Abstract:
A photoreceiver circuit includes (a) a photoelectric conversion element for converting incident light to a current, (b) an analog voltage amplifier circuit for amplifying a voltage corresponding to the current of the photoelectric conversion element and for producing an amplified voltage as an output of the photoreceiver circuit, and (c) an analog multiplier circuit for multiplying the amplified voltage produced by the voltage amplifier circuit by an adjusting voltage and for producing an output current with a component proportional to a product of the amplified voltage and the adjusting voltage. The output current of the analog multiplier circuit is supplied to the photoelectric converter element, thereby forming a feedback path of the voltage amplifier circuit. A voltage-lowering part and a current-leaking part may be additionally provided. The voltage-lowering part is connected between the voltage amplifier and the analog multiplier circuit, and the current-leaking part is connected in parallel to the voltage-lowering part.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a photoreceiver circuit and more particularly, to a photoreceiver or photoreceptor circuit equipped with a photoelectric conversion element and an amplifier circuit, which is capable of conversion-gain adjustment of the photoelectric conversion element and high-speed circuit operation. For example, this photoreceiver circuit is applicable to intelligent sensors for sensing a moving object or objects in an image formed by a photoelectric conversion element (i.e., a scene). 
     2. Description of the Prior Art 
     An example of the prior-art photoreceiver or photoreceptor circuits each having photoelectric conversion elements and amplifier circuits is disclosed in the U.S. Pat. No. 5,376,813 issued on Dec. 27, 1994, which is intended to expand the dynamic range with respect to the incident light, resulting in increase in response speed. The circuit configuration of this prior-art photoreceiver circuit thus patented is shown in FIG.  1 . 
     In FIG. 1, a photodiode  301  serves as a photoelectric conversion element. One terminal of the photodiode  301  is connected to the gate of an n-channel Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET)  302 . The other terminal of the photodiode  301  is connected to the ground. The source of the MOSFET  302  is connected to the ground. The drain of the MOSFET  302  is connected to the drain of a p-channel MOSFET  303 . The source of the MOSFET  303  is connected to a power supply (not shown) supplying a constant supply voltage V cc . The gate of the MOSFET  303  is applied with a suitable bias voltage V bias . 
     The combination of the MOSFETs  302  and  303  serves as an inverting, analog voltage amplifier circuit  310  for amplifying a voltage V a  at the output terminal  300 A of the photodiode  301  with respect to the ground (i.e., an input voltage V in  of the amplifier  310 ). The MOSFET  302  is operated in the saturation region. The MOSFET  303  serves as a load resistor of the MOSFET  302  in the amplifier circuit  310 . 
     An output voltage V out  of the voltage amplifier circuit  310 , which is an amplified voltage of the input voltage V in , is derived from the drain of the MOSEET  302  or an output terminal  300 B. The output voltage V out  is fed back to the input side of the amplifier circuit  310  through a voltage-lowering circuit  330  and an n-channel MOSFET  307 . The voltage-lowering circuit  330  comprises two capacitors  304  and  305 . The capacitor  304 , which as a capacitance C 1 , is connected to the output terminal  300 B and a terminal  300 D connected to the gate of the MOSFET  307 . The capacitor  305 , which has a capacitance C 2 , is connected to the terminal  300 D and the ground. 
     A current-leaking circuit  320 , which comprises a p-channel MOSFET  306 , is connected in parallel to the voltage-lowering circuit  330  between the terminals  300 B and  300 D. The gate and the drain of the MOSFET  306  are coupled together to be connected to the terminal  300 B. The source of the MOSFET  306 , which is connected to the substrate, is connected to the terminal  300 D. 
     The source of the MOSFET  307  is connected to the terminal of the photodiode  301  at the terminal  300 A. The drain of the MOSFET  307  is connected to the power supply and applied with the supply voltage V cc . 
