Abstract:
A capacitive load driving device applies a multi-level voltage to a capacitive load to drive the capacitive load. In the capacitive load driving device, a voltage control signal generator unit generates a voltage control signal. A voltage amplifier unit amplifies a voltage of the voltage control signal. A current amplifier unit amplifies a current of an output of the voltage amplifier unit to perform charging of the capacitive load. A falling control signal generator unit generates a falling pulse having a predetermined pulse width when a width of falling of the voltage control signal exceeds a predetermined value. A switching unit performs discharging of the capacitive load in response to the falling pulse received.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is based upon and claims the benefit of priority of Japanese patent application No. 2006-281536, filed on Oct. 16, 2006, the entire contents of which are herein incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention generally relates to a capacitive load driving device, and more particularly to a capacitive load driving device which applies a multi-level voltage to a capacitive load to drive the capacitive load. 
   2. Description of the Related Art 
   In an optical matrix switch, a multi-level high voltage is applied to an electro-optic effect device, and a refractive index of the electro-optic effect device is varied according to the applied voltage so that optical switching is carried out. The rise time and the fall time of the high-voltage pulse signal are set up to control the driving cycle of the optical matrix switch. High-speed switching is demanded for the optical matrix switch. 
   The electro-optic effect device is provided with the electrodes on both sides of the crystal. The electro-optic effect device is electrically regarded as a capacitor (or capacitive load), and a high voltage on the order of several hundred volts is applied between the electrodes. 
     FIG. 1  shows the composition of a conventional capacitive load driving device. As shown in  FIG. 1 , a control signal generator unit  1  which is composed of an ASIC (application-specific integrated circuit), such as FPGA (field programmable gate array), outputs a digital voltage control signal. After the digital voltage control signal output from the control signal generator unit  1  is converted into an analog signal by a D/A converter (DAC)  2 , the analog signal is supplied to a voltage amplifier  3 , and the voltage of the analog signal is amplified by the voltage amplifier  3 . The amplified voltage signal is supplied to one end of a capacitive load  4 . The other end of the capacitive load  4  is grounded. 
   Japanese Laid-Open Patent Application No. 2005-169737 discloses a capacitive load driving device as shown in  FIG. 2 . In the conventional capacitive load driving device of  FIG. 2 , a drive waveform signal output from a controller  5  is converted into an analog signal by a D/A converter  6 , and the voltage of the analog signal output from the D/A converter  6  is amplified by a voltage amplifier circuit  7 . The current of the amplified voltage signal output from the voltage amplifier circuit  7  is amplified by a current amplifier circuit  8 . The amplified current signal output from the current amplifier circuit  8  is supplied to a piezoelectric element  9  which is a capacitive load. 
   Japanese Laid-Open Patent Application No. 47-037057 discloses a capacitive load driving device wherein a first current switch and a second current switch are connected in series via a pair of diodes, and a capacitive load is connected to the middle point of the pair of diodes. In this capacitive load driving device, the potential of the junction point of the second current switch and the pair of diodes is raised beforehand when charging the capacitive load. And when discharging the capacitive load, the potential of the junction point of the first current switch and the pair of diodes is lowered beforehand. 
   Japanese Laid-Open Patent Application No. 04-260089 discloses a capacitive load driving device which is adapted to quickly perform charging and discharging of a capacitive load by changing the voltage between the terminals of the capacitive load with a first current value, and thereafter driving the capacitive load with a second current value larger than the first current value. 
   In the case of the conventional circuit of  FIG. 1 , it is necessary that the output impedance when the voltage of the voltage amplifier  3  is varied at high speed is about 10 kΩ, in order to apply a multi-level voltage ranging from 0V to 100V to the capacitive load  4  having an electrostatic capacitance of some nanofarads (nF) at high speed. The time constant which is equal to a product of the output impedance of the voltage amplifier  3  and the capacitance of the capacitive load  4  is on the order of several ten microseconds. For this reason, there is a difficulty in performing the variable control of the voltage applied to the capacitive load  4  at a very high speed on the microsecond order. 
   In the case of the conventional circuit of  FIG. 2 , the current of the amplified voltage signal output from the voltage amplifier circuit  7  is amplified by the current amplifier circuit  8 , and the amplified current signal output from the current amplifier circuit  8  is supplied to the capacitive load (piezoelectric element)  9 . Thus, charging of the capacitive load at high speed is possible. 
