Abstract:
An ink jet printer has a plurality of piezoelectric elements arranged in a matrix form in which each piezoelectric element is connected between one of TS lines and one of selection lines. The TS lines are connected, one by one and in a time sharing manner, to a voltage source E 1  so as to be enabled. The remaining TS lines are connected to a voltage source E 2  so as to be disabled. The selection lines are selectively connected to a voltage source E 3  so as to be enabled. Non-selected selection lines are connected to a voltage source E 4  so as to be disabled. A selected piezoelectric element connected between enabled TS line and enabled selection line is applied with a driving voltage. A non-selected piezoelectric element connected between enabled TS line and disabled selection line, or between disabled TS line and enabled selection line, or between disabled TS line and disabled selection line, is applied with a non-driving voltage. The voltages supplied from the respective voltage sources are selected so that the non-driving voltage is one third or minus one third of the driving voltage. With the thus determined non-driving voltage, no ink droplet is ejected from a nozzle corresponding to the non-selected piezoelectric element while ensuring a sufficient amount of ink to be ejected from a nozzle corresponding to the selected piezoelectric element.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a matrix driving circuit of an ink jet printer with a plurality of piezoelectric elements arranged in a matrix form. The invention also relates to a method of driving the matrix driving circuit of such an ink jet printer. 
     2. Description of the Related Art 
     FIG. 1 shows a cross-section of a nozzle unit in a conventional ink jet printer. A predetermined number of nozzle units are arranged in the direction perpendicular to the sheet of drawing. A pressure chamber 2 is generally defined by a nozzle plate 11, a partition membrane 3, an upper wall 12 and a lower wall 13. The pressure chamber 2 is in fluid communication with an ink tank (not shown) through an ink channel 9 and is filled with an ink. A predetermined number of nozzles 1 are formed in the nozzle plate 11. Each nozzle 1 has a decreasing diameter toward the outer surface of the nozzle plate 11. Ink droplets are ejected the nozzles. 
     A piezoelectric element 4 is disposed just behind the partition membrane 3 and oriented in such a direction that the longitudinal axis of the piezoelectric element 4 extends perpendicular to the surface of the membrane 3. One end of the piezoelectric element 4 is attached to the partition membrane 3 and the opposite end thereof is fixedly secured to a frame 10. The piezoelectric element 4 has a rectangular cross-section. A positive electrode 5 is attached to the upper side surface of the piezoelectric element 4 and a negative electrode 6 is attached to the lower side surface of the piezoelectric element 4. The piezoelectric element 4 is, for example, of an electric field controlled type. The piezoelectric element 4 shown in FIG. 1 is a D31 type in which the piezoelectric element 4 contracts in the longitudinal direction as indicated by an arrow when a voltage is applied between the positive and negative electrodes 5 and 6. The piezoelectric element 4 contracts in the longitudinal direction in approximately proportional to a voltage applied therebetween. 
     In operation, a voltage V 0  is applied across the piezoelectric element at the time of a standby condition and a voltage V 1  is applied thereacross at the time of ejection. In response to the voltage V 1 , the piezoelectric element 4 contracts in the longitudinal direction, whereby the volume of the pressure chamber 2 increases and ink is supplemented through the ink channel 9. When application of the voltage V 1  terminates, the volume of the pressure chamber 2 reverts to an initial condition. At this time, an ink droplet is ejected from the nozzle 1. Printing on a recording medium is thus carried out. 
     In recent years, it has been proposed a multi-printhead with a plurality of nozzle units arranged in parallel to increase a printing speed. An attempt to increase the printing speed by increasing the number of nozzle units encountered a problem such that as the number of nozzle units increases, the number of wirings for supplying currents to the respective piezoelectric elements 4 also increases. This makes it difficult to arrange the nozzle units. 
     To solve this problem, a matrix driving of the piezoelectric elements has been proposed as disclosed in Japanese Laid-Open Patent Publication (Kokei) No. HEI-6-64166. In the driving method according to this publication, a plurality of charging switching elements are provided to respective ones of a plurality of piezoelectric elements and also a single discharging switching element is provided commonly to all of the piezoelectric elements, so that the piezoelectric elements are repeatedly charged and discharged by the actions of the switching elements. In this arrangement, the more the configuration of the driving circuit is simplified, the more the number of nozzles to be operated simultaneously increases. The printing speed is also increased. 
