Abstract:
A rectifier circuit configured with a conventional configuration using an operational amplifier and a diode by a thin film transistor over an insulating substrate cannot exhibit the performance of a rectifier circuit due to the low stability of operational amplifier and the low high-frequency characteristic. Therefore, the rectifier circuit requires to be configured by using an IC outside of the insulating substrate in order to rectify a high-frequency signal. According to the invention, an amplifier circuit and a waveform shaping circuit are configured with a thin film transistor and a non-rectified signal is switched by a signal thereof, so that a rectifier circuit with the excellent high-frequency characteristic can be realized.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a semiconductor device, and in particular, relates to a semiconductor device having a rectifier circuit configured with a thin film semiconductor element. The invention also relates to an electronic apparatus using a semiconductor device having a rectifier circuit.  
         [0003]     2. Description of the Related Art  
         [0004]     In recent years, mobile phones have been widely used with the advance of communication technology. In future, transmissions of moving images and of more information are expected. In addition, through reduction in weight of personal computers, those adapted for mobile communication have been produced. Information terminals called PDAs originated in electronic notebooks have been also produced in large quantities and widely used. With the development of display devices, the majority of portable information devices are equipped with a flat panel display.  
         [0005]     In particular, a display device having both a pixel and a driver configured with a thin film transistor by using a technology called a low-temperature polysilicon for forming polysilicon over a glass by laser annealing has been widely used recently. In addition, circuits using the low-temperature polysilicon technology have been developed, and a CPU, a memory circuit, and the like using the technology are released. The application of the low-temperature polysilicon technology to an analog circuit may be realized sooner or later.  
         [0006]     By such an analog circuit, nonelectrical data from outside of a display device such as a sound and a pressure can be detected. A sound signal is converted into an electrical signal by a microphone and the electrical signal is amplified, and then converted into direct current by a rectifier circuit or a smoothing circuit. The nonelectrical data can be detected based on the direct current potential.  
         [0007]     In the case where the rectifier circuit can be configured with a thin film transistor over an insulating substrate, a display device using low-temperature polysilicon can be configured more systematically.  
         [0008]     A rectifier circuit is generally configured by a diode and an operational amplifier as shown in  FIG. 3 . Such rectifier circuit is well known and frequently employed due to its simple configuration in the case of structuring an electronic apparatus (see Non-Patent Document 1, for example).  
         [0009]     [Non-Patent Document 1] 
         [0010]     ‘Standard Text for OP-Amp Circuit Design’, CQ publisher, p. 316  
         [0011]     An operation of a circuit shown in  FIG. 3  is explained hereinafter. Considered first is the case where a signal with a potential higher than that of a power source  309  is inputted to an input terminal  307 . A potential of an inverting input terminal of an operational amplifier  301  is equal to that of a non-inverting input terminal and the non-inverting input terminal is connected to the power source  309 , therefore, the potential of the non-inverting input terminal is equal to that of the power source  309 . Accordingly, current flows from the input terminal  307  toward the inverting input terminal. The current flows into an output terminal of the operational amplifier  301  through a diode  303 . An output potential of the operational amplifier  301  becomes lower than the potential of the power source  309  by one VF of the diode  303 . Each potential at both ends of a resistor  305  is equal to that of the power source  309  because no current flows into the resistor  305  in this case.  
         [0012]     Considered next is the case where a signal with a potential lower than that of the power source  309  is inputted to the input terminal  307 . Current flows from the output terminal of the operational amplifier  301  into the input terminal  307  through a diode  302 , the resistor  305 , and a resistor  304  in this case. A signal of the opposite phase to the input signal is outputted to a connection point between the resistor  305  and the diode  302 .  FIG. 4A  shows an input signal and  FIG. 4B  shows an output after the rectification. It is noted that the reference numeral  306  refers to a smoothing circuit and  308  refers to an output terminal.  
       SUMMARY OF THE INVENTION  
       [0013]     The conventional rectifier circuit described above has the following problem when being configured with a thin film transistor and mounted over an insulating substrate.  
         [0014]     The conventional rectifier circuit basically includes an operational amplifier, and thus the alternating-current feedback is required in using the circuit. In the case of a thin film transistor, the current capability of each element is small, and thus a large element is required for improving the current capability. However, increase in the size of an element causes increase in the parasitic capacitance and the oscillation stability becomes worse, consequently, the alternating-current feedback cannot be performed.  