   However, in order to vary the multi-level voltage applied to the electro-optic effect device at high speed (the applied multi-level voltage ranging between 0V and 400V), discharging of the capacitive load must be performed at high speed in accordance with the falling edges of the applied voltage. 
   In the case of the conventional circuit of  FIG. 2 , the change of the applied voltage is limited to one pattern. However, in a case in which there are many patterns including a pattern for changing the applied voltage from 400V to 0V, a pattern for changing the applied voltage from 400V to 380V, and so on, it is difficult to control the discharging of the capacitive load in accordance with the changes of the applied voltage in a wider range at high speed. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the invention, there is provided a novel and useful capacitive load driving device in which the above-described problems are eliminated. 
   According to one aspect of the invention, there is provided a capacitive load driving device which is adapted to control the discharging of the capacitive load in accordance with the changes of the applied voltage in a wide range, thereby carrying out the variable control of the multi-level voltage applied to the capacitive load at high speed. 
   In an embodiment of the invention which solves or reduces one or more of the above-mentioned problems, there is provided a capacitive load driving device which applies a multi-level voltage to a capacitive load to drive the capacitive load, the capacitive load driving device comprising: a voltage control signal generator unit generating a voltage control signal; a voltage amplifier unit amplifying a voltage of the voltage control signal; a current amplifier unit amplifying a current of an output of the voltage amplifier unit to perform charging of the capacitive load; a falling control signal generator unit generating a falling pulse with a predetermined pulse width when a width of falling of the voltage control signal exceeds a predetermined value; and a switching unit performing discharging of the capacitive load in response to the falling pulse received from the falling control signal generator unit. 
   The above-mentioned capacitive load driving device may be configured so that a positive thermistor is provided between the current amplifier unit and the capacitive load. 
   The above-mentioned capacitive load driving device may be configured so that the falling control signal generator unit comprises: a table unit which is provided so that a table is accessed with a digital value of a previously supplied voltage control signal and a digital value of a currently supplied voltage control signal, and a specific value is read from the table when a width of falling from the previously supplied voltage control signal value to the currently supplied voltage control signal value exceeds a predetermined value; and a pulse generator unit which generates a falling pulse when the specific value is read from the table. 
   The above-mentioned capacitive load driving device may be configured so that the falling control signal generator unit comprises: a differential unit which differentiates an analog voltage control signal to output a differential signal; a first transistor which is provided so that the first transistor is turned off when a negative polarity pulse of the differential signal is less than a predetermined value; and a second transistor which is turned on when the first transistor is turned off, to output a falling pulse. 
   The above-mentioned capacitive load driving device may be configured so that the differential unit comprises a time-constant adjusting unit adjusting a time constant. 
   The above-mentioned capacitive load driving device may be configured so that the capacitive load driving device further comprises a bias adjustment unit adjusting a bias of the first transistor. 
   The above-mentioned capacitive load driving device may be configured so that the capacitive load is an electro-optic effect device. 
   According to the embodiment of the invention, it is possible to control the discharging of the capacitive load in accordance with the changes of the applied voltage in a wide range, and the variable control of the multi-level voltage applied to the capacitive load can be carried out at high speed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings. 
       FIG. 1  is a diagram showing the composition of a conventional capacitive load driving device. 
       FIG. 2  is a diagram showing the composition of a conventional capacitive load driving device. 
       FIG. 3  is a diagram showing the composition of an optical matrix switch to which a capacitive load driving device in an embodiment of the invention is applied. 
       FIG. 4  is a diagram showing the composition of a capacitive load driving device in an embodiment of the invention. 
       FIG. 5  is a diagram showing the composition of a falling control signal generator circuit in an embodiment of the invention. 
       FIG. 6  is a diagram showing the waveform of the applied voltage of a capacitive load and the waveform of a falling control signal. 
       FIG. 7  is a diagram showing the composition of a capacitive load driving device in an embodiment of the invention. 
       FIG. 8  is a diagram showing the composition of a falling control signal generator circuit in an embodiment of the invention. 
       FIG. 9  is a diagram showing the composition of a modification of the falling control signal generator circuit. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A description will now be given of an embodiment of the invention with reference to the accompanying drawings. 
