Abstract:
A circuit for shifting the potential level of an input signal toward higher potentials is added to a conventional differential ECL circuit in order to shift levels of emitter potentials of npn bipolar transistors forming a current switch toward higher potentials. Thus, the ECL circuit is improved to ensure a continuous flow of a current and to maintain stable operations even at an instant where base potentials of the npn bipolar transistors are switched by a standard ECL-level signal even when the power source voltage is around −2 V.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a semiconductor and, more particularly, to an ECL (Emitter Coupled Logic) integrated circuit. 
     Semiconductor integrated circuits using bipolar transistors for transmitting logic signals are generally classified into TTL circuits, ECL circuits, and others. In TTL circuits, high potential levels range from 2.4 V to 2.5 V, low potential levels range from 0 V to 0.4 V. In ECL circuits, potential levels are −0.7 V to −1.9 V, and amplitudes are around 1 V, which are largely different from those of TTL circuits and require different usages. 
     FIG. 4 is a circuit diagram of a conventional differential ECL circuit, and FIG. 5 is an explanatory diagram schematically showing potentials at internal nodes in the differential ECL circuit of FIG. 4 when the base-emitter voltage V BE  of npn bipolar transistors is 0.9 V and the reference potential GND is 0 V. 
     Construction of the differential ECL circuit is explained below. 
     Resistors R 1 , R 2  are supplied at their first ends with the reference potential GND, which is typically the ground potential. Connected to the other ends of the resistors R 1  and R 2  are collectors of npn bipolar transistors Q 1  and Q 2 . Introduced to bases of these two npn bipolar transistors Q 1 , Q 2  are input signals IN and /IN (/ denotes inverted signal) of potential levels between −0.9 V and −1.7 V. Commonly connected emitters of the npn bipolar transistors Q 1 , Q 2  are connected to one end of a resistor R 3 , and the power source potential V EE  is applied to the other end of the resistor R 3 . These elements form a current switch S 1 , and output from the current switch S 1  is used as input to an emitter follower. That is, bases of npn bipolar transistors Q 3 , Q 4  whose collectors are supplied with the reference potential GND are connected to the connection node of a resistor R 1  and the collector of the npn bipolar transistor Q 1  and the connection node of a resistor R 2  and the collector of the npn bipolar transistor Q 2 . Emitters of the npn bipolar transistors Q 3 , Q 4  are connected to given ends of resistors R 4 , R 5  whose other ends are supplied with the power source potential V EE . Output signals OUT and /OUT of the differential ECL circuit are taken out from connection nodes of the npn bipolar transistors Q 3 , Q 4  and given ends of resistors R 4 , R 5 . The resistors R 1  and R 2  have the same resistance value R. 
     In the conventional differential ECL circuit, input signals IN and /IN of potential levels from −0.9 V to −1.7 V are introduced to the current switch S 1 . Among two npn bipolar transistors Q 1 , Q 2  forming the current switch S 1 , one having a higher base voltage is turned ON, and emitter potentials of Q 1 , Q 2  represent values lower by corresponding base-emitter voltages V BE  than the higher base potential. 
     The differential ECL circuit operates as explained below. 
     One of the resistors R 1 , R 2  connected to one of the npn bipolar transistors Q 1 , Q 2  currently in its ON state permits a current I to flow from a reference voltage point for giving the reference potential GND to one end, and the potential of its other end decreases to −R X I. Further, in the emitter follower, a potential −R X I−V BE  which is lower than the potential −R X I by the base-emitter voltage V BE  of the ON-state npn bipolar transistor Q 3  or Q 4  is taken out from the output terminal as an output signal OUT or /OUT of a LOW level. On the other hand, the other of the resistors R 1 , R 2  connected to OFF-state one of the npn bipolar transistors Q 1 , Q 2  does not introduce the current from the reference potential point for giving the reference potential GND to one end, and maintains a potential substantially equal to the reference potential GND at the other end. Therefore, in the emitter follower, a potential GND−V BE  lower than the reference potential GND by the base-emitter voltage V BE  of the ON-state npn bipolar transistor Q 3  or Q 4  is taken out from the output terminal as an output signal OUT or /OUT of a HIGH level. 
     As a result, when, for example, the base potential of the npn bipolar transistor Q 1  is −0.9 V and the base potential of Q 2  is −1.7 V, the npn bipolar transistor Q 1  is turned ON, and emitter potentials of Q 1 , Q 2  become −0.9 V−V BE =−1.8 V. 