         [0015]     Therefore, in the case of configuring the conventional rectifier circuit with the conventional thin film transistor, many capacitors for preventing the oscillation are required to connect in the rectifier circuit in order to solve the problem of the oscillation. That reduces a frequency characteristic and a rectified waveform shown in  FIG. 4C  is obtained, which is not a desirable waveform.  
         [0016]     In view of the above problem, a rectifier circuit having an amplifying function, a waveform shaping function for shaping waveform of an amplified signal, and a switching function which can be controlled by the waveform shaped signal is provided, aiming to realize a semiconductor device mounting a rectifier circuit by a thin film element.  
         [0017]     According to the invention, in a semiconductor device having a rectifier circuit configured with a thin film transistor over an insulating substrate, the rectifier circuit comprises an amplifier circuit, a waveform shaping circuit for shaping waveform of an outputted signal of the amplifier circuit, and a switching circuit which is controlled by an outputted signal of the waveform shaping circuit.  
         [0018]     According to the invention, in a semiconductor device having a rectifier circuit configured with a thin film transistor over an insulating substrate, the rectifier circuit comprises an amplifier circuit, a waveform shaping circuit for shaping waveform of an outputted signal of the amplifier circuit, a switching circuit which is controlled by an outputted signal of the waveform shaping circuit, and a resistor which is connected to the switching circuit.  
         [0019]     According to the invention, in a semiconductor device having a rectifier circuit configured with a thin film transistor over an insulating substrate, the rectifier circuit comprises an input terminal, a power source, an amplifier circuit, a waveform shaping circuit for shaping waveform of an outputted signal of the amplifier circuit, a switching circuit which is controlled by an outputted signal of the waveform shaping circuit, and a buffer amplifier, the power source is connected to a first terminal of the switching circuit, and the input terminal is connected to a second terminal of the switching circuit and the amplifier circuit.  
         [0020]     According to the invention, in a semiconductor device having a rectifier circuit configured with a thin film transistor over an insulating substrate, the rectifier circuit comprises a first input terminal, a second input terminal, an amplifier circuit, a waveform shaping circuit for shaping waveform of an outputted signal of the amplifier circuit, a switching circuit which is controlled by an outputted signal of the waveform shaping circuit, and a buffer amplifier, the first input terminal is connected to a first terminal of the switching circuit and a first input terminal of the amplifier circuit, and the second input terminal is connected to a second terminal of the switching circuit and a second input terminal of the amplifier circuit.  
         [0021]     In the abovementioned configuration according to the invention, the switching circuit is an analog switching circuit.  
         [0022]     In the abovementioned configuration according to the invention, the analog switching circuit is configured by an n-type thin film transistor and a p-type thin film transistor, and further the respective gate width of the n-type thin film transistor and the p-type thin film transistor is approximately equal to each other.  
         [0023]     In the abovementioned configuration according to the invention, the waveform shaping circuit is an inverter circuit.  
         [0024]     The invention provides an electronic apparatus including the above-mentioned semiconductor device.  
         [0025]     As described above, in the semiconductor device according to the invention, a rectifier circuit with a preferable frequency characteristic can be realized by using a thin film transistor over an insulating substrate so as to amplify an input signal and shape waveform of the signal to control the switching of an input signal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]      FIG. 1  is a diagram showing an embodiment mode of the rectifier circuit in the semiconductor device of the invention.  
         [0027]      FIG. 2  is a diagram showing an embodiment mode of the rectifier circuit in the semiconductor device of the invention.  
         [0028]      FIG. 3  is a diagram showing an example of the rectifier circuit in the conventional semiconductor device.  
         [0029]      FIGS. 4A  to  4 C are diagrams showing waveforms of the rectifier circuit.  
         [0030]      FIG. 5  is a diagram showing an embodiment of the rectifier circuit in the semiconductor device of the invention.  
         [0031]      FIG. 6  is a diagram showing an embodiment of the rectifier circuit in the semiconductor device of the invention.  
         [0032]      FIGS. 7A and 7B  are views of electronic apparatuses applying the semiconductor device of the invention.  