     The current leaking circuit  320  serves to leak a current between the terminals  300 B and  300 D. Specifically, when a potential difference occurs between the terminals  300 B and  300 D, a current flows gradually (i.e., leaks) through the MOSFET  306  from the terminal  300 B to the terminal  300 D and vice versa, thereby eliminating the potential difference after a specific relaxation time. 
     Next, the operation of the prior-art photoreceiver circuit shown in FIG. 1 is explained below. 
     When the incident light PH applied to the photodiode  301  has a constant intensity with time, i.e., the photodiode  301  is in the steady state, the electric potentials or voltages at the terminals  300 B and  300 D with respect to the ground are equal to each other because of the current-leaking operation of the MOSFET  306 . On the other hand, a voltage V d  at the terminal  300 D (i.e., the gate voltage of the MOSFET  307 ) with respect to the ground is determined in such a way that a current flowing through the MOSFET  307  is equal to an output current I PH  of the photodiode  301 . 
     Thus, the output voltage V out  of the prior-art photoreceiver circuit of FIG. 1 produced at the output terminal  300 B is equal to the voltage V d  at the terminal  300 D, i.e., V out =V d , when the photodiode  301  is in the steady state. 
     On the other hand, when the intensity of the incident light PH applied to the photodiode  301  varies with time, i.e , the photodiode  301  is in the changing state, the magnitude of the output current I PH  of the photodiode  301  varies with time according to the intensity change of the light PH, thereby changing the magnitude of the voltage V a  at the terminal  300 A. The change of the voltage V a  at the terminal  300 A is applied to the amplifier circuit  310  as its input voltage V in  and is amplified therein, producing an amplified change of the output voltage V out  at the terminal  300 B. This amplified change of V out  is opposite in phase to the change of V a  and therefore, the latter is decreased if the former is increased, and vice versa. The amplified change of the output voltage V out  is sent to the gate of the MOSFET  307  through the voltage-lowering circuit  330 , causing an amplified change of the current flowing through the MOSFET  307 . Thus, the current flowing through the MOSFET  307  is equalized with the output current I PH  of the photodiode  301 . 
     As explained above, the output voltage V out  of the photoreceiver circuit is fed back to the input side of the amplifier circuit  310  through the voltage-lowering circuit  330  and the n-channel MOSFETs  307 , thereby suppressing the change of the voltage V a  at the terminal  300 A caused by the change of the output current I PH . As a result, the value of the voltage V a  is kept approximately constant independent of the intensity change of the incident light PH. 
     The above-described circuit operation of the prior-art photoreceiver circuit of FIG. 1 is unlike that of another prior-art photoreceiver circuit shown in FIG.  2 . The circuit in FIG. 2 is simply comprised of a photodiode  401  and an n-channel MOSFET  402  without any feedback path. An output terminal  400 A of the photodiode  401  is connected to the source of the MOSFET  402 . The gate of the MOSFET  401  is applied with a fixed bias voltage V b . An output voltage V out  of the photoreceiver circuit is derived from the output terminal  400 A. 
     In the circuit of FIG. 2, since no feedback path is provided, the output voltage V out  produced at the terminal  400 A varies largely in order to equalize the current flowing through the MOSFET  402  with the output current I PH  of the photodiode  401 . This is quite different from that of the prior-art photoreceiver circuit shown in FIG. 1 where the current flowing through the MOSFET  307  is equalized with the output current I PH  of the photodiode  301  by changing the gate voltage V d  of the MOSFET  307 . 
     In the circuit of FIG. 2, the parasitic capacitors existing in the vicinity of the terminal  400 A (e.g., the parasitic capacitors of the photodiode  401  and the source region of the MOSFET  402 ) need to be charged and discharged by the output current I PH  itself of the photodiode  401 . Since the output current I PH  is usually very small, it takes a long time to fully charge or discharge these parasitic capacitors. This means that the necessitated relaxation time of the photoreceiver circuit of FIG. 2 from the changing state to the steady state is extremely long. 