     In the matrix driving as disclosed in the publication No. HEI-6-64166, a half of the driving voltage on an enabled common or time sharing lines is distributedly applied to the non-driving piezoelectric elements. Due to this distributed voltage, an ink droplet is ejected from a particular nozzle corresponding to a non-driving piezoelectric element. 
     FIG. 3 shows an explanatory diagram for illustrating distributed voltages, and FIG. 4 shows a timing chart for describing the operation of the matrix driving circuit to which the distributed voltages are applied. 
     As shown in FIGS. 3 and 4, it is assumed that when a voltage V 0  is applied to the second time sharing line X 2 , a quarter of the voltage V 0 , i.e., 1/4·V 0 , is applied as a distributed voltage to the remaining time sharing lines X 1 , X 3 , and X 4  and that zero volt is applied to the selection lines Y 2  and Y 3  and 1/2·V, is applied to the remaining selection lines to drive the piezoelectric elements P 22  and P 23 . In this condition, the voltage V 0  is developed across the piezoelectric elements P 22  and P 23  so that ink droplets are ejected from the corresponding nozzles. The voltage of 1/4·V 0  is applied across the remaining piezoelectric elements except the piezoelectric element P 21 . With the voltage of 1/4·V 0 , no ink droplet is ejected from the corresponding nozzles. However, the piezoelectric element P 21  is applied with the voltage V 0  on its positive electrode and the voltage 1/2·V 0  on its negative electrode, thus a forward voltage of 1/2·V 0  is developed across the piezoelectric element P 21 , resulting in an ejection of ink droplet from the corresponding nozzle. Hereinafter, the voltage on the positive electrode relative to the voltage on the negative electrode of the piezoelectric element will be referred to as &#34;a differential voltage&#34;, the differential voltage for driving the piezoelectric element will be referred to as &#34;driving differential voltage&#34;, and the differential voltage for non-driving the piezoelectric element will be referred to as &#34;non-driving differential voltage&#34;. 
     FIG. 2 shows a relationship between a driving differential voltage dV applied across the piezoelectric element and an amount of ink ejected from the corresponding nozzle. As can be seen from the solid line in FIG. 2, ink will not be ejected from the nozzle if the driving differential voltage is below dVx. Therefore, it would be possible not to eject ink from the nozzle if the distributed voltage applied to the non-driving piezoelectric elements is below this critical voltage. 
     However, the characteristic curve regarding the driving differential voltage vs. amount of ink will vary in a range indicated by two two-dotted-chain lines. Therefore, in actuality, a small amount of ink may flow out from the nozzle when a half of the driving voltage is applied to the non-driving piezoelectric element. The ink thus flowed out from the nozzle causes clogging of the nozzle and bothers the subsequent ink ejection. 
     If the driving differential voltage is lowered to solve the above-mentioned problem, a sufficient amount of ink may not be ejected from the nozzle when the corresponding piezoelectric element is driven. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to solve the above-mentioned problems accompanying the conventional driving methods. Accordingly, it is an object of the present invention to provide a matrix driving circuit of an ink jet printer and also a method of driving the same wherein the driving differential voltage applied to the non-driving piezoelectric element is set to minimum so that ink is not flowed out or ejected from the nozzle corresponding to the non-driving piezoelectric element. 
     It is another object of the present invention to provide a matrix driving circuit and a method of driving the same wherein a sufficient amount of ink can be ejected from the nozzle corresponding to the driving piezoelectric element. 
     It is still another object of the present invention to provide a matrix driving circuit of the in jet printer which is highly reliable and less costly. 
