Patent Publication Number: US-6340911-B1

Title: Level conversion circuit having differential circuit employing MOSFET

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a level conversion circuit and, more particularly, to such a circuit that converts an ECL level or CML level signal to a MOS level signal. 
     2. Description of the Related Art 
     As a signal input circuit of a MOS-IC or MOS-LSI, a level conversion circuit is widely employed to convert an input signal having an ECL level or a CML level signal to into an internal signal having a MOS level signal. Such a circuit generally employs a differential circuit composed of bipolar transistors. 
     Specifically, as shown in FIG. 2, a conventional level conversion circuit includes an emitter follower section  101 , an amplitude amplification section  102 , and a level conversion section  103 . The emitter follower section  101  is an emitter follower constructed such that the base of an NPN transistor Q 1  is connected to an input terminal IN, the collector is connected to a high power-supply terminal VCC, and the connection node  001  of the emitter is connected to a first low power-supply terminal GND 1  through a resistor R 1 . 
     The amplitude amplification section  102  is a differential amplifier constructed such that the base of an NPN transistor Q 2  is connected to the node  001 , the connection node  002  of the collector is connected to the high power-supply terminal VCC through a resistor R 2 , and the emitter is connected to a node  004 . The base of an NPN transistor Q 3  is connected to a reference power-supply terminal VR, the connection node  003  of the collector is connected to the high power-supply terminal VCC through a resistor R 3 , and the emitter is connected to a node  004 . The base of an NPN transistor Q 4  is connected to a reference power-supply terminal VCSI, the collector is connected to the node  004 , and the emitter is connected to the first low power-supply terminal GND 1  through a resistor R 4 . 
     The level conversion section  103  is a level conversion section constructed such that the gate of a P-channel MOS transistor P 1  is connected to the node  003 , the source is connected to the high power-supply terminal VCC, and the drain is connected to a node  006 . The gate of a P-channel MOS transistor P 2  is connected to the node  002 , the source is connected to the high power-supply terminal VCC, and the drain is connected to an output terminal OUT. The gate and drain of an N-channel MOS transistor N 1  are connected to a node  006 , and the source is connected to a second low power-supply terminal GND 2 . The gate of an N-channel MOS transistor N 2  is connected to the node  006 , the source is connected to the second low power-supply terminal GND 2 , and the drain is connected to the output terminal OUT. 
     In operation, when the high level of the same current mode logic (CML) level as the high power-supply potential is input to the input terminal IN, the level of the signal is shifted by the amount of the ON-state base-emitter voltage of the NPN transistor by the emitter follower section  101 . The shifted signal is input to the node  001  which is the input of the amplitude amplification section  102 , and the NPN transistor Q 2  is turned on. The NPN transistor Q 3  is turned off and current flows through the resistor R 2 . A level lower by the amount of the voltage drop of the resistor than the high power-supply potential is output to the node  002 , and the high power-supply potential is output to the node  003 . These levels are input to the level conversion section  103 . The P-channel MOS transistor P 2  is turned on and the P-channel MOS transistor P 1  is turned off. The electric charges on the node  006  are pulled out by the N-channel MOS transistor N 1 , and the N-channel MOS transistor N 2  is turned off. The high level of the same CMOS level as the high power-supply potential is output to the output terminal OUT. 
     When the low level of the CML level lower by the amount of a logic amplitude than the high power-supply potential is input to the input terminal IN, the low level of the same CMOS level as the second low power-supply potential is output to the output terminal OUT. 
     In this conventional level conversion circuit, the levels on the nodes  002  and  003 , which are the outputs of the amplitude amplification section  102 , have to be sufficiently low in order to completely turn on one transistor of the P-channel MOS transistors P 1  and P 2  of the input section of the level conversion section  103  and completely turn off the other transistor. If this requirement is not met, the level conversion section will not be operated, or even if it were operated, the operating speed would become slow. 
     The low level on the output of the amplitude amplification section  102  must be a sufficiently low level so that one transistor of the input transistors of the level conversion section  103  can be completely turned on even when the value fluctuates and becomes the highest value. However, if the low level of the output is set to a sufficiently low value in view of the case where it is the maximum value, there will be the problem that the collector potentials of the NPN transistors Q 2  and Q 3 , which constitute the current switches of the amplitude amplification section  102 , will be overreduced and that the transistors will be saturated and the operating speeds will be reduced, when the low level of the output fluctuates conversely in the lowest direction. 
     In order to avoid the saturation of the NPN transistors Q 2  and Q 3  at the current switch section of the amplitude amplification section  102 , there is a method where a diode is inserted between the emitter of the NPN transistor Q 1  and the node  001  so that the input signal level to a transistor constituting the current switch section is further reduced by the amount of the ON-state base-emitter voltage of the transistor. When the first low power-supply terminal GND 2  is sufficiently low and the potential on the reference power-supply terminal VCSI is also low, there is no problem. However, when the first power-supply potential GND 1  is high and becomes equal to the potential of the second low power-supply terminal GND 2  and when the power-supply voltage, which is the potential difference between the high power-supply potential and the low power-supply potential, is low, there arises the problem that the collector potential of the NPN transistor Q 4  constituting a current source falls and that the NPN transistor Q 4  is saturated, if an input signal level is lowered. 
     Making the low level on the output of the amplitude amplification section  102  sufficiently large so that the MOS transistors P 1  and P 2  of the level conversion section  103  can be completely turned on or off and also preventing the saturation of the transistors of the current switch and current source sections become even more severe as the power-supply voltage becomes lower. 