     In contrast, in the photoreceiver circuit of FIG. 1, the voltage V a  at the terminal  300 A is always kept approximately constant because of the operation of the MOSFET  307 . Thus, the parasitic capacitances need not be charged nor discharged, which shortens the relaxation time. This creates an advantage of high-speed circuit operation. 
     In addition to the advantage of high-speed circuit operation, the prior-art photoreceiver circuit of FIG. 1 has another advantage that the gain of the photoreceiver circuit in the changing state is different from that in the steady state. Here, the term “gain” means the coefficient of photoelectric conversion in this photoreceiver circuit, in other words, it means the ratio of the magnitude of the output voltage V out  to the intensity of the incident light PH. Because of this variable gain, the prior-art photoreceiver circuit of FIG. 1 has a wider dynamic range than that of the prior-art photoreceiver circuit of FIG.  2 . 
     Subsequently, adjustment of the gain in the photoreceiver circuit of FIG. 1 is explained below. 
     As explained previously, the p-channel MOSFET  306  of the current-leaking circuit  320  has a function to leak the current between the terminals  300 B and  300 D. Thus, if the intensity of the incident light PH varies abruptly and therefore, the output voltage V out  generated at the terminal  300 B is abruptly changed, the voltage change at the terminal  300 B is transmitted to the terminal  300 D through the capacitors  304  and  305  of the voltage-lowering circuit  330 . In this case, the voltage V d  at the terminal  300 D is equal to [C 1 /(C 1 +C 2 )] times the output voltage V out  because it is divided by the capacitors  304  and  305 , where V d &lt;V out . As a result, to equalize the current flowing through the MOSFET  307  with the output current IPH of the photodiode  401 , the output voltage V out  at the terminal  300 B needs to be higher than a voltage required in the steady state. This means that the gain of the photoreceiver circuit of FIG. 1 in Lhe changing state is higher than that in the steady state. 
     However, the state where the voltage V d  at the terminal  300 D is equal to [C 1 /(C 1 +C 2 )] V out  occurs only in the changing state where the intensity of the incident light PH varies. After the intensity change of the light PH disappears and the circuit operation enters the steady state, the MOSFET  306  allows the current to leak from the terminal  300 B to the terminal  300 D and vice versa, resulting in the output voltage V out  at the terminal  300 B being equal to the voltage V d  at the terminal  300 D. 
     In other words, in the changing state, the feedback loop is constituted by the capacitors  304  and  305  and the MOSFET  307  and therefore, the divided voltage V d  is applied to the gate of the MOSFET  307 . Unlike this, in the steady state, the feedback loop is constituted by the p-channel MOSFET  306  and the MOSFET  307  and therefore, the output voltage V out  at the terminal  300 B is directly fed back to the gate of the MOSFET  307 . 
     As explained above, in the prior-art photoreceiver circuit of FIG. 1, the value of the photoelectric-conversion gain in the changing state is [(C 1 +C 2 )/C 1 ] times as much as that in the steady state, resulting in a wider dynamic range than that in the circuit of FIG.  2 . 
     With the prior-art photoreceiver circuit of FIG. 1, however, there is a problem that the gain value of the photoreceiver circuit is unable to be optionally adjusted from the outside. This problem occurs not only in the changing state where the intensity of the incident light PH varies (i.e., the capacitors  304  and  305  constitute the feedback path) but also in the steady state where the intensity of the light PH does not vary (i.e., the MOSFET  306  constitutes the feedback path). 
     The above problem is caused by the difficulty in gain adjustment of the analog voltage amplifier circuit  310 . The adjustment of the gain of the amplifier circuit  310  can be realized only by changing the value of the bias voltage V bias  applied to the gate of the MOSFET  203 . However, the change of the bias voltage V bias  is unable to be realized in practice, because the operation of the amplifier circuit  310  is extremely sensitive to the change of the bias voltage V bias . Accordingly, the bias voltage V bias  is usually fixed at a specific value and is unable to be changed from the outside. 
