Abstract:
An integrated circuit device includes a differential amplifier, an output driver and a control circuit. The differential amplifier is responsive to a pair of differential input signals and may include a pull-down current source responsive to a pair of bias signals. The output driver has a pull-up path and pull-down path therein. These paths are joined together at an output node. The output driver has a first input terminal electrically coupled to a first output terminal of the differential amplifier. The control circuit is electrically coupled to the differential amplifier and a second input terminal of the output driver. The control circuit is configured to dispose the pull-down path in a nonconductive state when the output node is being switched low-to-high.

Description:
REFERENCE TO PRIORITY APPLICATION 
   This application claims priority to Korean Application Serial No. 2004-89696, filed Nov. 5, 2004, the disclosure of which is hereby incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The present invention relates to integrated circuit devices and, more particularly, to differential amplifier circuits. 
   BACKGROUND OF THE INVENTION 
   A push-pull amplifier circuit including CMOS (complementary metal oxide semiconductor) transistors is a general and often used circuit. An amplifier circuit, which may be used as an audio amplifier, is typically an analog amplifier circuit or a digital amplifier circuit. Analog amplifier circuits are generally realized using a class A amplifier circuit, a class B amplifier circuit or a class AB amplifier, and digital amplifier circuits are generally realized using a class D amplifier circuit. Since linearity of an audio amplifier circuit is considered more important than high efficiency, a linear amplifier, which is an analog amplifier, is generally used as the audio amplifier. 
   Class A, B and AB amplifier circuits are generally used as analog amplifier circuits because of their higher linearity. However, these classes of amplifiers use significant quantities of power when implemented as amplifiers for high output. Thus, analog amplifiers typically have the advantage of high linearity, but the disadvantage of low power efficiency. Specifically, in a class A analog amplifier, much more power is dissipated than the maximum output of the amplifier, and frequently the efficiency of the amplifier is not more than 25%. A class B push-pull amplifier, which is often used to overcome the efficiency problems of the class A amplifier, has two transistors coupled to each other in an emitter follower configuration. The class B push-pull amplifier is more efficient than the class A amplifier, but crossover distortion typically occurs in the class B push-pull amplifier when a signal level is low. 
   Furthermore, when the transistors included in the class B amplifier are alternately turned on and off, the transistors are easily turned on and off while small currents flow, but the transistors cannot be rapidly turned on and off while large currents flow. Therefore, since no bias current flows in the class B amplifier when the amplifier is in an idle state, it is difficult to rapidly turn the transistors on/off in a large current area and hence the total harmonic distortion (THD) is increased. 
   In the class AB amplifier, small currents flow when the amplifier is in a static state. These currents are much smaller than those of the class A amplifier but larger than those of the class B amplifier. As more bias current flows, the features of the class AB amplifier become more similar to those of the class A amplifier, and as less bias current flows, the features of class AB amplifiers become more similar to those of the class B amplifier. 
     FIG. 1  is a circuit diagram of a general differential amplifier circuit  100 .  FIG. 2  is a diagram that illustrates the relationship between a waveform of an output signal of the differential amplifier circuit of  FIG. 1  and a pull-down transistor M 6 . Referring to  FIG. 1 , the differential amplifier circuit  100  includes a bias unit  110 , a voltage control unit  120 , a slew rate control unit  130 , a differential amplifying unit  140 , and an output unit  150 . The differential amplifying unit  140  amplifies a voltage level difference between input signals PINS and NINS and outputs it through a first control node N 1 . The output unit  150  generates an output signal S_OUT through an output node NOUT in response to a voltage level of the first control node N 1  and a voltage level of a second control node N 2 . When a voltage level of the input signal PINS is higher than that of the input signal NINS, the voltage level of the first control node N 1  becomes low and a pull-up transistor M 5  is turned on. When the pull-up transistor M 5  is turned on, the output signal S_OUT rises from a low level to a high level. 
   Moreover, due to currents generated by a current source IB 1  of the bias unit  110 , transistors M 7  and M 8  of the voltage control unit  120  are turned on, and the voltage level of the second control node N 2  remains constant at a level that maintains NMOS pull-down transistor M 6  in a conductive state. Unfortunately, since the pull-down transistor M 6  is kept in a conductive state while the output signal S_OUT rises from a low level to a high level, the current that flows through the pull-down transistor M 6  is wasted. 