     FIG. 3  shows the composition of an optical matrix switch to which a capacitive load driving device in an embodiment of the invention is applied. 
   In the optical matrix switch shown in  FIG. 3 , the light signals of a plurality of channels inputted from an optical fiber array  11  are converted into parallel light beams by a waveguide lens array  12  respectively, and they are supplied to a deflection element array  13 . 
   Each of a plurality of deflection elements, which constitute the deflection element array  13 , comprises two electro-optic effect devices  14   a  and  14   b . For example, in each of the electro-optic effect devices  14   a  and  14   b , electrodes are provided on the front and back surfaces of a ceramic PLZT (PLZT is the abbreviation of lead lanthanum zirconate titanate, which is also known as lanthanum-doped lead zirconate-lead titanate). When a voltage is applied between the electrodes, the refractive index of the PLZT is varied according to the applied voltage, so that the path of the light beam is changed and optical switching is carried out. 
   The light signals of the respective channels deflected by the deflection element array  13  are transmitted through a slab waveguide  15  to a deflection element array  16 . The light signals are deflected by the electro-optic effect devices  14   a  and  14   b  contained in the deflection element array  16 , and the deflected light signals are supplied to a waveguide lens array  17 . The light signals are converted into parallel light beams by the waveguide lens array  17  respectively, and they are outputted from an optical fiber array  18 . 
     FIG. 4  shows the composition of a capacitive load driving device in an embodiment of the invention. 
   In the capacitive load driving device of  FIG. 4 , a digital voltage control signal output from a control signal generator unit  21  (which is composed of a FPGA) is converted into an analog signal by a D/A converter (DAC)  22 . A voltage of the analog voltage control signal is amplified by a voltage amplifier circuit  23 . The amplified voltage control signal is supplied through a reverse-flow preventing diode D 1  to a base of a pnp transistor Q 24 . This pnp transistor Q 24  constitutes a current amplifier circuit  24 . 
   The base of the pnp transistor Q 24  is grounded via a resistor R 1 . This resistor R 1  is provided for discharging the electric charge on the base of the pnp transistor Q 24  when it is turned OFF. 
   Simultaneously, the digital voltage control signal output from the control signal generator unit  21  is supplied to a falling control signal generator circuit  25 . 
     FIG. 5  shows the composition of a control signal generator circuit in an embodiment of the shown. 
   As shown in  FIG. 5 , the falling control signal generator circuit  25  comprises a register  31 , a ROM  32 , and a pulse generator circuit  33 . A previously supplied voltage control signal which is composed of a predetermined number of bits is stored in the register  31 . A currently supplied voltage control signal which is composed of the predetermined number of bits is supplied to the ROM  32 . Simultaneously, the previously supplied digital voltage control signal from the register  31  is also supplied to the ROM  32 . 
   In the ROM  32 , a table is stored in accordance with a difference between the values of the voltage control signals received. Specifically, when a difference Vd between the previously supplied voltage control signal value and the currently supplied voltage control signal value is less than a predetermined threshold value Vt (e.g., Vt=−50 V), the value 1 which is a specific value is stored in the table of the ROM  32  at a corresponding address (when Vd&lt;Vt). The difference Vd indicates a width of falling of the voltage control signal output from the control signal generator unit  21 . Otherwise, the value 0 is stored in the table of the ROM  32  (when Vd&gt;=Vt). 
   Accordingly, when a width of falling from the previously supplied voltage control signal value to the currently supplied voltage control signal value exceeds 50V (i.e., when a width of falling of the voltage control signal exceeds 50V), the specific value 1 is read from the ROM  32 , and the signal of the value 1 is supplied to the pulse generator circuit  33 . 
   If the signal of the value 1 from the ROM  32  is supplied to the pulse generator circuit  33 , the pulse generator circuit  33  is triggered so that it outputs a falling pulse (high-level pulse). For example, this falling pulse has a pulse width of 5 microseconds. The falling pulse output from the pulse generator circuit  33  is supplied to a base of a pnp transistor Q 26  which constitutes the switching circuit  26  shown in  FIG. 4 . 
   In the capacitive load driving device of  FIG. 4 , a high voltage (for example, +400V) from a high-voltage power supply is supplied to the collector of the transistor Q 24 , and the emitter of the transistor Q 24  is connected to the collector of the transistor Q 24  to the collector of the transistor Q 26  via a positive thermistor  27 . This positive thermistor  27  has positive temperature characteristics. 