     A matrix driving circuit according to the present invention includes a plurality of piezoelectric elements, N-number time shearing lines, and M-number selection lines wherein N and M are integers equal to or greater than two. The piezoelectric elements are divided into N groups so that each of the N groups contains M-number piezoelectric elements. The piezoelectric elements are further divided into M sub-groups so that each of the M sub-groups contains N-number piezoelectric elements belonging to respective ones of the N groups individually. Each piezoelectric element has a first electrode and a second electrode. The N-number time shearing lines are provided in one-to-one correspondence to the N groups and connected to the first electrodes of the M-number piezoelectric elements belonging to corresponding groups. The M-number selection lines are provided in one-to-one correspondence to the M sub-groups and connected to the second electrodes of the N-number piezoelectric elements belonging to corresponding sub-groups. 
     In the driving method of the present invention, a driving differential voltage is applied to a selected piezoelectric element through a corresponding time shearing line and a corresponding selection line, and a non-driving differential voltage is applied to a non-selected piezoelectric element, wherein the non-driving differential voltage is one third or minus one third of the driving voltage. With the thus determined non-driving differential voltage, no ink droplet is ejected from a nozzle corresponding to the non-selected piezoelectric element whereas an ink droplet is ejected from a nozzle corresponding to the selected piezoelectric element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The particular features and advantages of the invention as well as other objects will become more apparent from the following description taken in connection with the accompanying drawings, in which: 
     FIG. 1 is a cross-sectional view showing a nozzle unit of a conventional ink jet printer; 
     FIG. 2 is a graphical representation showing a relationship between a driving differential voltage applied to a piezoelectric element and an amount of ink ejected from a nozzle; 
     FIG. 3 is an explanatory diagram for describing voltages distributed to non-driving piezoelectric elements; 
     FIG. 4 is a timing chart of the voltages applied to respective electrodes of the piezoelectric elements for describing the operation of piezoelectric elements shown in FIG. 3; 
     FIG. 5 is a circuit diagram showing a matrix driving circuit of an ink jet printer according to one embodiment of the present invention; 
     FIG. 6 is a timing chart of various signals applied to the components of the matrix driving circuit shown in FIG. 5 and also voltages developed across the piezoelectric components; 
     FIG. 7 is a circuit diagram showing a matrix driving circuit of an ink jet printer according to another embodiment of the present invention; and 
     FIG. 8 is a timing chart of various signals applied to the components of the matrix driving circuit shown in FIG. 7 and also voltages developed across the piezoelectric components. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described with reference to the accompanying drawings. 
     A first embodiment of the present invention will be described with reference to FIGS. 5 and 6. An ink jet printer used in the first embodiment is the same as that shown in FIG. 1, and therefore the description thereof will not be repeated here. Also, the basic operation of the printer remains the same. 
     FIG. 5 shows a matrix driving circuit according to the first embodiment of the present invention. As shown therein, a plurality of piezoelectric elements are arranged in a matrix form. The piezoelectric elements are divided into four groups, each containing M-number piezoelectric elements. The piezoelectric elements are further divided into M sub-groups, each containing four piezoelectric elements belonging to respective ones of four groups individually. The total number of piezoelectric elements making up the driving circuit is therefore 4M. The piezoelectric elements belonging to the first group are denoted by 41 1 , 41 2 , . . . , 41 M , the second group by 42 1 , 42 2 , . . . , 42 M , the third group by 43 1 , 43 2 , . . . , 43 M  (not shown in the figure), and the fourth group by 44 1 , 44 2 , . . . , 44 M . 
     Four time sharing lines (hereinafter referred to as &#34;TS lines&#34;) 11 1 , 11 2 , 11 3 , 11 4  are connected to four piezoelectric element groups, respectively. The first TS line 11 1  is connected to the positive electrodes 5 11  through 5 1M  of the piezoelectric elements 4 11  through 4 1M  ; the second TS line 11 2  to the positive electrodes of the piezoelectric elements 4 21  through 4 2M  ; the third TS line 11 3  to the positive electrodes of the piezoelectric elements 4 31  through 4 3M  ; and the fourth TS line 11 4  to the positive electrodes 5 41  through 5 4M  of the piezoelectric elements 4 41  through 4 4M . 
