Abstract:
An optical signal receiving circuit has a current-voltage converting circuit which receives the output current signal of a photoelectric converting circuit, converting an optical signal into the current signal, and converts the current signal into a voltage signal. A differential circuit in the subsequent stage to the current-voltage converting circuit uses a resistor as its current source to facilitate setting of an operating voltage level in the circuit. To eliminate an adverse effect of asymmetry of the output waveform from the differential circuit due to the use of the resistor, the reference voltage level as the other input to the reference circuit is generated from the output voltage signal of the current-voltage converting circuit by a voltage generating circuit incorporating a feed-forward-controlling connection. Thus, coexistence of high bandwidth characteristics and broad dynamic range having so far been difficult to attain by low voltage apparatus can be realized.

Description:
CLAIM OF PRIORITY 
   The present application claims priority from Japanese application JP 2006-061996 filed on Mar. 8, 2006, the content of which is hereby incorporated by reference into this application. 
   FIELD OF THE INVENTION 
   The present invention relates to an optical signal receiving circuit for converting an optical signal into differential voltage signals and, more particularly, relates to an optical signal receiving circuit operable with a single low-voltage power supply of 3.3 V or the like. 
   BACKGROUND OF THE INVENTION 
   With recent development of widespread use of optical communications, demands are becoming strong for higher-speed transmitting and receiving circuits. Also, small low-voltage and low-power-consumption circuits are highly demanded from the point of view of cost reduction and energy saving. 
   In optical signal receiving circuits and optical signal receiving apparatuses, such an amplifier is generally needed that performs photoelectric conversion of the optical signal by the use of a photoelectric converter, such as a photodiode, amplifies the weak current signal output therefrom, and converts the amplified signal into an electric signal, especially differential voltage signals. 
   Accordingly, operability with a single low-voltage power supply has come to be demanded to such an amplifier in view of cost reduction and energy saving. 
   In JP09-232877A is disclosed a preamplifier for optical communication operable with a single low-voltage power supply. Especially in FIG. 1 of JP09-232877A, there is shown an example in which the source and the drain of a field-effect transistor (FET) are connected to each end of a feedback resistor of a current-voltage converting circuit employing a trans-impedance amplifier (TIA), and thus the output voltage amplitude of the current-voltage converting circuit is controlled. 
   In the data sheet of the product ADN2821, Analog Devices, Inc., is described a trans-impedance amplifier for optical communication.  FIG. 1  in the data sheet, in particular, describes a feedback connection from the output of the differential amplifier to the input of the differential amplifier. 
   SUMMARY OF THE INVENTION 
   Prior to submission of this application, we made a consideration about a technology of semiconductor circuit operable with a single low-voltage power supply of 3.3 V or the like for converting an optical signal into differential voltage signals. In order to support high-speed communication employing an optical signal, operability in the range of bandwidth of the signal frequency is required to the current-voltage converting circuit, and in order to support long distance transmission, sensitivity to receive an attenuated, weak signal is required also.  FIG. 3  shows a circuit configuration that was considered by the inventors and  FIG. 4  shows a circuit example of its partial block. 
   As the current-voltage converting circuit, a trans-impedance amplifier  10  is used here. 
   Referring to the circuit of  FIG. 4 , an increase in the resistance value of the feedback resistor  11  has a beneficial effect on improvement in the noise characteristic determining the minimum input sensitivity. 
   However, the output voltage amplitude of the trans-impedance amplifier  10  is given by input current amplitude multiplied resistance value of feedback resistor  11 , and therefore, if the value of the feedback resistor  11  is increased, the output voltage amplitude becomes large, and this affects the operating limit of the circuit when a large signal is input thereto (this operating limit may hereinafter be called “overload limit.”) Accordingly, in order to keep a dynamic range of the circuit to satisfy the conditions for the demanded minimum input sensitivity and the overload limit, an issue arises how to suppress the effect of an increase in the value of the feedback resistor  11  on the overload limit. 