     In practice, even if the value of the bias voltage V bias  is changed within an extremely small range, the operating point of the amplifier circuit  310  readily deviates from its optimum point, resulting in abrupt decrease in gain. This means that the amplifier circuit  310  does not provide the desired amplification operation any more. 
     Thus, although the gain value of the amplifier circuit  310  can be adjusted from the outside by changing the bias voltage V bias  in theory, it is extremely difficult or impossible to be realized in practice. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention to provide a photoreceiver circuit that makes it possible to adjust readily and optionally the gain value of the photoreceiver circuit. 
     Another object of the present invention to provide a photoreceiver circuit capable of high-speed operation. 
     The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description. 
     A photoreceiver circuit according to the present invention is comprised of (a) a photoelectric conversion element for converting incident light to a current, (b) an analog voltage amplifier circuit for amplifying a voltage corresponding to the current of the photoelectric conversion element and for producing an amplified voltage as an output of the photoreceiver circuit, and (c) an analog multiplier circuit for multiplying the amplified voltage produced by the voltage amplifier circuit by an adjusting voltage and for producing an output current with a component proportional to a product of the amplified voltage and the adjusting voltage. 
     The output current of the analog multiplier circuit is supplied to the photoelectric converter element, thereby forming a feedback path of the voltage amplifier circuit. 
     With the photoreceiver circuit according to the present invention, the analog multiplier circuit is provided between the photoelectric converter element and the analog voltage amplifier circuit to form a feedback path of the voltage amplifier circuit. Therefore, the output (i.e., the amplified voltage) of the voltage amplifier circuit is determined in such a way that the output current of the multiplier circuit is equal to the output current of the photoelectric conversion element. 
     The analog multiplier circuit multiplies the amplified voltage from the voltage amplifier circuit and the adjusting voltage, thereby producing the output current with the component proportional to the product of the amplified voltage and the adjusting voltage. 
     Therefore, by changing the value of the adjusting voltage, the value of the output current of the multiplier circuit is changed, in other words, the gain of the photoreceiver circuit can be adjusted readily and optionally. 
     Moreover, since the voltage corresponding to the current of the photoelectric conversion element scarcely varies, the photoreceiver circuit is capable of high-speed operation. 
     In a preferred embodiment of the circuit according to the present invention, a voltage-lowering means and a current-leaking means are additionally provided. The voltage-lowering means is connected between the voltage amplifier circuit and the analog multiplier circuit. The current-leaking means is connected in parallel to the voltage-lowering means. 
     In another preferred embodiment of the circuit according to the present invention, the adjusting voltage is variable from the outside of the photoreceiver circuit. 
     In still another preferred embodiment of the circuit according to the present invention, the multiplier circuit has a same circuit configuration as that of the Gilbert multiplier circuit. However, any other configuration may be applied to the multiplier circuit if it has a multiplication function of two voltage inputs and produces a current output. 
     In a further preferred embodiment of the circuit according to the present invention, the current-leaking means has an operation that the amplified voltage of the voltage amplifier circuit is equalized with the output voltage of the photoreceiver circuit after a specific relaxation time has passed. The current-leaking means may have any form if it can realize this operation. 
     In still further preferred embodiment of the circuit according to the present invention, the voltage-lowering means is a voltage-dividing circuit including capacitors. However, the voltage-lowering means may be any other form if it can lower the amplified voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the present invention may be readily carried into effect, it will now be described with reference to the accompanying drawings. 
     FIG. 1 is a circuit diagram showing a prior-art photoreceiver circuit equipped with an adaptive gain adjusting circuit and a feedback path. 
     FIG. 2 is a circuit diagram showing a prior-art photoreceiver circuit with a simple configuration, which has only a photodiode and a MOSFET. 
     FIG. 3 is a block diagram showing the basic configuration of a photoreceiver circuit according to a first embodiment of the present invention. 