         [0033]      FIG. 8  is a diagram showing an embodiment of the rectifier circuit in the semiconductor device of the invention.  
         [0034]      FIG. 9  is a view showing an embodiment of the semiconductor device of the invention.  
         [0035]      FIG. 10  is a diagram showing the waveform of the rectifier circuit which is measured in Embodiment 1. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0036]     Hereinafter, embodiment modes of the invention are described in detail with reference to the accompanying drawings.  
         [heading-0037]     [Embodiment Mode 1] 
         [0038]      FIG. 1  shows a first embodiment mode of the invention. The embodiment mode shown in  FIG. 1  comprises an amplifier circuit  101 , a waveform shaping circuit  102 , a resistor  103 , a switch  104 , a buffer amplifier  105 , a smoothing circuit  106 , an input terminal  107 , and an output terminal  108 . This circuit is configured with a thin film transistor (abbreviated to a TFT hereinafter). An operation of this embodiment mode is explained below.  
         [0039]     In the beginning, a signal inputted to the input terminal  107  is amplified by the amplifier circuit  101 . An output of the amplifier circuit  101  is inputted to the waveform shaping circuit  102 , and thereby, waveform is shaped into a logic signal. The waveform shaped signal is inputted to the switch  104  as a control signal so as to turn ON/OFF the switch  104 . The signal inputted to the input terminal  107  is also inputted to the resistor  103  as well as the amplifier circuit  101 .  
         [0040]     When the switch  104  is OFF, the signal is inputted to the buffer amplifier  105  through the resistor  103 . An input impedance of the buffer amplifier  105  is preferably set much higher than a resistance value of the resistor  103  for reducing loss due to the resistor  103 , so that the inputted signal is inputted to the smoothing circuit  106  through the buffer amplifier  105 .  
         [0041]     When the switch  104  is ON, an input of the buffer amplifier  105  is connected to a power source  109 , and thus only a direct current potential is outputted from the buffer amplifier  105 . That waveform corresponds to  FIG. 4B . The rectified waveform is smoothed by the smoothing circuit  106 , then outputted to the output terminal  108 .  
         [0042]     In this manner, according to this embodiment mode, a rectifier circuit is configured with an amplifier circuit to which a feedback is not performed, a waveform shaping circuit, a resistor, and a switch to configure over an insulating substrate by using a TFT without deteriorating a frequency characteristic thereof.  
         [0043]     Accordingly, a semiconductor device such that a display portion and a rectifier circuit are integrally formed over an insulating substrate can be obtained.  
         [heading-0044]     [Embodiment Mode 2] 
         [0045]      FIG. 2  shows a second embodiment mode of the invention. The embodiment mode shown in  FIG. 2  comprises an amplifier circuit  201 , a waveform shaping circuit  202 , a switch  204 , a buffer amplifier  205 , a smoothing circuit  206 , an input terminal  207 , and an output terminal  208 . An operation of this embodiment mode is explained below.  
         [0046]     In the beginning, a signal inputted to the input terminal  207  is amplified by the amplifier circuit  201 . An output of the amplifier circuit  201  is inputted to the waveform shaping circuit  202 , and thereby, waveform is shaped into a logic signal. The waveform shaped signal is inputted to the switch  204  as a control signal so as to switch the connection of the switch  204 . The signal inputted to the input terminal  207  is also inputted to the switch  204  as well as the amplifier circuit  201 .  
         [0047]     When the switch  204  is connected to ‘B’, the inputted signal is inputted to the buffer amplifier  205  and the smoothing circuit  206  in this order.  
         [0048]     When the switch  204  is connected to ‘A’, an input of the buffer amplifier  205  is connected to a power source  209 , and thus only a direct current potential is outputted from the buffer amplifier  205 . That waveform corresponds to  FIG. 4B . The rectified waveform is smoothed by the smoothing circuit  206 , then outputted to the output terminal  208 .  
         [0049]     In this manner, according to this embodiment mode, a rectifier circuit is configured with an amplifier circuit to which a feedback is not performed, a waveform shaping circuit, and a switch to configure over an insulating substrate by using a TFT without deteriorating a frequency characteristic thereof.  
         [0050]     Accordingly, a semiconductor device such that a display portion and a rectifier circuit are integrally formed over an insulating substrate can be obtained.  