     The first TS line 11 is connected to a first voltage source E 1  and a second voltage source E 2  through respective drivers. The first voltage source E 1  supplies 24 volts and the second voltage source E 2  supplies 8 volts. An E 1  -to-11 1  driver connected between the first voltage source E 1  and the first TS line 11 1  includes a PNP transistor 20 1  and a diode 36 1  connected in opposite polarity between the emitter and collector of the transistor 20 1 . The emitter of the transistor 20 1  is connected to the voltage source E 1  and a collector thereof to the first TS line 11 1 . An E 2  -to-11 1  driver connected between the second voltage source E 2  and the first TS line 11 1  includes a series connection of a PNP transistor 21 1  and an NPN transistor 22 1 , and diodes 32 1  and 33 1  connected in parallel to the transistors 21 1  and 22 1 , respectively. The diodes 32 1  and 33 1  are connected in opposite polarity to the respective transistors similar to the transistor and diode pair of the E 1  -to-11 1  driver. The emitter of the transistor 21 1  is connected to the second voltage source E 2  and the collector thereof to the emitter of the transistor 22 1 . The collector of the transistor 22 1  is connected to the first TS line 11 1  and also to ground through an NPN transistor 25 1 . 
     The second to fourth TS lines 11 2  to 11 4  are also connected to the first and second voltage sources E 1  and E 2  through the similarly configured drivers. Specifically, an E 1  -to-11 2  driver connected between the first voltage source E 1  and the second TS line 11 2  includes a PNP transistor 20 2  and a diode 36 2 , and an E 2  -to-11 2  driver connected between the second voltage source E 2  and the second TS line 11 2  includes a PNP transistor 21 2 , an NPN transistor 22 2 , and diodes 32 2 , 33 2 . An E 1  -to-11 3  driver connected between the first voltage source E 1  and the third TS line 11 3  includes a PNP transistor 20 3  and a diode 36 3 , and an E 2  -to-11 3  driver connected between the second voltage source E 2  and the third TS line 11 3  includes a PNP transistor 21 3 , an NPN transistor 22 3 , and diodes 32 3 , 33 3  (not shown). An E 1  -to-11 4  driver connected between the first voltage source E 1  and the fourth TS line 11 4  includes a PNP transistor 20 4  and a diode 36 4 , and an E 2  -to-11 4  driver connected between the second voltage source E 2  and the fourth TS line 11 4  includes a PNP transistor 21 4 , an NPN transistor 22 4 , and diodes 32 4 , 33 4 . 
     The TS lines 11 1  through 11 4  are also connected to ground serving as a fifth voltage source E 5  through respective drivers. An E 5  -to-11 1  driver connected between the first TS line and the fifth voltage E 5 , i.e., ground, includes an NPN transistor 25 1  and a diode 37 1  connected in parallel with the transistor 25 1  in opposite polarity. The collector of the transistor 25 1  is connected to the first TS line 11 1  and the emitter thereof to the fifth voltage source E 5 . Likewise, transistor and diode pairs configured by respective one of NPN transistors 25 2  through 25 4  and respective one of diodes 37 2  through 37 4  are connected between the fifth voltage source E 5  and the respective TS lines 11 2  through 11 4 . 
     M-number selection lines 12 1  through 12 M  are provided to connect the piezoelectric elements in a matrix form in cooperation with the TS lines 11 1  to 11 4 . Each of the M-number selection lines is connected to the negative electrodes of the piezoelectric elements arranged in the same column of the four groups. Specifically, the selection line 12 1  is connected to the negative electrode 6 11  of the piezoelectric element 4 11  belonging to the first group. The selection line 12 1  is also connected to the negative electrodes of the piezoelectric elements belonging to the second to fourth groups. The selection line 12 2  is connected to the negative electrode of the piezoelectric element 4 12  belonging to the second group. The selection line 12 2  is also connected to the negative electrodes of the piezoelectric elements belonging to the second to fourth groups. Likewise, the selection line 12 M  is connected to the negative electrode 6 1M  of the piezoelectric element 4 1M  belonging to the M-th group. The selection line 12 M  is also connected to the negative electrodes of the piezoelectric elements belonging to the second to fourth groups. 