   To expand the range under the overload limit, the setting of the output voltage level of the trans-impedance amplifier  10  has a great significance. The amount of electric current flowing through the Tr.  13  is determined by the resistance value of the resistor  14 , the power supply voltage, and the voltage level at the base of Tr.  15 . It is essential for the operation of the trans-impedance amplifier that there is a flow of the current of a predetermined value or above. Therefore, the voltage level at the base of Tr.  15  should be set to be lower than the power supply voltage. The output voltage level of the trans-impedance amplifier  10  is equal to the voltage level at the base of the Tr.  15  less the base-emitter voltage (hereinafter referred to as VBE) of the Tr.  15 . In the case where a large signal is input, since it is needed to satisfy the condition with the voltage signal, the average voltage level of the output of the trans-impedance amplifier  10  need to be further reduced by ½, or above, of the voltage signal amplitude. Generally, the VBE of a transistor varies according to temperature change, and it becomes 1 V or so at low temperature. If a case is considered where the power supply voltage VCC is 3 V, and the output voltage amplitude of the trans-impedance amplifier  10  becomes 1.2 V when a large signal is input, the output average voltage level of the trans-impedance amplifier  10  needs to be set to
 
3 −VBE− 1.2÷2=3−1−0.6=1.4 [V]
 
or below.
 
   Meanwhile, in a differential circuit  20  at the subsequent stage, reference voltage level which is same level as the output average voltage level of the trans-impedance amplifier  10 , is applied to the transistor  22  as its base voltage level. Therefore, the minimum voltage applied to the current source  2  is determined by the output average voltage level of the trans-impedance amplifier  10  less the VBE of the Tr.  22 :
 
1.4 −VBE= 1.4−1=0.4 [V].
 
When a constant current source employing a transistor is used as the current source, it becomes impossible to keep a bias voltage necessary for operating the transistor at high speed and keep the operating bandwidth range of the circuit and, as a consequence, the overload limit becomes low.
 
   From the above, while the setting of the resistance value of the feedback resistor  11  need to be made on the basis of tradeoff between the noise characteristic and the overload limit, in the case of the single low-voltage power supply circuit that was considered prior to the present application, it was found difficult to set the resistance value of the feedback resistor  11  keeping necessary dynamic range. 
   A circuit with an AGC (Automatic Gain Control) circuit applied to a current-voltage converting circuit for keeping the dynamic range is shown in FIG. 1 in JP09-232877A. When this circuit is used, however, deterioration of bandwidth occurs due to parasitic capacitance of the field-effect transistor added to make the feedback resistance variable. Therefore, a tradeoff arises between acquirement of a dynamic range and the bandwidth characteristic. Hence, in the optical signal receiving circuit considered prior to the present application, it was found difficult to make them coexist. 
   One of typical examples of the optical signal receiving circuit of the invention will be disclosed as follows: The circuit has a current-voltage converting circuit receiving the current signal output from a photoelectric converting circuit, which converts an optical signal into the current signal, for converting the current signal into a voltage signal, a differential circuit provided in the stage subsequent to the current-voltage converting circuit employs a resistor as its current source, and a reference voltage level to be supplied to the other input of the differential circuit is generated from the voltage signal output from the current-voltage converting circuit. 
   By use of a resistor as the current source in the differential circuit provided in the stage subsequent to the current-voltage converting circuit, it is made possible to lower the output voltage level of the current-voltage converting circuit, and thus realization of required dynamic range can be facilitated and the adverse effect on the bandwidth characteristic can be reduced. 
   More concretely, the optical signal receiving circuit of the invention is an optical signal receiving circuit receiving an electric signal generated by photoelectric conversion of an optical signal, converting the electric signal into a differential voltage signal, and outputting the differential voltage signal. The optical signal receiving circuit is configured to be operable with a single voltage power supply and adapted to generate a differential voltage signal by single-differential conversion, through a feed-forward connection, of a common voltage signal generated by current-voltage conversion of the aforesaid current signal. 
   Further, the optical signal receiving circuit of the present invention includes a current-voltage converting circuit receiving the current signal output from a photoelectric converting circuit receiving an optical signal and converting the optical signal into the current signal, as an input thereto and converting the current signal into a voltage signal, a voltage generating circuit receiving the voltage signal output from the current-voltage converting circuit as an input thereto and generating a DC voltage from the voltage signal, and a first differential circuit receiving a first voltage signal output from the current-voltage converting circuit and a second voltage signal output from the voltage generating circuit as input thereto and generating a differential voltage signal from the first and second voltage signals. The first differential circuit includes at least a pair of differential transistors and a current source commonly connected to the pair of transistors and uses a resistor, not a transistor, as the current source. 