   The positive thermistor  27  is set up so that the resistance of the positive thermistor  27  at normal temperature is very low. The positive thermistor  27  is provided in order to prevent the flowing of a large current between the transistor Q 24  and the transistor Q 26  when the transistors Q 24  and Q 26  are turned on simultaneously. 
   One end of the capacitive load  28  (namely, one electrode of the electro-optic effect device) is connected to the junction point of the positive thermistor  27  and the collector of the transistor Q 26 , and the other end of the capacitive load  28  (namely, the other electrode of the electro-optic effect device) is grounded. 
     FIG. 6  shows the waveform of the applied voltage of the capacitive load and the waveform of the falling control signal. 
   Suppose the case in which the applied voltage having the waveform shown in  FIG. 6(A)  is supplied to the capacitive load  28  from the current voltage having the waveform shown in  FIG. 6(A)  is supplied to the capacitive load  28  from the current amplifier circuit  24  in accordance with the voltage control signal. In this case, the falling control signal generator circuit  25  outputs the falling control signal having the waveform shown in  FIG. 6(B) , to the switching circuit  26 . 
   At the time instant t 2 , the applied voltage falls to 380V from 400V, and the width of falling (or the voltage difference) is less than 50V. Thus, no falling pulse is generated at this time. This is because the change of the applied voltage from 400V to 380V may be effected at high speed only by the operation of the transistor Q 24  sufficiently, and it is not necessary to turn on the transistor Q 26 . 
   On the other hand, at each of the time instants t 3 , t 5 , t 6  and t 8 , the width of falling of the applied voltage exceeds 50V. Thus, a falling pulse is generated at each time instant and the transistor Q 26  is turned on, so that the waveform of the applied voltage to the capacitive load  28  changes at sufficiently high speed. 
     FIG. 7  shows the composition of a capacitive load driving device in an embodiment of the invention. 
   In  FIG. 7 , the elements which are the same as corresponding elements in  FIG. 4  are designated by the same reference numerals, and a description thereof will be omitted. 
   In the capacitive load driving device of  FIG. 7 , the digital voltage control signal output from the control signal generator unit  21  (which is composed of a FPGA) is converted into an analog signal by the D/A converter (DAC)  22 . A voltage of the analog voltage control signal is amplified by the voltage amplifier circuit  23 . The amplified voltage control signal is supplied through the reverse-flow preventing diode D 1  to the base of the pnp transistor Q 24  which constitutes the current amplifier circuit  24 . 
   The base of the pnp transistor Q 24  is grounded via the resistor R 1  which is provided for discharging the electric charge on the base of the pnp transistor Q 24  when it is turned OFF. 
   Simultaneously, the analog voltage control signal from the DAC  22  is supplied to a falling control signal generator circuit  40  in this embodiment. 
     FIG. 8  shows the composition of a control signal generator circuit in an embodiment of the invention. 
   The falling control signal generator circuit  40  of this embodiment comprises a differential circuit  41 , a bias circuit  42 , and a pulse generator circuit  43 . 
   The differential circuit  41  includes a capacitor C 1 , a series connection circuit having a switch S 1  and a capacitor C 2  (connected in parallel with the capacitor C 1 ), a series connection circuit having a switch S 2  and a capacitor C 3  (connected in parallel with the capacitor C 1 ), and a series connection circuit having a switch S 3  and a capacitor C 4  (connected in parallel with the capacitor C 1 ). The differential circuit  41  generates a differential waveform of the analog voltage control signal supplied from the DAC  22 , and supplies it to the bias circuit  42 . 
   The capacitors C 2 -C 4  are provided for adjusting the capacitance of the capacitor C 1  finely. At the time of initialization, control signals are supplied to the switches S 1 -S 3  so that the switching ON/OFF of the switches S 1 -S 3  in the differential circuit  41  may be set up. 
   Alternatively, a variable capacitance device (for example, a variable capacitance diode) which varies its electrostatic capacitance according to the applied voltage may be used as the differential circuit  41 . 