     The selection lines 12 1  through 12 M  are connected through respective drivers to ground serving as a third voltage source E 3 , and also as a sixth voltage source E 6 . The selection lines 12 1  through 12 M  are further connected to a fourth voltage source E 4  through respective drivers. The voltage source E 4  supplies 16 volts. An E 3  -to-12 1  driver connected between the first selection line 12 1  and the power source E 3 , i.e., ground, includes an NPN transistor 23 1  and a diode connected in parallel with the transistor 23 1  in opposite polarity. The emitter of the transistor 23 1  is connected to the third voltage source E 3 , i.e., ground, and a collector thereof to the selection line 12 1 . An E 4  -to-12 1  driver connected between the first selection line 12 1  and the fourth voltage source E 4  includes a PNP transistor 24 1  and a diode 35 1  connected in parallel with the transistor 24 1  in opposite polarity. The emitter of the transistor 24 1  is connected to the fourth voltage source E 4  and a collector thereof to the first selection line 12 1 . Likewise, transistor and diode pairs configured by respective one of NPN transistors 23 2  through 23 M  and respective one of diodes 34 2  through 34 M  are connected between the respective selection lines 12 2  through 12 M  and the third voltage source E 3 , i.e., ground. Another transistor and diode pairs configured by respective one of the PNP transistors 24 2  through 24 M  and respective one of diodes 35 2  through 35 M  are connected between the fourth voltage source E 4  and the respective selection lines 12 2  through 12 M . 
     As described, the first, second and fifth voltage sources E 1 , E 2 , and E 5  are connected to the TS lines 11 1  through 11 4  through the respective drivers, and the third, fourth, and sixth voltage sources E 3 , E 4 , and E 6  are connected to the selection lines 12 1  through 12 M  through the respective drivers. The drivers provided in association with the first voltage source E 1  sequentially applies 24 volts to selective one of the TS lines 11 1  through 11 4  in a time sharing manner. When the first voltage source E 1  is connected to the first TS line 11 1 , the first voltage source E 1  is not connected to the remaining TS lines but the second voltage source E 2  is connected thereto. Likewise, when the first voltage source E 1  is connected to the second TS line 11 2 , the first voltage source E 1  is not connected to the remaining TS lines but the second voltage source E 2  is connected thereto. 
     When a particular piezoelectric element is to be driven, the corresponding selection line is connected to the third voltage source E 3 , i.e., ground The selection lines corresponding to non-driving piezoelectric elements are connected to the fourth voltage source E 4  to supply 16 volts thereto. 
     To initialize the all the piezoelectric elements, the TS lines 11 1  through 11 4  are connected to the fifth voltage source E 5 , i.e., ground, and the selection lines 12 1  through 12 M  are connected to the sixth voltage source E 6 , i.e., ground. 
     As described, in the first embodiment, the third, fifth, and sixth voltage sources E 1 , E 5 , and E 6  apply zero volt. The first voltage source E 1  applies 24 volts that is three times as large as the 8 volts applied by the second voltage source E 2 . The fourth voltage source E 4  applies 16 volts that is twice as large as 8 volts applied by the second voltage source E 2 . 
     Referring next to FIG. 6, a driving method for the circuit shown in FIG. 5 will be described. FIG. 6 shows timing charts of signals applied to the bases of transistors included in the circuit shown in FIG. 5 and of the voltages developed across the piezoelectric elements. In FIG. 6, X 11  through X 4M  denote the voltages developed across the positive electrodes 5 11  through 54 M  and the negative electrodes 6 11  through 64 M  of the piezoelectric elements 41 1  through 44 M , respectively. 
     In the standby condition indicated by &#34;ST&#34; in the timing chart of FIG. 6, the transistors 25 1  through 25 4  are simultaneously rendered conductive (ON) so that all of the TS lines 11 1  through 11 4  are connected to the fifth voltage source E 5 , i.e., ground, through the transistor 25 1  through 25 4 . The transistors 23 1  through 23 M  are also rendered conductive so that the selection lines 12 1  through 12 M  are connected to the sixth voltage source E 6 , i.e., ground. The remaining transistors are held non-conductive (OFF). Thus, all the piezoelectric elements are initialized and placed in a standby condition. 