     FIG. 4 is a circuit diagram showing a detailed configuration off the photoreceiver circuit according to the first embodiment of FIG.  3 . 
     FIG. 5 is a block diagram showing the basic configuration of a photoreceiver circuit according to a second embodiment of the present invention. 
     FIG. 6 is a circuit diagram showing a detailed configuration of the photoreceiver circuit according to the second embodiment of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described in detail below while referring to the drawings attached. 
     FIRST EMBODIMENT 
     A photoreceiver or photoreceptor circuit according to a first embodiment of the present invention is explained below with reference to FIGS. 3 and 4. 
     FIG. 3 shows the basic configuration of the photoreceiver circuit according to the first embodiment. In FIG. 3, a photoelectric conversion element  101  receives incident light PH and converts it to an electric signal, thereby producing an output current I PH . The output current I PH  flows through an output terminal  100 A of the element  101 . The current I PH  has a magnitude or amplitude according to the intensity (i.e., illuminance) of the light PH thus received. A voltage V a  with respect to the ground at the terminal  100 A is determined by the magnitude of the current I PH . 
     An n-channel MOSFET  102  and a p-channel MOSFET  103  constitute an inverting, analog voltage amplifier circuit  110 . The gate of the MOSFET  102  is connected to the output terminal  100 A of the photoelectric conversion element  101 . The source of the MOSFET  102  is connected to the ground. The drain of the MOSFET  102  is connected to the drain of the p-channel MOSFET  103 . The source of the MOSFET  103  is connected to a power supply (not shown) supplying a supply voltage V cc . The gate of the MOSFET  103  is applied with a suitable bias voltage V bias . This voltage V bias  is fixed at a specific value. An output terminal  100 B of the photoreceiver circuit is provided at the connection point of the drains of the MOSFETs  102  and  103  in the amplifier circuit  110 . 
     Since the gate of the MOSFET  102  is applied with the voltage V a  at the output terminal  100 A of the photoelectric conversion element  101 , the voltage V a  is applied to the gate of the MOSFET  102  as an input voltage V in  of the amplifier circuit  110 , i.e., V in =V a . The amplifier circuit  110  amplifies the input voltage V a  (=V in ) and outputs an amplified voltage as an output voltage V out  at the output terminal  1005 . 
     The n-channel MOSFET  102  is operated in the saturation region, and the p-channel MOSFET  103  serves as a load resistor of the MOSFET  102 . This is the same as that of the prior-art photoreceiver circuit shown in FIG.  1 . 
     An analog multiplier circuit  104 , which is designed for outputting a current proportional to the product of two input voltages, is provided between the photoelectric conversion element  101  and the voltage amplifier circuit  110 , thereby forming a feedback path of the amplifier circuit  110 . An input terminal  104   a  of the multiplier circuit  104  is connected to the output terminal  100 B of the photoreceiver circuit and therefore, the output voltage V out  is applied to the multiplier circuit  104  as its first input. Another input terminal  104   b  of the multiplier  104  is used as a gain control terminal for controlling the gain of the photoreceiver circuit according to the first embodiment. To make it possible to adjust optionally the gain of the photoreceiver circuit from the outside, a variable gain-control voltage V m  is applied to the input terminal  104   b  as its second input. An output terminal  104   c  of the multiplier circuit  104  is connected to the output terminal  101 A of the photoelectric conversion element  101 . Therefore, an output current I o  of the multiplier circuit  104  is supplied to the output terminal  100 A of the element  101 . 
     The output current I o  of the multiplier circuit  104  has a magnitude proportional to the product of the two input voltages V out  and V m . In other words, the current I o  is expressed as 
     
       
         I o =A V out ·V m   (1) 
       
     
     where A is a constant. The voltage V out  serves as a “multiplicand” of the multiplication operation in the multiplier circuit  104  and the voltage V m  serves as a “multiplier” thereof. 