   A transistor M 12  of the slew rate control unit  130  is in a turned-off state due to the voltage at the gates of current mirror transistors M 3  and M 4  of the differential amplifying unit  140 . In addition, when the low level voltage of the first control node N 1  is applied to a gate of a transistor M 11 , the transistor M 11  is turned on. Then, a level of a gate of a transistor M 13  becomes high, and the transistor M 13  is kept in a turned-off state. 
   Alternatively, when the voltage level of the input signal NINS is higher than that of the input signal PINS, the voltage level of the first control node N 1  is pulled high and the pull-up transistor M 5  is turned off. When this occurs, the voltage level of the second control node N 2  remains constant due to currents generated by the current source IB 1  of the bias unit  110 . A transistor M 12  of the slew rate control unit  130  is in a turned-on state due to the voltage at the gates of the current mirror transistors M 3  and M 4 . In addition, when a high level voltage of the first control node N 1  is applied to the gate of the transistor M 11 , the transistor M 11  is turned off. Then, when the level of the gate of the transistor M 13  becomes low, the transistor is turned on, and a current IADD is applied to the second control node N 2  through the transistors M 12  and M 13 . When this occurs, the voltage level of the second control node N 2  goes up, and the pull-down transistor M 6  is turned on, and a logic level of the output signal S_OUT goes from high to low. 
   Because the gate voltage of the pull-down transistor M 6  generally remains constant when the pull-down transistor M 6  is turned on, the slew rate of the output signal S_OUT is reduced. Therefore, in the differential amplifier circuit  100  of  FIG. 1 , when the pull-down transistor M 6  is turned on, an additional current IADD is applied to the second control node N 2  to increase the voltage level of the second control node N 2  such that the slew rate of the output signal S_OUT is improved. However, since the pull-down transistor M 6  is kept turned on even when the pull-up transistor M 5  is turned on and the output signal S_OUT is increased from a low level to a high level, the differential amplifier circuit  100  of  FIG. 1  consumes significant power even during a stand-by power state. Accordingly, as illustrated by the timing diagram of  FIG. 2 , the pull-down transistor M 6  remains conductive during low-to-high and high-to-low output switching and during stand-by. This conductive state during all three modes of operation increases the static and dynamic power of the amplifier circuit  100 . Thus, as shown in  FIG. 2 , the pull-down transistor M 6  is constantly turned on regardless of changes in the level of the output signal S_OUT, thus consuming an excessively large amount of current. 
   SUMMARY OF THE INVENTION 
   The present invention provides a differential amplifier circuit capable of reducing current consumption. According to an embodiment of the present invention, there is provided a differential amplifier circuit with a differential amplifying unit for amplifying a voltage difference between input signals and outputting the voltage difference through a first control node. An output unit outputs the amplified voltage difference as an output signal through an output node in response to an output of the first control node and an output of a second control node. A control unit controls a voltage level of the second control node in response to the output of the first control node, and causes an operating current to not flow to the output unit when a level of the output signal goes from a second level to a first level. 
   The first level may be a high level and the second level may be a low level. The output unit includes a pull-up transistor having a first terminal connected to a power source, a gate connected to the first control node and a second terminal connected to the output node, and a pull-down transistor having a first terminal connected to the output node, a gate connected to the second control node and a second terminal connected to a ground voltage. The voltage level of the second control node is kept low when the level of the output signal goes from the second level to the first level. The low voltage level of the second control node is enough to turn off the pull-down transistor. 
   The control unit includes a first control transistor having a first terminal connected to a power source, a gate connected to the first control node and a second terminal connected to a third control node. A first bias transistor and a second control transistor are also provided. The first bias transistor has a first terminal connected to the third control node, a gate connected to a first bias voltage and a second terminal connected to a ground voltage. The second control transistor has a first terminal connected to the power source and a gate connected to gates of current mirror transistors of the differential amplifying unit. A switch transistor is provided. The switch transistor has a first terminal connected to the second terminal of the second control transistor, a gate connected to the third control node and a second terminal connected to the second control node. A second bias transistor is also provided, which has a first terminal connected to the second control node, a gate connected to a second bias voltage and a second terminal connected to the ground voltage. 
   According to another embodiment of the present invention, there is provided a differential amplifier circuit including a differential amplifying unit, which amplifies a voltage difference between input signals and outputs the voltage difference through a first control node and an output unit, which outputs the amplified voltage difference as an output signal through an output node, in response to an output of the first control node and an output of a second control node. A control is also provided, which keeps a voltage level of the second control node low in response to the output of the first control node when a level of the output signal goes from a second level to a first level. 