     In the above-explained differential ECL circuit, however, when the base potential of the npn bipolar transistor Q 1  is switched from −0.9 V to −1.7 V, and the base potential of Q 2  from −1.7 V to −0.9, the base potential of Q 1  and the base potential of Q 2  transitionally becomes equal, and the emitter potentials of Q 1  and Q 2  decrease to −1.3 V−V BE =−2.2 V, which causes the following problem. If the power source potential V EE  is higher than −2.2 V, then no current flows in the current switch S 1  for a while, where two outputs OUT and /OUT, which must be differential, both remain in the HIGH level. Therefore, the conventional differential ECL circuit cannot ensure stable operations unless the power source potential V EE  is lower than −2.2 V, and therefore needs an additional power source for generating the power source potential V EE  in addition to the power source for generating the a terminal voltage V TT  (=−2 V). 
     As reviewed above, there is a difficulty in maintaining stable operations required as a stable logic circuit by applying a typical ECL-level signal to a differential ECL circuit and by using a low voltage power source of approximately −2 V to activate the circuit. Not only for differential ECL circuit, the above discussion essentially applies also to single-end ECL circuits in which only structural difference lies in fixing one of input signals to a predetermined potential (typically, −1.3 V). 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a semiconductor integrated circuit, namely an ECL circuit, which can be operated in a stable mode even by a low voltage power source of approximately −2 V. 
     According to the invention, there is provided a semiconductor integrated circuit comprising an additional circuit for generating a differential input signal of a potential within a predetermined range in response to an input signal; and an ECL circuit to which the differential input signal is introduced. Since the circuit for shifting the potential level of the input signal toward higher potentials is added to a conventional differential ECL circuit so as to shift the levels of emitter potentials of npn bipolar transistors forming a current switch toward higher potentials, an improved ECL circuit can be realized, which can ensure a continuous flow of a current and can maintain stable operations even in an instant where base potentials of the npn bipolar transistors are switched by a standard ECL-level signal even when the power source voltage is around −2 V. 
     A second aspect of the invention is configured to introduce a second reference potential V REF , in lieu of an input signal V /IN ′, to bases of the npn bipolar transistors forming the current switch, but it is essentially based on the same theory as that of the first aspect. Therefore, also the semiconductor integrated circuit according to the second aspect can realize an ECL circuit capable of maintaining stable operations. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a semiconductor integrated circuit according to a first embodiment of the invention; 
     FIG. 2 is an explanatory diagram schematically showing potentials at internal nodes in the semiconductor integrated circuit of FIG. 1 when the base-emitter voltage V BE  of npn bipolar transistors is 0.9 V, reference potential GND is 0 V, and power source potential is V EE ; 
     FIG. 3 is a circuit diagram of a semiconductor integrated circuit according to a second embodiment of the invention; 
     FIG. 4 is a circuit diagram of a conventional differential ECL circuit; and 
     FIG. 5 is an explanatory diagram schematically showing potentials at internal nodes in the differential ECL circuit when the base-emitter voltage V BE  of npn bipolar transistors is 0.9 V, and the reference potential GND is 0 V. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Semiconductor integrated circuits embodying the invention are explained below with reference to the drawings. 
     Features of semiconductor integrated circuits according to the invention lie in adding to a conventional differential ECL circuit a circuit for shifting the potential level of the input signal toward higher potentials to shift the potential level of emitter potentials of npn bipolar transistors forming a current switch toward higher potentials. 
     Using this arrangement, an ECL circuit can be realized, which can be continuously supplied with a current even and can maintain stable operations even upon switching of base potentials of the npn bipolar transistors forming the current switch by a standard ECL-level signal, also when the power source voltage is approximately −2 V. 
     FIG. 1 is a circuit diagram of a semiconductor integrated circuit taken as a first embodiment of the invention, and FIG. 2 is an explanatory diagram schematically showing potentials at internal nodes in the semiconductor integrated circuit of FIG. 1 where the base-emitter voltage V BE  of npn bipolar transistors is 0.9 V, reference potential GND is 0 V, and power source potential is V EE . 
     The semiconductor integrated circuit according to the first embodiment has the following arrangement. 