     Following the standby condition &#34;ST&#34;, the piezoelectric elements are driven on a group basis. Because the piezoelectric elements are divided into four groups by the TS lines 11 1  through 11 4 ,four phases of drivings complete one cycle of driving. In the timing chart shown in FIG. 6, the first phase driving is indicated by &#34;D1&#34;, the second phase driving by &#34;D2&#34;, the third phase driving by &#34;D3&#34;, and the fourth phase driving by &#34;D4&#34;. 
     In the first phase driving D1, the transistors 25 1  through 25 4  are rendered non-conductive, and the transistor 20 1  is rendered conductive so that the voltage of the power source E 1  (24 volts) is applied to the first TS line 11 1 . At this time, the transistor 22 1  is rendered non-conductive to prevent the voltage source E 2  from being interfered by the voltage source E 1 . The transistors 21 2 , 21 3  (not shown) and 21 4  are rendered conductive so that the voltage of the power source E 2  (8 volts) is applied to the TS lines 11 2 , 11 3 , and 11 4 , respectively. Through the switching actions of the transistors, the first TS line 11 1  is applied with 24 volts and the remaining three TS lines 11 2  through 11 4  are applied with 8 volts. 
     When the TS line 11 1  is applied with 24 volts (the voltage of the power source E 1 ), the piezoelectric elements belonging to the first group are selectively driven, subject to connections of the corresponding selection lines to the third voltage source E 3 , i.e., ground. To this end, any of the transistors 23 1  through 23 M  corresponding to the piezoelectric elements to be driven are rendered conductive in response to a print signal. 
     For example, when the print signal indicates that the piezoelectric element 4 11  connected to the first selection line 12 1  is to be driven, the transistor 23 1  is rendered conductive so that the selection line 12 1  is connected to the third voltage source E 3 , i.e., ground. If the remaining piezoelectric elements belonging to the first group are not to be driven, the transistors 23 2  through 23 M  are rendered non-conductive. Instead, the transistors 24 2  through 24 M  are rendered conductive so that the voltage of the power source E 4  (16 volts) is applied to the selection lines 12 2  through 12 M , respectively. 
     As described, the driving piezoelectric element in the first group is applied with 24 volts on its positive electrode and zero volt on its negative electrode. The non-driving piezoelectric elements in the first group are applied with 24 volts on their positive electrodes and 16 volts on their negative electrodes. Consequently, 24 volts is applied across the driving piezoelectric element and 8 volts is applied across the non-driven piezoelectric elements belonging to the first group. To other non-driving piezoelectric elements belonging to the groups other than the first group, 8 volts is applied to their positive electrodes and zero volt is applied to their negative electrodes, or 8 volts is applied to their positive electrodes and 16 volts is applied to their negative electrodes. Consequently, 8 volts or -8 volts is applied to the non-driving piezoelectric elements in the second, third and fourth groups. 
     In the end of each phase of driving, all the piezoelectric elements are again initialized and placed in the standby condition by rendering the transistors 25 1  through 25 4  conductive. Thereafter, the second to fourth phase drivings are sequentially performed in the similar manner. 
     Next, a second embodiment of the present invention will be described while referring to FIGS. 7 and 8. 
     In the second embodiment shown in FIG. 7, the piezoelectric elements are arranged in a matrix form and divided into four groups and M-number sub-groups as in the first embodiment. The connections of four TS lines 11 1  through 11 4  and M-number selection lines 12 1  through 12 M  to the piezoelectric elements are also identical to those shown in FIG. 5. 
     In the second embodiment, the first TS line 11 1  is connected to a first voltage source E 1  and a second voltage source E 2  through respective drivers. The first voltage source E 1  supplies 16 volts and the second voltage source E 2  supplies 32 volts. An E 1  -to-11 1  driver connected between the first voltage source E 1  to the first TS line 11 1  includes a series connection of a PNP transistor 42 1  and an NPN transistor 43 1 , and diodes 48 1  and 49 1 . The diodes 48 1  and 49 1  are connected in parallel with the transistors 42 1  and 43 1 , respectively. 