         [heading-0051]     [Embodiment 1] 
         [0052]      FIG. 5  shows a first embodiment of the invention and shows the first embodiment mode shown in  FIG. 1  in more detail. The embodiment mode shown in  FIG. 5  comprises an amplifier circuit  501 , a waveform shaping circuit  502 , a resistor  503 , analog switching TFTs  517  and  518 , a buffer amplifier  505 , a smoothing circuit  506 , an input terminal  507 , and an output terminal  508 . Further, the amplifier circuit  501  comprises TFTs  510  to  513  and a current source  514 , the waveform shaping circuit  502  comprises inverters  515  and  516 , and the smoothing circuit  506  comprises a resistor  519  and a capacitor  520 . An operation of this embodiment is explained below.  
         [0053]     A signal inputted to the input terminal  507  is amplified by the amplifier circuit  501 . The amplifier circuit  501  comprises a differential circuit configured by the TFTs  512  and  513  and a current mirror circuit configured by the TFTs  510  and  511 . When a signal with a potential higher than that of a power source  509  is inputted to the input terminal  507 , the amount of current in the TFTs  512 ,  510 , and  511  becomes larger than that in the TFT  513 . Consequently, Hi is outputted from the amplifier circuit  501 . When a signal with a potential lower than that of the power source  509  is inputted to the input terminal  507 , the amount of current in the TFTs  512 ,  510 , and  511  becomes smaller than that in the TFT  513 . Consequently, Lo is outputted from the amplifier circuit  501 .  
         [0054]     The output of the amplifier circuit  501  is inputted to the waveform shaping circuit  502 , and thereby, waveform is shaped into a logic signal. The waveform shaping circuit  502  is configured with an inverter in this embodiment. The waveform shaped signal is inputted to the analog switching TFTs  517  and  518  as a control signal so as to turn ON/OFF the analog switching TFTs  517  and  518 .  
         [0055]     When the output of the amplifier circuit  501  is Hi, the analog switching TFTs  517  and  518  are turned OFF. When the output of the amplifier circuit  501  is Lo, the analog switching TFTs  517  and  518  are turned ON. The signal inputted to the input terminal  507  is also inputted to the resistor  503  as well as the amplifier circuit  501 .  
         [0056]     When the analog switching TFTs  517  and  518  are OFF, the signal is inputted to the buffer amplifier  505  through the resistor  503 . An input impedance of the buffer amplifier  505  is preferably set much higher than a resistance value of the resistor  503  for reducing loss due to the resistor  503 , so that the inputted signal is inputted to the smoothing circuit  506  through the buffer amplifier  505 .  
         [0057]     When the analog switching TFTs  517  and  518  are ON, an input of the buffer amplifier  505  is connected to the power source  509 , and thus only a direct current potential is outputted from the buffer amplifier  505 . That waveform corresponds to  FIG. 4B . The rectified waveform is smoothed by the smoothing circuit  506  configured by the resistor  519  and the capacitor  520 , then outputted to the output terminal  508 .  
         [0058]     It is preferable that the respective gate width of the p-type TFT  518  and the n-type TFT  517  is approximately equal to each other in order to prevent the influence of the gate capacitance of the switching TFTs upon the control signal.  
         [0059]     In this manner, according to this embodiment mode, a rectifier circuit is configured with an amplifier circuit to which a feedback is not performed, a waveform shaping circuit, a resister, and a switch to configure over an insulating substrate by using a TFT without deteriorating a frequency characteristic thereof.  
         [0060]     Accordingly, a semiconductor device such that a display portion and a rectifier circuit are integrally formed over an insulating substrate can be obtained.  
         [0061]      FIG. 10  shows an inputted sine wave and an output wave after the half-wave rectification, which are inputted for verifying the operation of the circuit explained in the first embodiment shown in  FIG. 5 . The input wave is a signal of sine wave of 1 kHz and 1 Vpp that is inputted to the input terminal  507 , whereas the output wave is a signal that is outputted from the output terminal  508 .  
         [0062]     The output wave becomes a constant potential during every second half cycle of the sine wave. That is, during the sine wave is in one half cycle at the positive side, the same waveform as the sine wave is outputted while during the sine wave is in the other half cycle at the negative side, a constant potential is outputted. A rectifier circuit with a preferable frequency characteristic is realized without dulling an output waveform in this manner, that ensures the effect of the invention.  