     The semiconductor integrated circuit embodying the invention uses an additional circuit located at an input side of the current switch in the conventional differential ECL circuit to shift the potential level of an input signal introduced to the current switch toward higher potentials. In the additional circuit, a reference potential GND, which is typically the ground potential, is supplied to the anode of a diode D 1  and to one end of a resistor R 6 . The cathode of the diode D 1  and the other end of the resistor R 6  are commonly connected to the collector of an npn bipolar transistor Q 6 . The bipolar transistor Q 6  is supplied with one of input signals, IN, through the base, and its emitter is connected to one end of a resistor R 7 . The other end of the resistor R 7  is supplied with the power source potential V EE . Similarly, the reference potential GND, typically the ground potential, is applied to the anode of a diode D 2  and to one end of a resistor R 8 . The cathode of the diode D 2  and the other end of the resistor R 8  are commonly connected to the collector of another npn bipolar transistor Q 5 . The bipolar transistor Q 5  is supplied with the other input signal /IN at the base, and its emitter is connected to one end of a resistor R 9 . The other end of the resistor R 9  is supplied with the power source potential V EE . The diodes D 1 , D 2  prevent an excessive current from flowing into the resistors R 6 , R 8  to prevent an acceptable decrease in potential at the other ends of the resistors R 6 , R 8 . 
     V IN ′ for use as one input signal of the differential ECL circuit is taken out from the connection node of the diode D 1  and the resistor R 6  with the collector of the npn bipolar transistor Q 6  in the additional circuit. Similarly, V /IN ′ for use as the other input signal of the differential ECL circuit is taken out from the connection node of the diode D 2  and the resistor R 8  with the collector of the npn bipolar transistor Q 5 . 
     The part of the differential ECL circuit has the same arrangement as that of the conventional circuit as explained below. The resistors R 1 , R 2  are supplied at their first ends with the reference potential GND, which is typically the ground potential. Connected to the other ends of the resistors R 1  and R 2  are collectors of npn bipolar transistors Q 1  and Q 2 . Introduced to bases of these two npn bipolar transistors Q 1 , Q 2  are input signals V IN ′ and V /IN ′ taken out from the additional circuit. Commonly connected emitters of the npn bipolar transistors Q 1 , Q 2  are connected to one end of the resistor R 3 , and the power source potential V EE  is applied to the other end of the resistor R 3 . These elements form the current switch S 1 , and output from the current switch S 1  is used as input to the emitter follower. That is, bases of npn bipolar transistors Q 3 , Q 4  whose collectors are supplied with the reference potential GND are connected to the connection node of the resistor R 1  with the collector of the npn bipolar transistor Q 1  and the connection node of the resistor R 2  with the collector of the npn bipolar transistor Q 2 . Emitters of the npn bipolar transistors Q 3 , Q 4  are connected to given ends of resistors R 4 , R 5  whose other ends are supplied with the power source potential V EE . Output signals OUT and /OUT of the differential ECL circuit are taken out from connection nodes of the npn bipolar transistors Q 3 , Q 4  with given ends of the resistors R 4 , R 5 . The resistors R 1  and R 2  have the same resistance value R. 
     Explained below are behaviors of the semiconductor integrated circuit according to the first embodiment of the invention. This embodiment employs a relatively low power source potential of V EE =−2 V. 
     When the input signal IN having the potential of −0.9 V is applied to the base of the npn bipolar transistor Q 6  and the input signal /IN having the potential of −1.7 V to the base of Q 5 , the base-emitter voltage V BE  of the npn bipolar transistor Q 6  becomes 0.9 V or more in relation to the power source potential V EE . Thus, Q 6  is turned ON, and a current flows in the resistor R 7 . While the current flows into the resistor R 7 , in the resistor R 6  supplied with the reference potential GND at one end, the potential at its other end becomes −0.9 V due to restriction of the forward voltage of the diode D 1 . A potential signal appearing at the other end of the resistor R 6  is introduced as one input signal V IN ′ to the base of the npn bipolar transistor Q 1 . On the other hand, since the base-emitter voltage V BE  of the npn bipolar transistor Q 5  becomes less than 0.9 V in relation with the power source potential V EE , Q 5  is turned OFF, and no current flows in the resistor R 9 . Therefore, in the resistor R 8  supplied with the reference potential GND at one end, the potential at its other end becomes substantially equal to the reference potential GND. A potential signal appearing at the other end of the resistor R 8  is introduced as the other input signal V /IN ′ to the base of the npn bipolar transistor Q 2 . 
