AN EMITTER COUPLED LOGIC CIRCUIT

Abstract: The ECL circuit includes a plurality of input terminals (1, 2, 3), a current source (8), a reference transistor (12) having an emitter connected to the current source, a base (7) receiving a reference voltage and a collector connected to a first resistor (15), a plurality of input transistors (9, 10, 11) each having an emitter connected to the current source (8), a collector connected to a second resistor (14) and a base connected to one of the input terminals (1, 2, 3), a plurality of pull-down resistors (57, 58, 59) each connected between the base and the emitter of one of the input transistors, an emitter follower transistor (13) having a base connected to the collector of the reference transistor (12) or a common connection point of collectors of the input transistors and an emitter connected to an output terminal (4) and a third resistor (16).

Description: BACKGROUND OF THE INVENTION Field of the Invention: This invention relates to an emitter coupled logic (ECL) circuit having one or more input transistors and pull-down resistors, and more particularly to an improvement of the connection of the pull-down resistors. ECL circuits are widely used in logic circuits operable at a high speed. The ECL circuits are basically composed of a differential type comparator and one or two emitter-followers. The differential type comparator has a reference transistor applied with a reference voltage at its base, at least one input transistors receiving input signals at respective bases, and a constant current source supplying a constant current to emitters of the reference and input transistors. The input signals are compared with the reference voltage. Usually, the input signals in an ECL circuit have a high level (V H ) between -0.88 and -1.025 volts and a low level (V L ) between -1.62 and -1.81 volts. The reference voltage is -1.3 volts. Transistors operating in this voltage range do not reach a saturated condition and do not store any storage carriers. The emitter-follower transistor(s) is also driven in a non-saturation condition. Therefore, the ECL circuit operates at a high speed, compared to any other type of logic. ECL gates in an ECL-IC have many input transistors. However, in many cases, all the input transistors are not always used. For example, a three-input ECL gate is sometimes used as a two-input OR/NOR gate wherein two input transistors are actually utilized with the remaining one being held in a cut-off condition. ECL gates in the prior art are provided with pull-down resistors to drive the non-used transistors into the cut-off condition. The pull-down resistors are respectively inserted between a base of each input transistor and a power supply terminal held at a lower potential voltage. Due to the pull-down resistor, the input transistor having a base which is not connected to any outside circuit remains in a cut-off condition. The logic operation is determined by input transistors whose bases are connected to the outside circuit. The above-explained pull-down resistors are disclosed on pages 1 - 8 of "THE SEMICONDUCTOR DATA LIBRARY" series A, volume IV, distributed by Motorola Inc., in 1974 and page 3 - 4 of "ECL DATA BOOK" distributed by Fairchild Camera and Instrument Corporation in 1977. Heretofore, a logic IC was composed of relatively large-sized transistors which have a relatively deep base-emitter junction and a relatively large breakdown voltage of the base-emitter junction. The conventional ECL gates having the pull-down resistors performed their functions without any trouble. In recent years, however, the integration density of transistors in the logic IC has greatly increased. The transistors used therein have become more and more small in size. The base-emitter junctions are designed shallow in depth. Due to such miniaturization, the breakdown voltage of the base-emitter junction becomes low. The base of an input transistor having no connection to an outside circuit is lowered to the lower potential of the power supply by the pull-down resistor, whilst the emitter thereof is held at a potential lower than the reference voltage of -1.3 volts by a base-emitter forward voltage of 0.8 volts. Since the lower potential of the power supply in ECL logic is about -5.0 volts, the base-emitter junction of the input transistor is backward biased with -2.9 volts by which transistors having the shallow base-emitter junction are broken. As the base-emitter junction becomes shallower, the break-down of the base-emitter junction becomes a more important problem to be solved. A logic circuit according to the preamble part of claim 1 is disclosed in "PATENT ABSTRACTS OF JAPAN", Vol. 8, No. 53(E-231)(1490) and JP-A-58 207 722. In this device a series connection of a resistor and the emitter-base path of a transistor is provided between the base and the emitter of an emitter coupled logical circuit transistor to decrease the voltage applied between the emitter and the base of this transistor. However, this series connection has no function of a pull-down resistor. Furthermore, provision of a further transistor is a hindrance in enhancing the integration density. SUMMARY OF THE INVENTION It is therefore an object of the invention to provide an ECL-gate composed of transistors having a low base-emitter breakdown voltage and which is suitable to be formed in