     FIG. 4 shows a detailed configuration of the photoreceiver circuit according to the first embodiment of FIG. 3, in which the multiplier circuit  104  has a same configuration as that of the well-known Gilbert multiplier circuit and at the same time, a photodiode  10 A utilizing a p-n junction of a semiconductor material is used as the photoelectric conversion element  101 . 
     As seen from FIG. 4, the Gilbert multiplier circuit is comprised of a differential pair of two source-coupled, n-channel MOSFETs N 1  and N 2  driven by a constant current sink  111 , a differential pair of two source-coupled, n-channel MOSFETs N 3  and N 4 , and a differential pair of two source-coupled, n-channel MOSFETs N 5  and N 6 . The gates of the MOSFETs N 1  and N 2  are applied with two input voltages V m1  and V m2 , respectively, where V m1 =V m2 =V m . The coupled sources of the MOSFETs N 3  and N 4  are connected to the drain of the MOSFET N 1 . The coupled sources of the MOSFETs N 5  and N 6  are connected to the drain of the MOSFET N 2  The gates of the MOSFETs N 3  and N 6  are commonly applied with the output voltage V out  of the amplifier circuit  110  through the input terminal  104   a . The gates of the MOSFETs N 4  and N 5  are commonly applied with a constant voltage V. through the input terminal  104   b . The drains of the MOSFETs N 3  and N 5  are coupled together. The drains of the MOSFETs N 4  and N 6  are coupled together. 
     Two p-channel MOSFETs P 1  and P 2  constitute a current mirror circuit, which serves as an active load of the Gilbert multiplier circuit. The gate and drain of the MOSFET P 1  are coupled together to be connected to the coupled drains of the MOSFETs N 3  and N 5 . The source of the MOSFET PI is connected to the power supply of V cc . The gate of the MOSFET P 2  is connected to the gate of the MOSFET  21 . The drain of the MOSFET P 2  is connected to the coupled drains of the MOSFETs N 4  and N 6 . The source of the MOSFET P 2  is connected to the power supply of V cc . The output terminal  104   c  of the multiplier circuit  104 , from which its output current I o  is derived, is provided at the connection point of the coupled drains of the MOSFETs N 4  and N 6  and the drain of the MOSFET P 2 . 
     The variable adjusting voltage V m  (=V m1 −V m2 ) is differentially applied across the gates of the MOSFETs N 1  and N 2  and at the same time, the voltage (V out −V g is differentially applied across the coupled gates of the MOSFETs N 3  and N 6  and those of the MOSFETs N 4  and N 5 . Thus, the output current I o  is proportional to the product of the voltages V m  and (V out −V g ), i.e., V m ·(V out −V g ), resulting in the following equation (2). 
     
       
         I o =A(V out −V g )(V m1 −V m2 )=A(V out −V g )V m   (2) 
       
     
     If V g =0 in the equation (2), I o =A V m ·V out  is obtained, which is the same as the above-described equation (1). 
     No further detailed explanation about the Gilbert multiplier circuit is omitted here, because it is well-known. This is disclosed in, for example, the book written by Carver Mead, entitled “ANALOG VLSI AND NEURAL SYSTEMS”, and published by ADDISON-WESLEY PUBLISHING COMPANY. 
     Next, the circuit operation of the photoreceiver circuit according to the first embodiment shown in FIG. 4 is explained below. 
     The magnitude of the output current I PH  of the photodiode  101 A is proportional to the intensity of the incident light PH. Also, the output current I PH  is kept equal to the output current I o  of the multiplier circuit  104 ; in other words, the output voltage V out  of the photoreceiver circuit, which is applied to the multiplier circuit  104 , is determined in such a way that the two currents I PH  and I o  are equal to each other. Therefore, if the value of the control voltage V m  (=V m1 −V m2 ) is set as positive and small in the above equation (2), the output voltage V out  has a comparative large value in order to equalize I o  with I PH . On the other hand, if the value of the control voltage V m  is set as positive and large, the output voltage Vout has a small value in order to equalize I o  with I PH . 