   The bias circuit  42  includes resistors R 4  and R 5  which are connected in series between the power supply Vcc (voltage: +12V) and the ground, a series connection circuit having a switch S 4  and a resistor R 6  (which is connected in parallel with the resistor R 5  and connected in series with the switch S 4 ), a series connection circuit having a switch S 5  and a resistor R 7  (which is connected in parallel with the resistor R 5  and connected in series with the switch S 5 ), and a series connection circuit having a switch S 6  and a resistor R 8  (which is connected in parallel with the resistor R 5  and connected in series with the switch S 6 ). In the bias circuit  42 , a divided voltage of the power supply voltage is generated, and a bias supplied to the base of an npn transistor Q 31  in the pulse generator circuit  43  is determined. 
   The resistors R 6 -R 8  are provided for adjusting the resistance of the resistor R 5  finely. At the time of initialization, control signals are supplied to the switches S 4 -S 6  so that the switching ON/OFF of the switches S 4 -S 6  in the bias circuit  42  may be set up so as to change the divided voltage of the power supply voltage and adjust the bias (which is supplied to the base of the transistor Q 31 ) finely. 
   In the pulse generator circuit  43 , the collector of the transistor Q 31  is connected to the power supply Vcc via a resistor R 10 , and the emitter of the transistor Q 31  is grounded. The collector of the transistor Q 31  is also grounded via resistors R 11  and R 12  which are connected in series). The junction point of the resistors R 11  and R 12  is connected to the base of an npn transistor Q 32 . 
   In the pulse generator circuit  43 , the collector of the transistor Q 32  is connected to the power supply Vcc, and the emitter of the transistor Q 32  is grounded via a resistor R 13 . And the emitter of the transistor Q 32  is connected to the base of the transistor Q 26 . 
   The transistor Q 31  is turned OFF only when the differential waveform is turned into a negative polarity pulse at the time of falling of the voltage control signal so that the potential of the base of the transistor Q 31  falls. The higher the bias voltage is set up, the shorter the period for which the transistor Q 31  is OFF. Therefore, the bias supplied to the base of the transistor Q 31  is set up so that the period for which the transistor Q 31  is turned OFF by a negative polarity pulse of the differential waveform which appears when the width of falling from the previously supplied voltage control signal value to the currently supplied voltage control signal value exceeds 50V is set to 5 microseconds. 
   When the transistor Q 31  is turned OFF, the transistor Q 32  is turned ON so that a falling pulse (which is the falling control signal set to the high-level) is generated at the collector of the transistor Q 32 . This falling pulse is supplied to the base of the pnp transistor Q 26  which constitutes the switching circuit  26  in the capacitive load driving device of  FIG. 7 . 
     FIG. 9  shows the composition of a modification of the control signal generator circuit  40  in an embodiment of the invention. 
   In  FIG. 9 , the elements which are the same as corresponding elements in  FIG. 8  are designated by the same reference numerals, and a description thereof will be omitted. 
   As shown in  FIG. 9 , the control signal generator circuit  40  in this embodiment uses a pulse generator circuit  44  which is composed of a Schmitt trigger circuit, instead of the pulse generator circuit  43  shown in  FIG. 8 . 
   In the pulse generator circuit  44  of  FIG. 9 , the collector of the npn transistor Q 41  is connected to the power supply Vcc via a resistor R 20 , and the emitter of the transistor Q 41  is grounded. The collector of the transistor Q 41  is grounded via resistors R 21  and R 22  which are connected in series and the junction point of the resistors R 21  and R 22  is connected to the base of the npn transistor Q 42 . 
   The collector of the npn transistor Q 42  is connected to the power supply Vcc via a resistor R 23 , and the emitter of the transistor Q 42  is grounded. The collector of the transistor Q 42  is grounded via resistors R 24  and R 25  which are connected in series, and the junction point of the resistors R 24  and R 25  is connected to the base of the npn transistor Q 43 . 
   The collector of the transistor Q 43  is connects to the power supply Vcc via a resistor R 26 , and the collector of the transistor Q 43  is connected to the base of the transistor Q 26 , and the emitter of the transistor Q 43  is grounded. 
   The transistors Q 41  and Q 42  have the emitters both grounded in common, and the transistors Q 41  and Q 42  constitute the Schmitt trigger circuit. Thereby, it is possible for the capacitive load driving device of this embodiment to realize steep rising and steep falling of the output trigger signal. 