   An optical signal receiving apparatus of the invention includes a photoelectric converting circuit receiving an optical signal and converting the signal into a current signal and an optical signal receiving circuit for generating a reference voltage signal from the current signal output from the photoelectric converting circuit and outputting the signal. The optical signal receiving circuit includes a current-voltage converting circuit receiving the current signal output from the photoelectric converting circuit as an input thereto and converting the current signal into a voltage signal, a voltage generating circuit receiving the voltage signal output from the current-voltage converting circuit as an input thereto and generating a DC voltage from the voltage signal, and a first differential circuit receiving a first voltage signal output from the current-voltage converting circuit and a second voltage signal output from the voltage generating circuit as inputs thereto and generating a differential voltage signal from the first and second voltage signals. The first differential circuit includes at least a pair of differential transistors and a current source commonly connected to the pair of transistors and uses a resistor, not a transistor, as the current source. 
   A typical advantage obtained from the above means is that an optical signal receiving circuit and apparatus adapted operable with a single low voltage power supply can be realized. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a drawing showing an overview of the entire configuration of an optical signal receiving circuit  100  and an optical signal receiving apparatus including the same; 
       FIG. 1B  is a circuit diagram showing an example of the differential circuit  50  in  FIG. 1A ; 
       FIG. 1C  is a circuit diagram showing an example of the voltage generating circuit  60  in  FIG. 1A ; 
       FIG. 2  is a configuration diagram of an optical signal receiving circuit to which the invention is applied; 
       FIG. 3  is a configuration diagram showing an optical signal receiving circuit considered by the inventors; 
       FIG. 4  is a circuit diagram showing an example of the current-voltage converting circuit  10  and the differential circuit  20  in  FIG. 3 ; 
       FIG. 5  is a circuit diagram showing an example of the current-voltage converting circuit  10  and the differential circuit  20  in  FIGS. 1 and 2 ; 
       FIG. 6  is a circuit diagram of a differential circuit  20  to which the invention is applied; 
       FIG. 7  is an example of input waveform to the differential circuit of  FIG. 6 ; 
       FIG. 8  shows an output waveform responsive to the input waveform of  FIG. 7  input to the differential circuit of  FIG. 6 ; 
       FIG. 9  is an example of input waveform to the differential circuit of  FIG. 6 ; 
       FIG. 10  shows an output waveform responsive to the input waveform of  FIG. 9  input to the differential circuit of  FIG. 6 ; 
       FIG. 11A  is an input waveform to the differential circuit  40 ; 
       FIG. 11B  shows an output waveform, before waveform shaping, responsive to the input waveform of  FIG. 11A  input to the differential circuit  40 ; and 
       FIG. 11C  shows an output waveform of the output waveform of  FIG. 11B  after waveform shaping through the differential circuit  40 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The embodiments of the present invention will be described in detail with reference to the accompanying drawings. The circuit elements constituting each block of the embodiments may be formed by known integrated circuit technology, though not limited thereto, on a single semiconductor substrate of such a material as single crystal silicon or, in the case of a part high-frequency characteristics thereof are not required, may be separately provided as an external part. While, in some embodiments, a circuit using an NPN-bipolar transistor is shown, this is not limitative but the present invention may be applicable to circuits employing common semiconductor devices. Each of the devices shown may also be realized to exhibit the characteristics required thereof by parallel or series connections of the same devices or the same kind of devices. 
   First Embodiment 
     FIG. 1  shows a first embodiment of the optical signal receiving circuit to which the invention is applied. The optical signal receiving circuit  100  of the present embodiment includes a current-voltage converting circuit  10  receiving the current signal output from a photoelectric converting circuit PD, photodiode, receiving an optical signal and converting the signal into the current signal, as an input signal thereto and converting the current signal into a voltage signal, a voltage generating circuit  60  receiving the voltage signal output from the current-voltage converting circuit  10  as an input thereto and generating a DC voltage from the voltage signal, and a first differential circuit  50  receiving a first voltage signal output from the current-voltage converting circuit  10  and a second voltage signal output from the voltage generating circuit  60  as inputs thereto and generating a differential voltage signal from the first and second voltage signals. The first differential circuit  50  includes one pair of differential transistors  21 ,  22  and a current source  3  commonly connected to the pair of transistors  21 ,  22 , in which a resistor is used as the current source  3 . 
   Further, the optical signal receiving apparatus of the present embodiment includes a photoelectric converting circuit PD receiving an optical signal and converting the signal into a current signal and the optical signal receiving circuit  100  generating a differential voltage signal from the current signal output from the photoelectric converting circuit PD and outputting the signal. 