an integrated circuit having a large integration density. A further object of the invention is to provide an ECL-gate having an improved connection of pull-down resistors. This object is achieved by a logic circuit including first and second power lines supplying a power voltage therebetween, a current source connected to said second power line to supply a substantially constant current, a reference transistor having an emitter connected to said current source, a base receiving a reference voltage and a collector, and at least one input transistor having an emitter connected to said current source, a base receiving an input signal and a collector, a first load connected between said collector of said reference transistor and said first power line and a second load connected between said collector of said input transistor and said first power line, wherein a pull-down resistor of 10 to 50 kΩ is connected between said base and emitter of said at least one input transistor. In a development of the invention, emitter-collector paths of a plurality of input transistors are connected in parallel with each other between the current source and the second load wherein each of the input transistors respectively has a pull-down resistor between its own base and emitter. According to the invention, the pull-down resistor or resistors are connected between the base and the emitter of the input transistor or transistors. When any signal is not applied to an input terminal, the base of the transistor connected to the opened input terminal is lowered to the potential at its own emitter by the pull-down resistor. The base is not lowered below its emitter voltage, because the pull-down resistor is connected between the base and emitter. This means that, if the input transistor is driven into a cut-off condition, the base-emitter junction is not backward biased. Therefore, the ECL logic according to the present invention may be formed by transistors having a base-emitter junction of low breakdown voltage. In other words, the ECL logic according to the present invention may be formed in an integrated circuit of a large-scale integration. BRIEF DESCRIPTION OF THE DRAWINGS The above and further objects, features and advantages of the present invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings, wherein: Fig. 1 is a circuit diagram showing an ECL circuit in the prior art; and Fig. 2 is a circuit diagram showing an ECL circuit according to a preferred embodiment of the present invention. An ECL circuit in the prior art has a circuit shown in Fig. 1. A power terminal 5 is supplied with a voltage V CC which is generally a ground potential. Another power terminal 6 is supplied with a voltage V EE which is about -5.0 volts, and more specifically a voltage from -4.5 volts to -5.2 volts. All the circuit elements are formed on a semiconductor chip. Input terminals 1, 2 and 3 are respectively connected to bases of input transistors 9, 10 and 11. Emitters of the input transistors 9, 10 and 11 are commonly connected to a constant current source 8 together with an emitter of a reference transistor 12. Collectors of the input transistors 9, 10 and 11 are commonly connected to a load resistor 14 which is in turn connected to the power terminal 5. Collector of the reference transistor 12 is connected to another load resistor 15 which is in turn connected to the power terminal 5. A reference voltage of -1.3 volts is applied to a base of the reference transistor 12 through a terminal 7. Pull-down resistors 17, 18 and 19 are respectively connected between the bases of the input transistors 9, 10 and 11 and the power terminal 6. A base of an emitter-follower transistor 13 is connected to the collector of the reference transistor 12. A collector of the emitter-follower transistor 13 is connected to the power terminal 5. A resistor 16 is connected between an emitter of the emitter-follower transistor 13 and the power terminal 6. The base of the emitter-follower transistor 13 may be connected to the connecting point of collectors of the input transistors 9, 10 and 11, instead of the collector of the reference transistor 12. An output terminal 4 is connected to the emitter of the emitter-follower transistor 13. The ECL circuit shown in Fig. 1 represents a 3-input OR gate. The same ECL circuit may be used as a 2-input OR gate by applying input signals to two of the three input terminals 1, 2 and 3. The remaining one input terminal is held opened without being connected to outside circuit. The input transistor having a base connected to the opened input terminal is driven into a cut-off condition by lowering the base potential to the potential at the power terminal 6 by the pull-down resistor. While the base potential is lowered, the emitter of the same transistor is maintained at a voltage lower than the reference voltage of -1.3 volts by a base-emitter forward voltage of 0.8 volts, that is,at a voltage of -2.1 volts. As a result, the base-emitter junction of the input transistor driven into a cut-off condition is backward biased with about 2.9 volts. In the past, transistors in an integrated circuit were so large that the base-emitter junction may have a large breakdown voltage. The breakdown voltage of the base-emitter junction is determined by a difference in impurity density between emitter and base regions and an impurity profile in the