1. Technical Field
The present invention relates to a liquid jet apparatus and printing apparatus arranged to print predetermined letters and images by emitting microscopic droplets of liquids from a plurality of nozzles to form the microscopic particles (dots) thereof on a printing medium.
2. Related Art
An inkjet printer as one of such printing apparatuses, which is generally low-price and easily provides high quality color prints, has widely used not only in offices but also for general users often in conjunction with the widespread use of personal computers and digital cameras. In recent inkjet printers, printing in fine tone is required. Tone denotes a state of density of each color included in a pixel expressed by a liquid dot, the size of the dot corresponding to the color density of each pixel is called a tone grade, and the number of tone grades capable of being expressed by a liquid dot is called a tone number. Fine tone denotes that the tone number is large. In order to change the tone grade, it is required to modify a drive pulse to an actuator provided in a liquid jet head. When a piezoelectric element is used as the actuator the tone grade of the liquid dot can be changed very accurately because the amount of displacement of the piezoelectric element (distortion of a diaphragm, to be precise) becomes large when the voltage value applied to the piezoelectric element becomes large.
Therefore, in JP-A-10-81013, a plurality of drive pulses with different wave heights are combined and joined, the drive pulses are commonly output to the piezoelectric elements of the nozzles of the same color provided to the liquid jet head, a drive pulse corresponding to the tone grade of the liquid dot to be formed is selected for every nozzle out of the plurality of drive pulses, and the selected drive pulses are supplied to the piezoelectric elements of the corresponding nozzles to emit droplets of the liquid in different amounts, thereby achieving the required tone grade of the liquid dot.
The method of generating the drive signals (or the drive pulses) is described in FIG. 2 of JP-A-2004-306434. Specifically, the data is retrieved from a memory storing the data of the drive signal, the data is converted into analog data by a D/A converter, and the drive signal is supplied to the liquid jet head through a voltage amplifier and a current amplifier. The circuit configuration of the current amplifier is, as shown in FIG. 3 of JP-A-2004-306434, composed of push-pull connected transistors, and the drive signal is amplified by a so called linear drive. However, in the current amplifier with such a configuration, the linear drive of the transistor is inefficient. A large-sized transistor is required as a measure against heating of the transistor. Moreover, a heat radiation plate for cooling the transistor is required. Thus, a disadvantage of growth in the circuit size arises, and among others, the size of the heat radiation plate constitutes a great barrier to layout design.
To overcome this disadvantage, a digital power amplifier, i.e., a class D amplifier can be used for amplified output of the drive signal. The digital power amplifier has better power amplification efficiency than an analog power amplifier, has little power loss, and can cope with high-speed rising or falling of the drive signal. However, when the drive signal is amplified using a digital power amplifier, the voltage value of the output drive signal changes with a change of the power supply voltage. In the ink-jet printer described in JP-A-2005-329710, correction is performed by returning the output drive signal, i.e., by applying feedback.
In the method of feeding back the drive signal, however, parts, such as a D/A converter to generate an analog signal to be compared with the fed back drive signal, and a feedback circuit are required which increases the number of parts and the cost. Since the drive signal generated by a pulse modulator, a digital power amplifier and a low-pass filter is fed back, the correction cannot follow the rapid change of the power supply voltage. In addition, since the phase of the drive signal changes according to the number of actuators to be driven, a filter with a single phase characteristic cannot cope with the phase change of the drive signal to be fed back.