   To be more concrete, the receiving circuit of the present embodiment is configured such that a trans-impedance amplifier  10  as the current-voltage converting circuit is connected to the output of a photodiode (PD) as the photoelectric converting circuit, the output of the trans-impedance amplifier  10  is connected to the voltage generating circuit  60  and to one input of the differential circuit  50 , and the output of the voltage generating circuit  60  is connected to the other input of the differential circuit  50 . Another differential circuit  40  may further be provided such that the output of the differential circuit  50  is connected to the differential circuit  40 . Here, the voltage generating circuit  60  may be configured of a low-pass filter (LPF) employing, for example, a resistor and a capacitor as shown in  FIG. 1C . Further, the differential circuit  50  may be configured, for example, as shown in  FIG. 1B , in which the input signals are connected to a pair of transistors Tr.  21 , Tr.  22 , the transistors are respectively connected to collector resistors  25 ,  26  and emitter resistors  23 ,  24 , and the emitter resistors  23 ,  24  are commonly connected to the current source  3  determining the current flowing through the differential circuit  50 . This configuration differs from  FIG. 3  in that the output of the trans-impedance amplifier  10  is used as the input to the voltage generating circuit  60  and, further, the circuit configuration differs from  FIG. 4  in that the current source  3  is configured of a resistor. In other words, the current source  3  is configured in such a way that a current passing through the current source  3  varies according to the voltage signal input to INT. 
   Operation of the first embodiment will be described below. 
   The optical signal receiving circuit  100  receives a current signal, generated based on an input optical signal by photoelectric conversion, at its terminal IN, converts the current signal into a differential voltage signal through the trans-impedance amplifier  10  and differential circuit  50 , and outputs the signal. The optical signal receiving circuit  100  is configured to be operable with a single voltage power supply. Especially, the trans-impedance amplifier  10  and the differential circuit  50  are supplied with the same power supply voltage VCC (see  FIG. 5 .). Furthermore, the optical signal receiving circuit  100  is configured to be adapted to generate a differential voltage signal by differential conversion through a feed-forward connection of a common voltage signal that is generated from the voltage signal made by current-voltage conversion of the current signal. 
   A more concrete description will be given as follows. An optical signal input to the PD is converted by the PD into an electric signal and the electric signal is converted by the trans-impedance amplifier  10  into a voltage signal. While this voltage signal is input to the differential circuit  50 , it is also input to the voltage generating circuit  60  so that an average voltage level is generated from the DC voltage component of the input voltage signal. The average voltage level is input to the differential circuit  50  as a reference voltage level and thus the differential voltage signal is generated in the differential circuit  50 . In the case where the differential circuit  40  is additionally provided, the differential voltage signal output from the differential circuit  50  is input to the differential circuit  40  so that the differential voltage waveform is wave-shaped or amplified. 
     FIG. 6  shows an example of a differential circuit using a resistor as the current source. In other words, the current source  3  has no transistors. In this case, INT receives the voltage signal output from the current-voltage converting circuit in the preceding stage, while INB receives the reference voltage level for converting a single phase signal into a differential signal. 
     FIG. 7  shows the input waveforms occurring at this time and the voltage waveform at the node A. Since the input to INB has a constant voltage level, the waveform at the node A is determined, when INT is at “H” level, on the basis of the “H” level, whereas the same is determined, when INT is at “L” level, on the basis of the reference voltage level at INB. Accordingly, the amount of current generated by the current source  3  differs on different occasions. Therefore, the amount of current flowing through the resistor  25  and the resistor  26  determining the “L” level in the differential output differs, thereby causing a phenomenon in which the “L” levels output to OUTT and to OUTB differ. 
     FIG. 8  shows the output waveform occurring at this time at OUTT, OUTB. When the waveform becomes asymmetric, like in this case, the method of generating the reference voltage level causes an issue. For example, when a method of generating a reference voltage level from the output of a differential circuit for use as the input to the same differential circuit, as shown in  FIG. 1  in the data sheet of the product ADN2821, Analog Devices, Inc., as a conventional art example different from this embodiment, the reference voltage level is generated so as to satisfy
 ∫( V out t−V out b ) dt= 0. 
Thus, a phenomenon occurs in which the generated reference voltage level is mismatched from the center of the amplitude that is input to INT.
