units are added. The light receiving device for spatial light communication according to claim 1.

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