     SUMMARY OF THEE INVENTION 
     Accordingly, it is an object of the present invention to provide a level conversion circuit which operates at high speeds even at a low power-supply voltage. 
     A level conversion circuit according to the present invention includes an emitter follower section for receiving an ECL or a CML level signal and outputting a level-shifted signal, a differential amplitude amplification section provided with a pair of MOS transistors of one channel type, their sources being connected in common and also being connected to a first low power-supply terminal through a current source, an output of said emitter follower section being input to the gate of one transistor of the pair of MOS transistor, a complementary signal of the output of the emitter follower section or an input reference voltage being input to the gate of the other transistor, and their drains being connected to a high power-supply terminal through road devices, respectively and outputting differentially. The circuit further includes a level conversion section provided with a current mirror circuit which comprises a pair of MOS transistors of an opposite channel type, where their sources are connected to the high power-supply terminal and their gates receive a differential output of the amplitude amplification section and a pair of MOS transistors of the one channel type having their drains respectively connected to the drains of the MOS transistors, sources connected to a second low power-supply terminal, and gates connected in common and also connected to one drain thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described in further detail with reference to the accompanying drawings, in which: 
     FIG. 1 is a circuit diagram showing an embodiment of a level conversion circuit of the present invention; and 
     FIG. 2 is a circuit diagram showing an example of a conventional level conversion circuit. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, a level conversion circuit according to an embodiment of the present invention includes an emitter follower section  101 , an amplitude amplification section  102 , and a level conversion section  103 . The emitter follower section  101  is an emitter follower constructed such that the base of an NPN transistor Q 1  is connected to an input terminal IN, the collector is connected to a high power-supply terminal VCC, and the connection node  001  of the emitter is connected to a first low power-supply terminal GND 1  through a resistor R 1 . 
     The amplitude amplification section  102  is a differential amplifier constructed such that the gate of an N-channel MOS transistor M 1  is connected to the node  001 , the connection node  002  of the drain is connected to the high power-supply terminal VCC through a resister R 2 , and the source is connected to a node  004 . The gate of an N-channel MOS transistor M 2  is connected to a reference power-supply terminal VR, the connection node  003  of the drain is connected to the high power-supply terminal VCC through a resistor R 3 , and the source is connected to a node  004 . The base of an NPN transistor Q 4  is connected to a reference power-supply terminal VCSI, the collector is connected to the node  004 , and the emitter is connected to the first low power-supply terminal GND 1  through a resistor R 4 . 
     The level conversion section  103  is a level conversion section constructed such that the gate of a P-channel MOS transistor P 1  is connected to the node  003 , the source is connected to the high power-supply terminal VCC, and the drain is connected to a node  006 . The gate of a P-channel MOS transistor P 2  is connected to the node  002 , the source is connected to the high power-supply terminal VCC, and the drain is connected to an output terminal OUT, The gate and drain of an N-channel MOS transistor N 1  are connected to a node  006 , and the source is connected to a second low power-supply terminal GND 2 . The gate of an N-channel MOS transistor N 2  is connected to the node  006 , the source is connected to the second low power-supply terminal GND 2 , and the drain is connected to the output terminal OUT. 
     In operation, when the high level of the same current mode logic (CML) level as the high power-supply potential is input to the input terminal IN, the level of the signal is shifted by the amount of the ON-state base-emitter voltage of the NPN transistor by the emitter follower section  101 . The shifted signal is input to the node  001  which is the input of the amplitude amplification section  102 , and the N-channel MOS transistor M 1  is turned on. The N-channel MOS transistor M 2  is turned off and current flows through the resistor R 2 . A level lower by the amount of the voltage drop of the resistor than the high power-supply potential is output to the node  002 , and the high power-supply potential is output to the node  003 . These levels are input to the level conversion section  103 . The P-channel MOS transistor P 2  is turned on and the P-channel MOS transistor P 1  is turned off. The electric charges on the node  006  are pulled out by the N-channel MOS transistor N 1 , and the N-channel MOS transistor N 2  is turned off. The high level of the same CMOS level as the high power supply potential is output to the output terminal OUT. 
     When the low level of the CML level lower by the amount of a logic amplitude than the high power-supply potential is input to the input terminal IN, the low level of the same CMOS level as the second low power-supply potential is output to the output terminal OUT. 
     In the level conversion circuit of the embodiment of the present invention, MOS transistors are used in the transistors which constitute the current switches of the amplitude amplification section, Therefore, in the case where an output level is set to a sufficiently low value so that the input transistors P 1  and P 2  of the level conversion section  103  can be completely turned on or off when the low level on the output of the amplitude amplification section  102  fluctuates and assumes the highest value, the output level can be set to a sufficiently low value without causing a reduction in an operating speed which is caused due to saturation, unlike bipolar transistors, even when the low level on the output fluctuates conversely in the lowest direction, and consequently-the high-speed operation of the level conversion section  103  also becomes possible. Because a low level can be output at the logic amplitude section without reducing the level of an input signal, it is possible to operate the NPN transistor of the current source without saturating it even at a low voltage. 
     As has been described above, the level conversion circuit according to the present invention constitutes the current switches of the differential amplifier, which is the amplitude amplification section, by MOS transistors, whereby there is no reduction in the speed which is caused by the saturation of the current switches resulting from a fluctuation in the output level in the amplitude amplification section and the output level can be set so as to be sufficiently low. Accordingly, there is the advantage that high-speed operation is made possible even when power-supply voltage is reduced. 
     While the invention has been described with reference to a preferred embodiment thereof, the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claim.