Abstract:
A driving method for improving power efficiency of an operational transconductance amplifier. The operational transconductance amplifier comprises a first current route and a second current route symmetrical to the first current route. Both of the first current route and the second current route comprise a plurality of transistors. Each of the transistors of the first current route has a smaller width/length ratio than the corresponding transistors of the second current route. The driving method comprises turning on the transistors of the first current route for outputting a reference current so that the second current route outputs a mirror current, which is greater than the reference current, corresponding to the reference current.

Description:
BACKGROUND OF INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a driving method of an operational transconductance amplifier, and more particularly, to a driving method for improving power efficiency of an operational transconductance amplifier.  
           [0003]    2. Description of the Prior Art  
           [0004]    Advantages of the liquid crystal display (LCD) include lighter weight, less electrical consumption, and less radiation contamination. Thus, the LCD has been widely applied to several portable information products such as notebooks, PDAs, etc. The LCD is gradually replacing the CRT monitors of conventional desktop computers. Incident light will produce different polarization or refraction when alignments of these liquid crystal molecules are different. The LCD utilizes the characteristics of the liquid crystal molecules to control the light transmittance and produce gorgeous images.  
           [0005]    Please refer to FIG. 1. FIG. 1 is a schematic diagram of a conventional thin film transistor (TFT) liquid crystal display (LCD) monitor  10 . The LCD monitor  10  comprises an LCD panel  12 , a controller  14 , a first driving circuit I 6 , a second driving circuit I 8 , a first voltage generator  20 , and a second voltage generator  22 . The LCD panel  12  comprises two substrates. An LCD layer is filled in the space between these two substrates. One substrate is disposed with a plurality of first data lines  24 , a plurality of second data lines  26  which are perpendicular to the first data lines  24 , and a plurality of thin film transistors  28 . The other substrate is disposed with a common electrode (not shown) for providing a stable voltage (Vcom) by the first voltage generator  20 . For convenience, only four thin film transistors  28  are illustrated in FIG. 1. In fact, the thin film transistors  28  are disposed on the LCD panel  12  in a matrix format. That is, each of the thin film transistors  28  is disposed on the intersection of each of the first data lines  24  and each of the second data lines  26 . Each first data line  24  corresponds to a column of the LCD panel  12 , each second data line  24  corresponds to a row of the LCD panel  12 , and each thin film transistor  28  corresponds to a pixel. Additionally, the circuit characteristic formed by the substrates can be deemed an equivalent capacitor  30 .  
           [0006]    A driving principle for the conventional LCD monitor  10  is described as follows. When the controller  14  receives horizontal synchronization signals or vertical synchronization signals, the controller  14  provides corresponding control signals respectively to the first driving circuit I 6  and to the second driving circuit I 8 . Then the first driving circuit I 6  and the second driving circuit I 8  provide input signals to the first data lines  24  and the second data lines  26  by determining the control signals. Next, the input signals received by the first data lines  24  and the second data lines  26  change the states of the thin film transistors  28  and the voltage of the equivalent capacitor  30 . Finally, the alignment of the liquid crystal molecules and the light transmittance are changed. Therefore, changing the voltage of the input signals provided from the first driving circuit I 6  and from the second driving circuit I 8  can change the gray level of the corresponding pixel. For example, if the second driving circuit  26  transmits a pulse to the second data lines  18  to turn on the thin film transistor  28 , the first driving circuit I 6  can transmit signals to the equivalent capacitor  30  through the first data lines  24  and the thin film transistors  28  to control the gray level of a corresponding pixel. Additionally, the signals, transmitted from the first driving circuit I 6 , of the first data lines  24  are generated from the second voltage generator  22 .  
           [0007]    Please refer to FIG. 2. FIG. 2 is a schematic diagram illustrating an operational amplifier buffer (op buffer)  40  circuit of the conventional LCD monitor  10  shown in FIG. 1. The op buffer  40  is a class-A driver amplifier. The op buffer  40  is used to drive the LCD monitor  10  so that each pixel on the LCD monitor  10  can reach a predetermined gray level. When a voltage Vin turns on a transistor  41  and a bias voltage Vb turns on transistors  42 ,  43 , a first stage circuit  44  of the op buffer  40  will drive a second stage circuit  45  of the op buffer  40  to generate a corresponding output voltage Vout with current I 3 . The voltage Vout is used to drive the LCD monitor  10 . Because the op buffer  40  is a class-A driver amplifier, it bears a high power efficiency. That is, most power-consumption of the op buffer  40  is used to drive the LCD monitor  10 . For example, the sum of currents I 1 , and I 2  is assumed to be 10 uA and the current I 3  derived from the op buffer  40  might be 100 uA. That is, the current I 3  is much greater than the currents I 1 , and I 2 . In other words, most electric power consumed by the op buffer  40  is used for driving the LCD monitor  10 .  