   The transistor Q 41  is turned OFF only when the differential waveform is turned into a negative polarity pulse at the time of falling of the voltage control signal so that the potential of the base of the transistor Q 41 . The higher the bias voltage is set up, the shorter the period for which the transistor Q 41  is OFF. Therefore, the bias supplied to the base of the transistor Q 41  is set up so that the period for which the transistor Q 41  is turned OFF by a negative polarity pulse of the differential waveform which appears when the width of falling from the previously supplied voltage control signal value to the currently supplied voltage control signal value exceeds 50V is set to 5 microseconds. 
   When the transistor Q 41  is turned OFF, the transistor Q 42  is turned ON. And when the transistor Q 42  is turned ON, the transistor Q 43  is turned OFF so that a falling pulse (which is the falling control signal set to the high level) is generated at the collector of the transistor Q 43  by the switching OFF of the transistor Q 43 . This falling pulse is supplied to the base of the pnp transistor Q 26  which constitutes the switching circuit  26  in the capacitive load driving device shown in  FIG. 7 . 
   In the capacitive load driving device of  FIG. 7 , +400V from the high voltage power supply is supplied to the collector of the transistor Q 24 , and the emitter of the transistor Q 24  is connected to the collector of the transistor Q 26  via the positive thermistor  27  which has the positive temperature characteristics. 
   The positive thermistor  27  is set up such that the resistance of the positive thermistor  27  in normal temperature is very low. The positive thermistor  27  is provided in order to prevent a large amount of current from flowing through the transistors Q 24  and Q 25  when the transistors Q 24  and Q 25  are turned ON simultaneously. 
   One end of the capacitive load  28  (or, one electrode of the electro-optic effect device) is connected to the junction point of the positive thermistor  27  and the collector of the transistor Q 26 , and the other end of the capacitive load  28  (that is, the other electrode of the electro-optic effect device) is grounded. 
   When the voltage having the waveform as shown in  FIG. 6(A)  is supplied from the current amplifier circuit  24  to the capacitive load  28  by the voltage control signal, the falling control signal generator circuit  25  generates the falling pulse as shown in  FIG. 6(B) . 
   At the time instant t 2 , the applied voltage falls to 380V from 400V, and the width of falling (or the voltage difference) is less than 50V. Thus, no falling pulse is generated at this time. This is because the change of the applied voltage from 400V to 380V may be effected at high speed enough only by the operation of the transistor Q 24 , and it is not necessary to turn on the transistor Q 26 . 
   On the other hand, at each of the time instants t 3 , t 5 , t 6  and t 8 , the width of falling of the applied voltage exceeds 50V. Thus, a falling pulse is generated at each time instant, and the transistor Q 26  is turned on, so that the waveform of the applied voltage to the capacitive load  28  changes at sufficiently high speed. 
   Alternatively, the capacitive load which is driven by the capacitive load driving device of the invention may be any other capacitive load, such as a piezoelectric element, different from the electro-optic effect device as in the above-described embodiment. 
   The control signal generator unit  21  in the above-described embodiment is equivalent to a voltage control signal generator unit in the claims. The voltage amplifier circuit  23  in the above-described embodiment is equivalent to a voltage amplifier unit in the claims. The current amplifier circuit  24  in the above-described embodiment is equivalent to a current amplifier unit in the claims. 
   The falling control signal generator circuits  25  and  40  in the above-described embodiment are equivalent to a falling control signal generator unit in the claims. The switching circuit  26  in the above-described embodiment is equivalent to a switching unit in the claims. 
   The ROM  32  in the above-described embodiment is equivalent to a table unit in the claims. The pulse generator circuit  33  in the above-described embodiment is equivalent to a pulse generator unit in the claims. The differential circuit  41  in the above-described embodiment is equivalent to a differential unit in the claims. 
   The transistors Q 31  and Q 32  in the above-described embodiment are equivalent to first and second transistors in the claims. The capacitors C 2 , C 3 , C 4 , and the switches S 1 , S 2 , S 3  in the above-described embodiment are equivalent to a time-constant adjusting unit in the claims. The resistors R 6 , R 7 , R 8 , and the switches S 4 , S 5 , S 6  in the above-described embodiment are equivalent to a bias adjustment unit in the claims. 
   The present invention is not limited to the specifically disclosed embodiment, and variations and modifications may be made without departing from the scope of the present invention.