emitter region. The impurity profile in the emitter region is not steep, if the emitter region has a sufficient depth. Therefore, the circuit in the prior art had no problem if any input terminal 1, 2 or 3 were held open. In the recent years, however, the integration density of logic ICs has been improved. The transistors held in a cut-off condition by opening the corresponding input terminal are often broken at the base-emitter junction. The cause for this destruction will be next considered. The transistors used in the recent large-scale integrated circuits are greatly miniaturized by using a shallow emitter region, the depth being 0.08 µm, for example. The impurity density of the emitter region requires a value more than 10²⁰ cm⁻³, similar to the conventional transistors. Accordingly, the impurity profile in the emitter region becomes steep, causing a low breakdown voltage at the base-emitter junction. DESCRIPTION OF THE PREFERRED EMBODIMENT The ECL circuit according to the preferred embodiment shown in Fig. 2 has a similar circuit to the conventional circuit shown in Fig. 1, except for pull-down resistors 57, 58 and 59. The pull-down resistors 57, 58 and 59 have a resistance between 50 kiloohms and 10 kiloohms, preferably about 20 kiloohms, and are respectively connected between the base and emitter of the input transistors 9, 10 and 11. If the pull-down resistors 57, 58 and 59 have a resistance larger than 50 kiloohms, the base voltage of the transistor held in a cut-off condition is easily affected by external noise, causing a malfunction. On the other hand, if the pull-down resistors have a resistance smaller than the 10 kiloohms, the voltage level of input signal changes to alter the output level. The resistance of the pull-down resistors 57, 58 and 59 are so selected from the range between 50 and 10 kiloohms that the voltage drop across the resistor 15 becomes nearly 600 milivolts. This value is controlled with the resistance value of the load resistors 14 and 15. One example of the resistance of the load resistors 14 and 15 is 900 ohms. The constant current supplied from the current source 8 may be 0.7 mA. The resistance of the resistor 16 may be 7.4 kiloohms. When the ECL circuit shown in Fig. 2 is used as a two-input OR gate, one of the input terminals 1, 2 and 3 is held open. For example, the input signals are applied to the input terminals 2 and 3, the input terminal 1 being kept unconnected. The base of the input transistor 9 is lowered by the pull-down resistor 57. However, since the pull-down resistor 57 is connected between the base and emitter of the input transistor 9, the voltage at the base does not lower below the voltage at its emitter. Accordingly, the base-emitter junction of the input transistor 9 is not backward biased but biased to drive the input transistor 9 into a cut-off condition. Since the base-emitter junctions of the input transistors 9, 10 and 11 are not biased backward, the ECL circuit shown in Fig. 2 may be formed of transistors having a base-emitter junction of a small breakdown voltage. In other words, the ECL circuit may be formed of transistors having shallow emitter regions and may be formed in a large-scale integrated circuit in which very miniaturized transistors are used. The breakdown of the base-emitter junction is also generated in the ECL circuit in the prior art, when the connection to the input terminal 1, 2 or 3 is disconnected while the power voltage is supplied to the ECL circuit. Against such loose-connection, the input transistors in the ECL circuit according to the present invention is protected from being broken down. Therefore, the present invention may be applied to a comparator having one input transistor. The base of the emitter-follower transistor may be connected to a connection point of collectors of the input transistors 9, 10 and 11. By this connection, the ECL circuit operates as a NOR gate. The same features and advantages as the OR gate can be obtained.

Claims: A logic circuit including first and second power lines supplying a power voltage therebetween, a current source (8) connected to said second power line to supply a substantially constant current, a reference transistor (12) having an emitter connected to said current source (8), a base receiving a reference voltage and a collector, at least one input transistor (9, 10, 11) having an emitter connected to said current source (8), a base receiving an input signal and a collector, a first load (15) connected between said collector of said reference transistor (12) and said first power line and a second load (14) connected between said collector of said input transistor (9, 10, 11) and said first power line, characterized in that a pull-down resistor (57, 58, 59) of 10 to 50 KΩ is connected between said base and emitter of said at least one input transistor (9, 10, 11). A logic circuit as claimed in claim 1 , characterized in that emitter-collector paths of a plurality of input transistors (9, 10, 11) are connected in parallel with each other between said current source (8) and said second load (14), each of said input transistors (9, 10, 11) respectively having a pull-down resistor (57, 58, 59) connected between its own base and emitter.

Inventors: UMEKI, YOSHITAKA C/O NEC CORPORATION
Applicants: NEC CORPORATION
Application year: 1986
Main classification: H03K19
IPC codes: H03K19