   According to still another embodiment of the present invention, there is provided a differential amplifier circuit including an amplifying control unit for amplifying a voltage difference between input signals and outputting the voltage difference and an output unit, which includes a pull-up transistor controlled by a first control node and a pull-down transistor controlled by a second control node, for outputting an output signal through an output node. The amplifying control unit turns off the pull-down transistor when the output signal goes from a second level to a first level. 
   Still further embodiments of the invention include an integrated circuit device with a differential amplifier, an output driver and a control circuit therein. The differential amplifier is responsive to a pair of differential input signals. The differential amplifier may also include a pull-down current source responsive to a pair of bias signals. The output driver has a pull-up path and pull-down path therein. These paths are joined together at an output node (e.g., S_OUT). The output driver has a first input terminal electrically coupled to a first output terminal of the differential amplifier. The control circuit is electrically coupled to the differential amplifier and a second input terminal of the output driver. The control circuit is configured to dispose the pull-down path in a nonconductive state when the output node is being switched low-to-high. According to aspects of these embodiments, the first output terminal of the differential amplifier is fed back as an input to the control circuit and the control circuit is responsive to at least one of the pair of bias signals. In some of these embodiments, the pull-up path of the output driver may include a PMOS pull-up transistor having a gate terminal electrically connected to the first output terminal of the differential amplifier and the pull-down path of the output driver may include an NMOS pull-down transistor having a gate terminal electrically connected to an output terminal of the control circuit. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit diagram of a conventional differential amplifier circuit; 
       FIG. 2  is a diagram for explaining the relationship between a waveform of an output signal of the differential amplifier circuit and a pull-down transistor of  FIG. 1 ; 
       FIG. 3  is a circuit diagram of a differential amplifier circuit according to an embodiment of the present invention; and 
       FIG. 4  is a diagram for explaining the relationship between a waveform of an output signal of the differential amplifier circuit and a pull-down transistor of  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thickness of layers and regions are exaggerated for clarity. Like numbers refer to like elements throughout. It will be understood that when an element such as a layer, region or substrate is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. Moreover, each embodiment described and illustrated herein includes its complementary conductivity type embodiment as well. 
     FIG. 3  is a circuit diagram of a differential amplifier circuit  300  according to an embodiment of the present invention, and  FIG. 4  is a diagram for explaining a relationship between a waveform of an output signal of the differential amplifier circuit  300  and a pull-down transistor PDTR of  FIG. 3 . Referring to  FIG. 3 , the differential amplifier circuit  300  includes a differential amplifying unit  310 , an output unit  320 , and a control unit  330 . The differential amplifying unit  310  amplifies a voltage difference between input signals PINS and NINS, and outputs it through a first control node N 1 . The amplifying unit  310  includes a pair of NMOS input transistors M 1  and M 2 , a pair of PMOS load transistors M 3  and M 4  and a current source defined by NMOS transistors M 5  and M 6 . 
   The output unit  320  outputs an amplified voltage difference as an output signal S_OUT through an output node NOUT, in response to outputs of the first control node N 1  and a second control node N 2 . More specifically, the output unit  320  includes a pull-up transistor PUTR and a pull-down transistor PDTR. The pull-up transistor PUTR has a first terminal connected to a power source VDD, a gate connected to the first control node N 1 , and a second terminal connected to the output node NOUT. The pull-down transistor PDTR has a first terminal connected to the output node NOUT, a gate connected to the second control node N 2 , and a second terminal connected to a ground voltage VSS. 
   The control unit  330  controls a voltage level of the second control node N 2  in response to the output of the first control node N 1 , so that an operating current does not flow through the output unit  320  when the level of the output signal S_OUT goes from a second level to a first level. In the illustrated embodiment, the first level is a high level, and the second level is a low level. More specifically, the control unit  330  includes a first control transistor CTR 1 , a second control transistor CTR 2 , a first bias transistor BTR 1 , a second bias transistor BTR 2 , and a switch transistor STR. The first control transistor CTR 1  has a first terminal connected to the power source voltage VDD, a gate connected to the first control node N 1 , and a second terminal connected to a third control node N 3 . The first bias transistor BTR 1  has a first terminal connected to the third control node N 3 , a gate connected to a first bias voltage BIAS 1 , and a second terminal connected to the ground voltage VSS. The second control transistor CTR 2  has a first terminal connected to the power source voltage VDD and a gate connected to gates of current mirror transistors M 3  and M 4  in the differential amplifying unit  310 . The switch transistor STR has a first terminal connected to a second terminal of the second control transistor CTR 2 , a gate connected to the third control node N 3 , and a second terminal connected to the second control node N 2 . The second bias transistor BTR 2  has a first terminal connected to the second control node N 2 , a gate connected to a second bias voltage BIAS 2 , and a second terminal connected to the ground voltage VSS. 