     Accordingly, even if the intensity of the incident light PH (i e., the magnitude of the output current I o ) is constant, the magnitude of the output voltage V out  can be changed. This means that the gain of the voltage amplifier circuit  110  (i.e., the photoreceiver circuit according to the first embodiment) can be readily and optionally adjusted from the outside of the photoreceiver circuit. 
     Moreover, because of the existence of the feedback path comprising the analog multiplier circuit  104 , similar to the prior-art photoreceiver circuit shown in FIG. 1, the voltage V a  at the output terminal  100 A of the photo diode  101 A (i.e., the input voltage V in  of the amplifier circuit  110 ) is scarcely changed. Thus, the photoreceiver circuit according to the first embodiment of FIG. 4 operates at extremely high speed, which enables the quick response of the photoreceiver circuit to the intensity change of the incident light PH. 
     SECOND EMBODIMENT 
     A photoreceiver circuit according to a second embodiment of the present invention is explained below with reference to FIGS. 5 and 6. 
     FIG. 5 shows the basic configuration of the photoreceiver circuit according to the second embodiment, in which the value of the gain in the changing state where the intensity of the incident light PH varies is different from that in the steady state where the intensity of the light PH is constant. 
     The photoreceiver circuit according to the second embodiment of FIG. 5 has the same configuration as that of the first embodiment of FIG. 3 other than that a current-leaking means  207  and two capacitors  205  and  206  are additionally provided between the output terminal  100 B of the photoreceiver circuit and the input terminal  104   a  of the analog multiplier circuit  104 . Therefore, the explanation about the same configuration is omitted here by attaching the same reference characters as those in the first embodiment of FIG. 3 for the sake of simplification of description. 
     The capacitor  205 , which has a capacitance C 1 , is connected to the output terminal  100 B of the photoreceiver circuit and the input terminal  104   a  of the multiplier circuit  104 . The capacitor  206 , which has a capacitance C 2 , is connected to the input terminal  104   a  and the ground. The two capacitors  205  and  206  constitute a voltage-lowering means  210  for lowering the output voltage V out  of the photoreceiver circuit to a specific voltage, producing a lowered (or divided) output voltage V out ′. This voltage V out ′ is applied to the input terminal  105   a.    
     The current-leaking means  207  has the following operation When a potential difference or voltage is generated between the output terminal  100 B and the input terminal  104   a , the current leaking means  207  allows a small current to gradually flow (i.e., leak) from the output terminal  100 B to the input terminal  104   a  and vice versa. Thus, the potential difference or voltage between the terminals  100 B and  104   a  disappears, resulting in V out ′=V out , after a specific relaxation time has passed. 
     FIG. 6 shows a detailed configuration of the photoreceiver circuit according to the second embodiment of FIG. 5, in which the multiplier circuit  104  has a same configuration as that of the well-known Gilbert multiplier circuit, a photo diode  101 A is used as the photoelectric conversion element  101 , and a p-channel MOSFET  207 A is used as the current-leaking means  207 . 
     The gate and the drain of the MOSFET  201 A are coupled together to be connected to the output terminal  100 B of the photoreceiver circuit. The source of the MOSFET  207 A, which is connected to the substrate, is connected to the input terminal  104   a  of the multiplier circuit  104 . The capacitor  205  is connected in parallel to the MOSFET  207 A. 
     Next, the circuit operation of the photoreceiver circuit according to the second embodiment shown in FIG. 6 is explained below. 
     When the incident light PH applied to the photodiode  101 A has a constant intensity with time, i.e., the photoreceiver circuit is in the steady state, the electric potentials at the output terminal  100 B and the input terminal  104   a  are kept equal to each other (i.e., V out ′=V out ) because of the operation of the current-leaking means  207  (i.e., the MOSFET  207 A). Therefore, because of the same reason as that shown in the first embodiment, the value of the gain of the photoreceiver circuit can be changed by adjusting the value of the adjusting voltage V m (=V m1 −V 2 ) even if the intensity of the incident light PH is not changed. Thus, the gain of the photoreceiver circuit according to the second embodiment can be adjusted readily and optionally from the outside of the photoreceiver circuit. 