     An E 2  -to-11 1  driver connected between the second voltage source E 2  and the first TS line 11 1  includes a PNP transistor 41 1  and a diode 54 1  connected in parallel with the transistor 41 1  in opposite polarity. The second to fourth TS lines 11 2  to 11 4  are also connected to the voltage sources E 1  and E 2  through the similarly configured drivers. The second voltage source E 2  and its associated driver is commonly used as a fifth voltage source E 5  and its associated driver. 
     The selection lines 12 1  through 12 M  are connected to a third voltage source E 3  through respective drivers. The first voltage source E 1  is commonly used as the third voltage source E 3 , thus supplying 16 volts. An E 3  -to-12 1  driver connected between the first selection line 12 1  to the third voltage source E 3  includes a PNP transistor 45 1  and a diode 51 1  connected in parallel to the transistor 45 1  in an opposite polarity. The second to M-th selection lines 12 1  through 12 M  are also connected to the third voltage source E 3  through the similarly configured drivers. 
     Further, the selection lines 12 1  through 12 M  are connected to ground serving as a fourth voltage source E 4  through respective drivers. An E 4  -to-12 1  driver connected between the first selection line 12 1  to the third voltage source E 3 , i.e., ground, includes an NPN transistor 44 1  and a diode 50 1  connected in parallel to the transistor 44 1  in an opposite polarity. The second to M-th selection lines 12 2  through 12 M  are also connected to the fourth voltage source E 4 , i.e., ground, through the similarly configured drivers. 
     The selection lines 12 1  through 12 M  are connected to a sixth voltage source E 6  through respective drivers. The sixth voltage source E 6  supplies 8 volts. An E 6  -to-12 1  driver connected between the first selection line 12 1  to the sixth voltage source E 6  includes a series connection of a PNP transistor 46 1  and an NPN transistor 47 1 , and diodes 52 1  and 52 1 . The diodes 52 1  and 52 1  are connected in parallel to the transistors 46 1  and 47 1  in opposite polarity, respectively. The second to M-th selection lines 12 2  through 12 M  are also connected to the sixth voltage source E 6  through the similarly configured drivers. 
     A current may flow in the direction A when the transistor 46 1  is rendered conductive, so the diode 52 1  is connected in opposite polarity in parallel with the transistor 46 1 . Further, to allow a current to flow in the direction B, the diode 53 1  is connected in parallel with the transistor 47 1 . The transistor 47 1  is provided so that a current does not flow into the diode 52 1  from the X point. Because the X point is applied with 16 volts when the transistor 46 1  is rendered non-conductive and the transistor 45 1  is rendered conductive. The same is true with respect to the transistors 47 2  through 47 M . 
     In the second embodiment, first to sixth voltage sources E 1  through E 6  are provided. The first and third voltage sources E 1  and E 3  supply 16 volts, the second and fifth voltage sources E 2  and E 5  supply 32 volts, the fourth voltage source E 4  supplies zero volt, and the sixth voltage source E 6  supplies 8 volts. 
     Referring next to FIG. 8, a driving method for the circuit shown in FIG. 7 will be described. FIG. 8 shows timing charts of signals applied to the bases of transistors included in the circuit shown in FIG. 7 and of the voltages developed across the piezoelectric elements. 
     In the standby condition indicated by &#34;ST&#34; in the timing chart of FIG. 8, the transistors 41 1  through 41 4  are simultaneously rendered conductive so that all of the TS lines 11 1  through 11 4  are connected to the fifth voltage source E 5  (32 volts). The transistors 46 1  through 46 M  and 47 1  through 47 M  are also rendered conductive so that the selection lines 12 1  through 12 M  are connected to the sixth voltage source E 6  (8 volts). The remaining transistors are held non-conductive. All the piezoelectric elements are applied with 32 volts on their positive electrodes and 8 volts on their negative electrodes, so 24 volts is applied across each of the piezoelectric element. In this manner, all the piezoelectric elements are initialized and placed in a standby condition. 