 
     FIG. 9  shows the input waveform at this time; thus, the voltage waveform output from the differential circuit  50  becomes further distorted. 
     FIG. 10  shows the output waveform at this time. A time domain distortion is generated in the differential output signal due to -the mismatch of the reference voltage level from the center of the amplitude and, thereby, a phenomenon occurs in which the duty of the output waveform(the ratio between “H” level time and “L” level time in an alternating signal) deviates from 50%. Thus, if these output waveforms are used as the inputs, there is possibility of trouble occurring in the operation in the subsequent circuits. 
   The problem of “duty” is caused by the generation of the reference voltage level from the output of the circuit subsequent to the differential circuit outputting asymmetric waveform. The problem may be dissolved by generating the reference voltage level by using, as in the present embodiment, the output of the current-voltage converting circuit incorporating a feed-forward connection. 
     FIG. 11A  shows the waveform, when the differential circuit  40  is provided, of the input to the differential circuit  40 .  FIG. 11B  shows the output waveform obtained by amplifying the input waveform through the differential circuit  40 . By setting small (appropriately) the limiting voltage amplitude of the differential circuit  40  to be applied to the above referred amplified output amplitude, waveform shaping to eliminate the asymmetry between the low and high levels of the pair of differential signals can be realized. 
     FIG. 11C  shows an output waveform obtained when the limiting voltage amplitude for the differential circuit  40  is set as shown in  FIG. 11B . 
   By providing the differential circuit  40  in this way, the asymmetry of the waveform in the amplitude can be eliminated. 
   By using the method to generate the reference voltage level by the use of the output of the current-voltage converting circuit incorporating the above referred feed-forward connection and using the method of suitably setting the limiting voltage amplitude for the differential circuit  40 , it becomes possible to generate an output waveform free from distortion even when a differential circuit, which employs a resistor as the current source as shown in  FIG. 6 , is used as the differential circuit  50 . 
   According to the present embodiment, it becomes possible to set the bias voltage needed for the current source of the differential circuit  50  to a low value and to lower the input voltage to the differential circuit  50 . Thereby, the adjustment range of the output voltage of the trans-impedance amplifier  10  can be enlarged and, by attaining the optimum setting, the overload limit can be enlarged. Further, without limiting the output amplitude in the trans-impedance amplifier  10 , the output can be input to the differential circuit  50  in the subsequent stage. Thus, adverse effects on the noise characteristic and frequency bandwidth characteristic can be eliminated. 
   Second Embodiment 
     FIG. 2  shows a second embodiment of the optical signal receiving circuit to which the invention is applied. It differs from the first embodiment in that the voltage generating circuit  60  is configured of a feedback loop using an error amplifier. The voltage generating circuit  60  includes a resistor  81  and a capacitor  83  constituting a low-pass filter, a differential circuit  80 , an input-voltage-offset compensating resistor  82  of the differential circuit  80 , and an error amplifier  70 . 
   The voltage signal input to the voltage generating circuit  60  is turned into an average voltage level by extraction of its DC component through the low-pass filter configured of the resistor  81  and the capacitor  83 , and the signal is input to one input terminal of the differential circuit  80 . At this time, since the amplitude is limited by the low-pass filter and, in addition, the feedback loop does not need wide bandwidth, the differential circuit  80  does not need to use a resistor for its current source as the differential circuit  50  in the main path, but it may be provided by a common constant current source employing a transistor. The output of the differential circuit  80  is connected to the input of the error amplifier  70 . The output of the error amplifier is connected to the input of the differential circuit  50  as the reference voltage level and, at the same time, it is connected to the other input of the differential circuit  80  through the resistor  82  and thus the feedback loop is constituted. Here, by using a resistor ideally of the same resistance value as the resistor  81  for the resistor  82 , it becomes possible to equalize the offset voltage caused by the base currents at the input terminals occurring in the case where a bipolar transistor circuit is used for the differential circuit  80 . 
   According to this embodiment, it is made possible to increase the value of the resistor  81  constituting the low-pass filter and to decrease the capacitance of the capacitor  83 ; accordingly, it is made easy to form the capacitor  83  on the same semiconductor substrate as other circuits. Further, by having a small capacitor element incorporated in the error amplifier  70  and decreasing the bandwidth of the feedback loop, it is made easy to configure the entire circuit on the same semiconductor substrate.