           [0008]    Concerning a dot inversion driving applied on the LCD monitor  10 , a positive driving buffer is used for pulling up voltage of a pixel from a negative polarity to a positive polarity, and a negative driving buffer is used for pushing down voltage of the pixel from the positive polarity to the negative polarity. Therefore, each of the positive driving buffer and the negative driving buffer is only responsible for driving pixels toward a positive or a negative polarity according to the dot inversion driving. The class-A operational amplifier with small bias current is generally adopted to be the required positive or negative driving buffer owing to great power efficiency on driving single polarity. Although the op buffer  40 , which is a class-A operational amplifier, bears high power efficiency, yet it still needs a compensating capacitor  46  and an output resistor  47  to control the output slew rate of the op buffer  40 . Thus, a bigger layout area and a higher manufacturing cost of the op buffer  40  are inevitable.  
           [0009]    Please refer to FIG. 3. FIG. 3 is a schematic diagram illustrating a conventional operational transconductance amplifier (OTA)  50  circuit. A voltage Vin turns on a transistor  51 . A bias voltage Vb turns on a transistor  52  and keeps the transistor  52  in a saturation state. Because the voltage at node D is not large enough to turn on a transistor  53  in the beginning, the transistor  53  is cut-off and current I 5  equals current I 4 . Although the OTA  50  bears many advantages such as a smaller size, a simpler structure, and a good slew rate (no extra compensating capacitors or output resistors are necessary), yet the power efficiency of the OTA  50  is not high. As described previously, since the current I 5  is equal to the current I 6  before the voltage at node D is equal to the voltage Vin to turn on the transistor  53 , the power efficiency of the OTA is only 50% (power efficiency=I 6 /(I 5 +I 6 )).  
           [0010]    In conclusion, contrary to the op buffer  40 , the OTA  50  bears advantages of smaller size and simpler structure. However, the low power efficiency for the OTA  50  prevents it from being applied to the LCD monitor  10 .  
         SUMMARY OF INVENTION  
         [0011]    It is therefore a primary objective of the claimed invention to provide an operational transconductance amplifier with simpler structure, smaller size, but higher power efficiency to solve the above-mentioned problems.  
           [0012]    The claimed invention provides a driving method for improving power efficiency of an operational transconductance amplifier. The operational transconductance amplifier comprises a first current route and a second current route symmetrical to the first current route. Both of the first current route and the second current route comprise a plurality of transistors. Each of the transistors of the first current route has a smaller width/length ratio than the corresponding transistors of the second current route. The driving method comprises turning on the transistors of the first current route for outputting a reference current so that the second current route outputs a mirror current, which is greater than the reference current, corresponding to the reference current.  
           [0013]    It is an advantage of the claimed invention that the operational transconductance amplifier can achieve both high power efficiency and good slew rate by only adjusting the ratio between the W/L ratio of the transistors disposed on the first current route and the W/L ratio of the transistors disposed on the second current route. Therefore, a great amount of current intensity is generated at an output terminal of the operational transconductance amplifier until the voltage level of the output terminal approaches a required value, and high power efficiency is acquired as well.  
           [0014]    These and other objectives of the claimed invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0015]    [0015]FIG. 1 is a schematic diagram of a prior art TFT LCD monitor.  
         [0016]    [0016]FIG. 2 is a schematic diagram of an op buffer circuit of the prior art TFT LCD monitor.  
         [0017]    [0017]FIG. 3 is a schematic diagram of a prior art operational transconductance amplifier circuit.  
         [0018]    [0018]FIG. 4 is a schematic diagram of an operational transconductance amplifier (OTA) circuit according to first embodiment of the present invention.  
         [0019]    [0019]FIG. 5 is a schematic diagram of an operational transconductance amplifier circuit according to the second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0020]    Please refer to FIG. 4. FIG. 4 is a schematic diagram of an operational transconductance amplifier (OTA) circuit  60  according to a first embodiment of the present invention. The OTA  60  comprises a plurality of transistors,  62 ,  63 ,  64 ,  65 , and  66 . The transistors  62 ,  63 , and  64  are n-channel metal oxide semiconductor transistors (NMOS transistors); the transistors  65 ,  66  are p-channel metal oxide semiconductor transistors (PMOS transistors). The transistors  63 ,  65  form a first current route  68 , and the transistors  64 ,  66  form a second current route  70 . The transistors  63 ,  65  and the transistors  64 ,  66  respectively form a complementary metal oxide semiconductor transistor (CMOS transistor). In the preferred embodiment, the first current route  68  connects to the second current route  70  through a current mirror formed by the transistors  65 ,  66 , and a width/length ratio (W/L ratio) of the transistors  63 ,  65  (disposed on the first current route  68 ) is smaller than that of the transistors  64 ,  66  (disposed on the second current route  70 ). When the transistor  62  is maintained in a saturation state, a reference current I 7  (corresponding to a bias voltage Vb) will flow through the transistor  62 .  