     In the first phase driving DR1, the transistors 42 1  and 43 4  are rendered conductive so that 16 volts of the first voltage source E 1  is applied to the first TS line 11 1 . At this time, the transistor 41 1  is rendered non-conductive to prevent the voltage source E 1  from being interfered by the voltage source E 2  (or E 5 ). The transistors 41 2 , 41 3  (not shown) and 41 4  are rendered conductive so that the voltage of the power source E 2  (32 volts) is applied to the TS lines 11 2 , 11 3 , and 11 4 , respectively. The transistors 42 2  through 42 4  and 43 2  through 43 4  are rendered non-conductive. Through the switching actions of the transistors, the first TS line 11 1  is applied with 16 volts and the remaining three TS lines 11 2  through 11 4  are applied with 32 volts. 
     When the TS line 11 1  is applied with 16 volts (the voltage of the power source E 1 ), the piezoelectric elements belonging to the first group are selectively driven, subject to connections of the corresponding selection lines to the third voltage source E 3  (16 volts). To this end, any of the transistors 23 1  through 23 M  corresponding to the piezoelectric elements to be driven are rendered conductive in response to a print signal. 
     For example, when the print signal indicates that the piezoelectric element 4 1M  connected to the M-th selection line 12 M  is to be driven, the transistor 45 M  is rendered conductive and the transistor 44 M  is rendered non-conductive so that the selection line 12 M  is connected to the third voltage source E 3  (16 volts). If the remaining piezoelectric elements belonging to the first group are not to be driven, the transistors 45 1  through 45 M-1  are rendered non-conductive and the transistors 44 1  through 44 M-1  are rendered conductive so that the selection lines 11 1  through 12 M-1  are connected to ground serving as the fourth voltage source E 4 . 
     As described, the driving piezoelectric element in the first group is applied with 16 volts on its positive electrode and 16 volt on its negative electrode. Three types of non-driving piezoelectric elements exist. The first type non-driving piezoelectric element is the one whose TS line is selected but selection line is not selected. The second type non-driving piezoelectric element is the one whose TS line is not selected but selection line is selected. The third type non-driving piezoelectric element is the one whose TS line and selection line are not selected. Therefore, the voltage V 2  applied across the first type non-driving piezoelectric element is E 1  -E 4 , The voltage V 3  applied across the second type non-driving piezoelectric element is E 2  -E 3 . The voltage V 4  applied across the third type non-driving piezoelectric element is E 2  -E 4 . 
     Specifically, to the first type non-driving piezoelectric elements, 16 volts is applied to their positive electrodes and zero volt is applied to their negative electrodes. To the second type non-driving piezoelectric elements, 32 volts is applied to their positive electrodes and 16 volt is applied to their negative electrodes. To the third type non-driving piezoelectric elements, 32 volts is applied to their positive electrodes and zero volt is applied to their negative electrodes. Consequently, 16 volts, 16 volts, and 32 voltages are applied to the first to third types of non-driving piezoelectric elements, respectively. 
     The driving differential voltage dV 1  is given by V 1  -V 0 . The non-driving differential voltage dV 2  for the first type non-driving piezoelectric element is given by V 2  -V 0 . The non-driving differential voltage dV 3  for the second type non-driving piezoelectric element is given by V 3  -V 0 . The non-driving differential voltage dV 4  for the third type non-driving piezoelectric element is given by V 4  -V 0 . 
     The driving differential voltage dV, applied across the driving piezoelectric element is 0-24=-24 volts. The non-driving differential voltage dV 2  applied across the first type non-driving piezoelectric element is 16-24=-8 volts. The non-driving differential voltage dV 3  applied across the second type non-driving piezoelectric element is 16-24=-8 volts. The non-driving differential voltage dV 4  applied across the third type non-driving piezoelectric element is 32-24=8 volts. 
     In the end of each phase of driving, all the piezoelectric elements are again initialized and placed in the standby condition by rendering the transistors 41 1  through 41 4 , 46 1  through 46 M , and through 47 1  through 47 M  conductive. Thereafter, the second to fourth phase drivings are sequentially performed in the similar manner. 
     While exemplary embodiments of this invention have been described in detail, those skilled in the art will recognize that there are many possible modifications and variations which may be made in these exemplary embodiments while yet retaining many of the novel features and advantages of the invention. Accordingly, all such modifications and variations are intended to be included within the scope of the appended claims.