     When the incident light PH applied to the photodiode  101 A varies with time, i.e., the photoreceiver circuit is in the changing state, the current I PH  of the photodiode  101 A varies according to the intensity change of the light PR, thereby causing change of the voltage V a  at the output terminal  100 A. When a small change of the voltage V a  occurs at the terminal  100 A, it is amplified by the voltage amplifier circuit  110  to produce a large change in the output voltage V out . The large change in the output voltage V out  is sent to the input terminal  100   a  of the multiplier circuit  104  through the voltage-lowering means  210  comprising the capacitors  205  and  206 . 
     The output voltage V out  is lowered by the voltage-lowering means  210  to she lowered voltage V out ′. Here, the output voltage V out  is divided by the capacitors  205  and  206  to produce the lowered voltage V out ′ given as V out ′=[C 1 /(C 1  +C 2 )] V out . This is because the current-leaking operation of the current-leaking means  207  is carried out very slowly. Thus, to equalize the output current I o  of the multiplier circuit  104  with the outout current I PH  of the photodiode  101 A, the output voltage V out  of the amplifier circuit  110  in the changing state needs to have a value higher than that necessitating in the steady state. This means that the gain of the amplifier circuit  110  or the photoreceiver circuit in the changing state has a value greater than that in the steady state. 
     When the intensity change of the incident light PH ceases after a specific time period (i.e., the relaxation time) has passed, the voltage V out ′ at the input terminal  104   a  becomes equal to the voltage V out  at the output terminal  100 B because of the current-leaking operation of the current-leaking means  207 . In other words, the lowered output voltage V out ′ becomes equal to the original output voltage V out , i.e., V out ′=V out . 
     As a result, the value of the gain in the changing state is [(C 1 +C 2 )/C 1 ] times as much as that in the steady state. This may be explained in the following way. 
     In the changing state, the feedback path of the amplifier circuit  110  is formed by the capacitors  205  and  206  of the voltage-lowering means  210  and the multiplier circuit  104 . Therefore, the divided voltage V out ′=[C 1 /(C 1 +C 2 )] V out  is applied to the multiplier circuit  104 . On the other hand, in the steady state, the feedback path of the amplifier circuit  110  is formed by the MOSFET  207 A of the current-leaking means  207  and the multiplier circuit  104 . Therefore, the original output voltage V out  is directly applied to the multiplier circuit  104 . 
     Moreover, because of the existence of the feedback loop comprising the analog multiplier circuit  104 , similar to the prior-art photoreceiver circuit shown in FIG. 1, the voltage V a  at the output terminal  100 A of the photodiode  101 A (i.e., the input voltage V in  of the amplifier circuit  110 ) is scarcely changed. Thus, the photoreceiver circuit according to the second embodiment of FIG. 6 operates at extremely high speed, in other words, it is capable of quick response to the intensity change of the incident light PH. 
     In the above-described second embodiment, the current-leaking means  207  is formed by the p-channel MOSFET  207  and the voltage-lowering means  210  is formed by the capacitors  205  and  206 . However, the current-leaking means  207  maybe formed by any other device/devices or a circuit if it has the current-leaking function described above. Also, the voltage-lowering means  210  may be formed by any other device/devices or a circuit if it has the voltage-lowering function described above. 
     Additionally, in the above-described first and second embodiments, the analog multiplier circuit  104  has a same configuration as that of the well-known Gilbert multiplier circuit. However, any other configuration may be applied to the multiplier circuit  104  if it has a multiplication function of two voltage inputs and produces a current output. 
     While the preferred forms of the present invention have been described, it is to be understood that modifications will be apparent to those skilled in the art without departing from the spirit of the invention. The scope of the invention, therefore, is to be determined solely by the following claims.