     While the base potential of the npn bipolar transistor Q 5  changes from −0.9 V to −1.7, and the base potential of Q 6  from −1.7 V to −0.9 V, when their base potential values pass through V EE +V BE =−1.1 V, approximately, npn bipolar transistors Q 5  and Q 6  change between ON and OFF. In this process, the potential at the other end of the resistor R 6  and the potential at the other end of the resistor R 8  cross at the intermediate potential −0.45 V between 0 V and −0.9 V. Although the potential at the other end of the resistor R 6  and the potential at the other end of the resistor R 8  are introduced to the base of the npn bipolar transistor Q 1  and the base of Q 2 , respectively, emitter potentials of the npn bipolar transistors Q 1  Q 2  never decrease below −0.45 V−V BE =−1.35 V, even transitionally, and these potentials are always maintained above −1.35 V. Therefore, even if the power source potential V EE  is −2 V, the current flowing in the current switch S 1  is never blocked transitionally, and never causes unstable transmission of the logic signal. Since a stable behavior is ensured even when the power source potential V EE , a single electric power source can be commonly used as both the electric power source for generating the terminal voltage V TT  (=−2 V) and the electric power source for generating the power source voltage V EE . 
     In the semiconductor integrated circuit according to the invention described above, diodes D 1 , D 2  and resistors R 6 , R 8  are used to form the circuit for shifting the potential level of the input signal to the current switch of the ECL circuit toward higher potentials. However, diodes D 1 , D 2  may be omitted from the circuit arrangement in applications with small fluctuations in power source potential V EE . In other applications, npn bipolar transistors may be used in lieu of the diodes, so as to for the anode by short-circuiting the base and the collector and to use the emitter as the cathode. 
     FIG. 3 is a circuit diagram of another semiconductor integrated circuit taken as a second embodiment of the invention. 
     The semiconductor integrated circuit according to the second embodiment additionally uses a circuit located in an input-side of the single-end ECL circuit to shift the potential level of the input signal introduced to the current switch toward high potentials. The additional circuit has the construction explained below. A first reference potential GND, which is typically the ground potential, is supplied to the anode of a diode D 1  and to one end of a resistor R 6 . The cathode of the diode D 1  and the other end of the resistor R 6  are commonly connected to the collector of the npn bipolar transistor Q 6 . The bipolar transistor Q 6  is supplied with one of input signals, IN, through the base, and its emitter is connected to one end of a resistor R 7 . The other end of the resistor R 7  is supplied with the power source potential V EE . Thus, V IN ′, as the input signal to the single end ECL circuit, is taken out from the connection node of the diode D 1  and the resistor R 6  in the additional circuit with the collector of the npn bipolar transistor Q 6 . 
     The part of the single-end ECL circuit has the same arrangement as that of a typical single-end ECL circuit. The resistors R 1 , R 2  are supplied at their first ends with the reference potential GND, which is typically the ground potential. Connected to the other ends of the resistors R 1  and R 2  are collectors of npn bipolar transistors Q 1  and Q 2 . The input signal V IN ′ taken out from the additional circuit is introduced to the base of Q 1  of these two npn bipolar transistors Q 1 , Q 2 . On the other and, a second reference potential V REF  is applied to the base of the other npn bipolar transistor Q 2 . Emitters of the npn bipolar transistors Q 1 , Q 2  are connected together and commonly connected to one end of the resistor R 3 . The other end of the resistor R 3  is supplied with the power source potential V EE . These elements form the current switch S 1 , and output from the current switch S 1  is used as input to the emitter follower. That is, bases of npn bipolar transistors Q 3 , Q 4  whose collectors are supplied with the reference potential GND are connected to the connection node of the resistor R 1  with the collector of the npn bipolar transistor Q 1  and the connection node of the resistor R 2  with the collector of the npn bipolar transistor Q 2 . Emitters of the npn bipolar transistors Q 3 , Q 4  are connected to given ends of resistors R 4 , R 5  whose other ends are supplied with the power source potential V EE . Output signals OUT and /OUT of the differential ECL circuit are taken out from connection nodes of the npn bipolar transistors Q 3 , Q 4  with given ends of the resistors R 4 , R 5 . 
     Comparing the semiconductor integrated circuit according to the second embodiment with that of the first embodiment, only difference is that the second reference potential V REF =−0.45 V, in lieu of input signal V /IN ′, is introduced to the base of the npn bipolar transistor Q 2  forming the current switch S 1 , and its essential behaviors are identical to those of the semiconductor integrated circuit according to the first embodiment. Therefore, also the semiconductor integrated circuit according to the second embodiment can realize an ECL circuit that can maintain stable operations. 
     In short, the semiconductor integrated circuit according to the invention promises an ECL circuit stably operative with an ECL signal of a standard potential level and operative with a low voltage power source. Therefore, the invention can reduce the power consumption of ECL circuits advantageous for high-speed operations, and makes it possible to use a single electric power source for generating both the power source voltage V EE  and the terminal voltage V TT  (=−2 V) which required different power sources in conventional circuits.