         [heading-0063]     [Embodiment 2] 
         [0064]      FIG. 6  shows a second embodiment of the invention and shows the second embodiment mode shown in  FIG. 2  in more detail. The embodiment mode shown in  FIG. 6  comprises an amplifier circuit  601 , a waveform shaping circuit  602 , analog switching TFTs  617  to  620 , a buffer amplifier  605 , a smoothing circuit  606 , an input terminal  607 , and an output terminal  608 . Further, the amplifier circuit  601  comprises TFTs  610  to  613  and a current source  614 , the waveform shaping circuit  602  comprises inverters  615  and  616 , and the smoothing circuit  606  comprises a resistor  621  and a capacitor  622 . An operation of this embodiment is explained below.  
         [0065]     A signal inputted to the input terminal  607  is amplified by the amplifier circuit  601 . The amplifier circuit  601  comprises a differential circuit configured by the TFTs  612  and  613  and a current mirror circuit configured by the TFTs  610  and  611 . When a signal with a potential higher than that of a power source  609  is inputted to the input terminal  607 , the amount of current in the TFTs  612 ,  610 , and  611  becomes larger than that in the TFT  613 . Consequently, Hi is outputted from the amplifier circuit  601 . When a signal with a potential lower than that of the power source  609  is inputted to the input terminal  607 , the amount of current in the TFTs  612 ,  610 , and  611  becomes smaller than that in the TFT  613 . Consequently, Lo is outputted from the amplifier circuit  601 .  
         [0066]     The output of the amplifier circuit  601  is inputted to the waveform shaping circuit  602 , and thereby, waveform is shaped into a logic signal. The waveform shaping circuit  602  is configured with an inverter in this embodiment. The waveform shaped signal is inputted to the analog switching TFTs  617  to  620  as a control signal so as to turn ON/OFF the analog switching TFTs  617  and  620 .  
         [0067]     When the output of the amplifier circuit  601  is Hi, the analog switching TFTs  617  and  618  are turned OFF while TFTs  619  and  620  are turned ON. When the output of the amplifier circuit  601  is Lo, the analog switching TFTs  617  and  618  are turned ON while TFTs  619  and  620  are turned OFF. The signal inputted to the input terminal  607  is also inputted to the analog switching TFTs  617  and  618  as well as the amplifier circuit  601 .  
         [0068]     When the analog switching TFTs  617  and  618  are ON while TFTs  619  and  620  are OFF, the signal is inputted to the buffer amplifier  605  and the smoothing circuit  606  in this order. When the analog switching TFTs  617  and  618  are OFF while TFTs  619  and  620  are ON, an input of the buffer amplifier  605  is connected to the power source  609 , and thus only a direct current potential is outputted from the buffer amplifier  605 . That waveform corresponds to  FIG. 4B . The rectified waveform is smoothed by the smoothing circuit  606  configured by the resistor  621  and the capacitor  622 , then outputted to the output terminal  608 .  
         [0069]     It is preferable that the respective gate width of the p-type TFTs  618  and  619  and the n-type TFTs  617  and  620  of analog switching circuits is approximately equal to one another in order to prevent the influence of the gate capacitance of the switching TFTs upon the control signal.  
         [0070]     In this manner, according to this embodiment mode, a rectifier circuit is configured with an amplifier circuit to which a feedback is not performed, a waveform shaping circuit, a resister, and a switch to configure over an insulating substrate by using a TFT without deteriorating a frequency characteristic thereof.  
         [0071]     Accordingly, a semiconductor device such that a display portion and a rectifier circuit are integrally formed over an insulating substrate can be obtained.  
         [heading-0072]     [Embodiment 3] 
         [0073]      FIG. 8  shows a third embodiment mode of the invention and shows an embodiment of a full-wave rectifier circuit. The embodiment mode shown in  FIG. 8  comprises an amplifier circuit  801 , a waveform shaping circuit  802 , a switch  804 , a buffer amplifier  805 , a smoothing circuit  806 , input terminals  807  and  809 , and an output terminal  808 . An operation of this embodiment mode is explained below.  