   When the first and second bias voltages BIAS  1  and BIAS 2  are applied to transistors M 5  and M 6 , the transistors M 5  and M 6  are turned on and the differential amplifying unit  310  operates. When the voltage level of the input signal PINS is higher than that of the input signal NINS, since a transistor M 2  of the differential amplifying unit  310  is turned on such that more current flows through M 2  than through a transistor M 1 , the voltage level of the first control node N 1  goes down and the pull-up transistor PUTR is turned on. Then, the output signal S_OUT at a high level is output through the output node NOUT. 
   When the first and second bias voltages BIAS 1  and BIAS 2  are respectively applied to the first and second bias transistors BTR 1  and BTR 2 , the first and second transistors BTR 1  and BTR 2  are turned on and the control unit  330  operates. Since the level of the first control node N 1  is low, the first control transistor CTR 1  of the control unit  330  is turned on and the voltage level of the third control node N 3  goes high. The second control transistor CTR 2  is kept in a turned-off state by a gate voltage of the load transistors M 3  and M 4  of the differential amplifying unit  310 . 
   Then, the switch transistor STR is turned off by a high level voltage of the third control node N 3 , and the level of the second control node N 2  goes low because the second bias transistor BTR 2  remains turned-on. Since the voltage level of the second control node N 2  is low, the pull-down transistor PDTR of the output unit  320  is turned off. Due to the low voltage level of the second control node N 2 , the pull down transistor PDTR can be sufficiently turned off to thereby reduce overall power consumption. In the differential amplifier circuit  300  of  FIG. 3 , unlike the differential amplifier circuit  100  of  FIG. 1 , when the level of the output signal S_OUT goes from a low level to a high level, the voltage level of the second control node N 2  remains low. Therefore, the pull-down transistor PDTR is turned off. 
   Accordingly, during an output signal S_OUT rising period in which the output signal S_OUT is switched from a low level to a high level, current can be prevented from flowing through the pull-down transistor PDTR and current consumption can be reduced. When the voltage level of the input signal PINS is less than that of the input signal NINS, since the transistor M 1  of the differential amplifying unit  310  is turned on such that more current flows through M 1  than through the transistor M 2 , the level of the first control node N 1  becomes high and the pull-up transistor PUTR is turned-off. If the level of the first control node N 1  is high, the first control transistor CTR 1  of the control unit  330  is turned off. Then, because the first bias transistor BTR 1  is in a turned-on state, the level of the third control node N 3  becomes low. The switch transistor STR is thereby turned on by the low level voltage of the third control node N 3 , and the level of the second control node N 2  becomes high since the second control transistor CTR 2  and the switch transistor STR are in a turned-on state. In response, the pull-down transistor PDTR of the output unit  320  is turned on because the level of the second control node N 2  is high, and the output signal S_OUT is transferred from a high level to a low level. At this point, since the voltage level of the second control node N 2  can be raised to a power source level by the power source VDD connected through the second control transistor CTR 2  and the switch transistor STR, the pull-down transistor PDTR is turned on such that a maximum current can flow therethrough and thus the output signal S_OUT is rapidly switched from a high level to a low level, as shown in  FIG. 4 . This rapid switching results in improved slew rate. 
   The output unit  320  of the differential amplifier circuit  300  of  FIG. 3  can include compensating capacitors C 1  and C 2  between the output node NOUT and the first control node N 1  and between the output node NOUT and the second control node N 2 , respectively. Moreover, the differential amplifier circuit  300  of  FIG. 3  may be installed in a driver circuit of a liquid crystal display device, and thus, the operating current consumption and static current consumption of the driver circuit can be reduced. Also, since the differential amplifier circuit  300  of the  FIG. 3  includes a small number of elements (transistors), the circuit size of the driver circuit can be decreased. 
   A differential amplifier circuit according to another exemplary embodiment of the present invention includes a differential amplifying unit, an output unit, and a control unit. The differential amplifying unit amplifies a voltage difference between the input signals and outputs it through a first control node. The output unit outputs the amplified voltage difference as an output signal to an output node in response to an output of the first control node and an output of a second control node. The control node keeps a voltage level of the second control node low in response to the output of the first control node when a level of the output signal goes from a second level to a first level. Due to the low voltage level of the second control node N 2 , a pull down transistor can be sufficiently turned off. 
   While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.