         [0021]    In the preferred embodiment, the W/L ratio of the transistors  64 ,  66  is assumed to be ten times as large as that of the transistors  63 ,  65 . The voltage Vb turns on the transistor  62  and simultaneously generates the reference current I 7 . In the beginning (initial phase), an input voltage Vin is greater than an output voltage Vf at node F. Because the voltage Vin is temporarily greater than the voltage Vf, the transistor  63  is turned on, and the transistor  64  is kept off. A current I 8  passing through the first current route  68  is equal to the reference current I 7 . In addition, an output current I 9  flowing through node F due to the current mirror is ten times as large as the current I 8 . The reason is that the W/L ratio of the transistor  66  is ten times as large as that of the transistor  65 . The current I 9 , therefore, is also ten times as large as the current I 8 . At the moment the current I 10  is equal to the current I 9 . The current I 10  is provided to a load, such as a liquid crystal cell. The power efficiency of the OTA  60  is (I 9 /(I 8 +I 9 ))=91%. For example, if the reference current I 7  is 10 uA, the current I 8  is 10 uA and the current I 9  is 100 uA. In other words, the total current consumption is 110 uA, and the actual current on driving is 100 uA.  
         [0022]    When the output voltage Vf finally equals the input voltage Vin, the OTA  60  steps into a stable state (stable phase). Therefore, the output voltage Vf equaling Vin is large enough to turn on the transistor  64 . That is, the sum of currents I 8 , and I 9  is nearly equal to the reference current I 7 . Because the transistor  62  is used for generating a reference circuit I 7 , the transistor  62  is always maintained in a saturation state, and the reference current I 7  will not change no matter if the OTA  60  is stable or not. That is, the output current I 9  is still ten times as large as the input current I 8 , and the sum of the current I 8  and the output current I 9  still equals the reference current I 7  when the OTA  60  reaches a stable state (Vf is equal to Vin for example). If the reference current I 7  is 10 uA in the beginning, the input current I 8  is 10 uA as well. The output current I 9  is 100 uA due to the current mirror and the predetermined W/L ratio relation. When the OTA  60  becomes stable, the reference current I 7  is still 10 uA, and the output current I 9  is ten times as large as the current I 8 . However, the sum of the currents I 8 , and I 9  equals I 7  (10 uA). Therefore, the output current I 9  is 100/11 uA, and the current I 8  becomes 10/11 uA eventually. It is noteworthy that the voltage Vf is equal to voltage Vin in the end. That is, a required voltage level (Vin) at node F is obtained because of the conductive transistor  64 .  
         [0023]    As described previously, the power efficiency of the OTA  60  is measured by the following equation. (I 9 /(I 8 +I 9 ))=N/(N+1), wherein N=(W/L ratio of the transistors on the second current route  70 )/(W/L ratio of the transistors on the first current route  68 ). Therefore, a user can acquire the desired power efficiency of the OTA  60  by altering a corresponding N value.  
         [0024]    Please refer to FIG. 5, which is a schematic diagram of an operational transconductance amplifier circuit  80  according to asecond embodiment of the present invention. The operational transconductance amplifier circuit  80  is similar to the operational transconductance amplifier circuit  60 . In the operational transconductance amplifier circuit  60 , the transistors  62 ,  63 , and  64  are NMOS transistors, and the transistors  65 , and  66  are PMOS transistors. However, the transistors  62   a ,  63   a , and  64 ain FIG. 5 are PMOS transistors instead of NMOS in FIG. 4, and the transistors  65   a , and  66 ain FIG. 5 are NMOS transistors instead of PMOS transistors in FIG. 4. In addition, the transistors  63   a  and  65   a  form a first current route  68   a , and the transistors  64   a  and  66   a  form a second current route  70   a  as well. In the preferred embodiment, the first current route  68   a  connects to the second current route  70   a  through a current mirror formed by the transistors  65   a  and  66   a . The operation of the operational transconductance amplifier circuit  80 , therefore, is identical to that of the operational transconductance amplifier circuit  60 , and it is not repeated again for simplicity.  
         [0025]    In contrast to the prior art, the claimed invention provides a method for improving the power efficiency of the OTA  60  of the LCD monitor  10  by adjusting the W/L ratio of the transistors on current routes. The OTA  60 , according to the present invention, does not need any extra output resistance or compensation capacitor to control the output slew rate, so a smaller layout is possible. Because the OTA  60  bears high power efficiency, it is suitable to be used for driving an LCD panel.  
         [0026]    Following the detailed description of the present invention above, those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.