         [0074]     Signals of opposite phases to each other are inputted to the input terminals  807  and  809 , then amplified by the amplifier circuit  801 . The amplifier circuit  801  is an amplifier circuit of differential input type. An output of the amplifier circuit  801  is inputted to the waveform shaping circuit  802 , and thereby, waveform is shaped into a logic signal. The waveform shaped signal is inputted to the switch  804  as a control signal so as to switch the connection of the switch  804 . The signals inputted to the input terminals  807  and  809  are also inputted to the switch  804  as well as the amplifier circuit  801 .  
         [0075]     When the switch  804  is connected to ‘A’, the inputted signal of the input terminal  807  is inputted to the buffer amplifier  805  and the smoothing circuit  806  in this order.  
         [0076]     When the switch  804  is connected to ‘B’, the inputted signal of the input terminal  809  is inputted to the buffer amplifier  805  and the smoothing circuit  806  in this order. The full-wave rectified waveform is smoothed by the smoothing circuit  806 , then outputted to the output terminal  808 .  
         [0077]     In this manner, according to this embodiment mode, a rectifier circuit is configured with an amplifier circuit to which a feedback is not performed, a waveform shaping circuit, and a switch to configure over an insulating substrate by using a TFT without deteriorating a frequency characteristic thereof.  
         [0078]     Accordingly, a semiconductor device such that a display portion and a rectifier circuit are integrally formed over an insulating substrate can be obtained.  
         [heading-0079]     [Embodiment 4] 
         [0080]      FIG. 9  shows a semiconductor device  901  having a liquid crystal display portion. The semiconductor device  901  comprises a TFT substrate  907 , a counter substrate  908 , a source signal line driver circuit  902 , a gate signal line driver circuit  903 , a pixel portion  904 , a sound signal circuit  905 , and an FPC  906  that are formed over the TFT substrate  907 . The sound signal circuit  905  for operating a speaker comprises a main amplifier  909 , a preamplifier  910 , and a rectifier circuit  911 .  
         [0081]     In the sound signal circuit  905 , a small sound signal is externally inputted and amplified to operate a speaker. In the rectifier circuit  911 , the inputted sound signal is rectified to detect the signal level. In this manner, the sound volume can be adjusted automatically, for example.  
         [0082]     As described above, according to the invention, the rectifier circuit with a preferable frequency characteristic can be configured over an insulating substrate by using a TFT. Described above is the case where a sound signal is rectified, however, other signals can be rectified as well as the sound signal according to the invention and a sensor and the like can be structured.  
         [heading-0083]     [Embodiment 5] 
         [0084]     The semiconductor device of the invention configured as described hereinbefore is applicable to various kinds of electronic apparatus. Such electronic apparatuses using the semiconductor device of the invention are illustrated hereinafter.  
         [0085]     Such electronic apparatuses include a video camera, a digital camera, a head mounted display (goggle-type display), a game machine, a car navigation system, a personal computer, a portable information terminal (mobile computer, mobile phone, electronic book, etc), a television, and the like.  
         [0086]      FIG. 7A  illustrates a laptop personal computer including a body  3201 , a housing  3202 , a display portion  3203 , a keyboard  3204 , an external connection port  3205 , a pointing mouse  3206 , and the like. The semiconductor device of the invention can be applied to the display portion  3203  and the like. The semiconductor device having the rectifier circuit of the invention can be structured a more systematic electronic apparatus.  
         [0087]      FIG. 7B  illustrates a portable information terminal including a body  3301 , a display portion  3302 , operating keys  3304 , an infrared port  3305 , and the like. The semiconductor device of the invention can be applied to the display portion  3302  and the like. The semiconductor device having the rectifier circuit of the invention can be structured a more systematic electronic apparatus.  
         [0088]     The applicable range of the invention is so wide that it can be applied to electronic apparatuses in every field in addition to the laptop personal computer and the portable information terminal. The electronic apparatuses in this embodiment can be realized by any configuration using Embodiment Modes  1  and  2  and Embodiments 1 to 3.  
         [0089]     This application is based on Japanese Patent Application serial No. 2003-310767 filed in Japan Patent Office on 2th, Sep., 2003, the contents of which are hereby incorporated by reference.  
         [0090]     Although the invention has been fully described by way of Embodiment Modes and with reference to the accompanying drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications depart from the scope of the invention hereinbefore defined, they should be constructed as being included therein.