LOGIC LEVEL CONTROL FOR CURRENT SWITCH EMITTER FOLLOWER LOGIC

Abstract: A logic circuit network with circuitry for independently controlling at least one of the logic levels generated thereby, comprising, in one embodiment, a logic circuit with an output current node, a complement output current node, and at least one input line, the circuit for generating an output voltage level at the output current node which depends on the amount of current drawn therethrough, and for generating a complement output voltage level at the complement output current node which depends on the amount of current drawn therethrough; in combination with a current drawing means for drawing a controlled amount of current through one of those nodes to adjust the voltage level at that node. In one embodiment, this current drawing means is connected to a voltage reference level VR1, and operates to draw an amount of current from whichever current node is at a voltage level which is closest to a predetermined constant plus this voltage reference level VR1. The amount of current drawn is controlled to cause this closest node voltage level to approach this predetermined constant plus VR1. In a preferred embodiment this current drawing means comprises a separate differential amplifier connected to each of the output current nodes.

Description: Background of Invention The present invention relates generally to logic circuit networks, and more particularly to a circuit and method for independently controlling one or more of the logic levels in the logic circuit network while reducing circuit switching delay. Logic circuits for producing one or more prescribed logic levels in response to circuit input signals are well known. For logic networks using one or more of such logic circuits, it is imperative that the prescribed logic levels generated by each circuit be standardized to permit viable communication between the various logic circuits. Without such level control, an UP level for example in one logic circuit, may be read as a DOWN level by an adjacent logic circuit. However, logic levels tend to vary from logic chip to logic chip due to variations in the power supply voltage applied to the chip, due to differences among chips such as differences in resistor values and the like, and due to temperature variations between different chip locations in the network. One standard logic circuit which is especially susceptible to such logic level variation is the current switch emitter-follower (CSEF) logic circuit, also known as emitted coupled logic. A typical prior art CSEF circuit is disclosed in Fig. 1 of US-A-4,575,647. This CSEF circuit comprises a set of input transistors connected in a differential amplifier configuration with a reference transistor. The emitters of these transistors are coupled to a common current source. The collectors of the input transistors are connected through a first resistor to a power supply VCC, while the collector of the reference transistor is connected through a second resistor to the power supply VCC. A first emitter follower circuit is connected to the collectors of the input transistors to provide a first logic level, while a second emitter follower circuit is connected to the collector of the reference transistor to provide a complement logic output. In operation, when at least one of the input signals applied to the base of the input transistors is at an UP level, its corresponding transistor is turned on, thereby causing most of the current that flows through the common current source to flow through that input transistor and its first resistor. This current flow causes a voltage drop at the collector of the input transistor, which voltage level is shifted by the first emitter follower circuit to provide the logical NOR output. The reference transistor is turned off by the current flow in the input transistor, thus eliminating the voltage drop through the connected second resistor. Accordingly, the voltage at the collector of the reference transistor is at an UP level. The UP voltage level is shifted by the second emitter follower circuit to provide the logical OR output. US-A-4 575 647 utilizes a specially designed circuit to control the current through the common current source to thereby control the voltage drop through the resistor drawing current, and thus control the DOWN logic level. A pair of cross-connected Schottky diode disposed between the collectors of the input transistors and the collector of the reference transistor set the UP level to be approximately one Schottky diode drop above the DOWN level plus a resistance voltage drop. The use of these cross-connected Schottky diodes to set the UP level has a number of drawbacks. First, many Schottky diode fabrication processes do not yield accurate repeatable Schottky barriers, resulting in UP level variations from chip to chip. More importantly, the Schottky barrier diodes have a predetermined capacitance which causes a considerable increase in switching delay because they are coupled in the critical switching area between the collectors of the transistor switching circuit. Additionally, the minimum voltage swing between the UP and the DOWN levels cannot be lower than approximately the minimum Schottky diode forward voltage drop. This limitation also impacts the switching delay because larger voltage swings between the UP and the DOWN levels require longer switching times. The invention is intended to remedy the above-described problems in controlling logic levels. The advantages offered by the present invention are that higher speed switching can be obtained in conjunction with the accurate and independent control of the voltage logic levels in the logic circuit. In one embodiment, the UP level may be compensated for variations in temperature and in the power supply, while independent UP level adjustment is used to permit control of the amount of voltage swing between the UP and the DOWN levels. Summary of the Invention Briefly, the present invention comprises a logic circuit network for producing logic levels in response to at least one input signal which includes: a current switch emitter follower circuit having an output current node (10), a complement output current node (12) and at least one input line (14), and means (32, VCS) for controlling the DOWN logic level in the logic circuit network, means (60, 62, 64) for controlling the UP logic level in the logic circuit network. The UP logic level controlling means further includes: a first differential amplifier circuit (60, 62) connected to said output current node (10) and having a reference side thereof connected to a voltage reference level VR1, said first differential amplifier circuit (60, 62) drawing an amount of current from said output current node (10) when its voltage level is too high with respect to said voltage reference level VR1 thereby adjusting the UP voltage level of said output current node (10), and a second differential amplifier circuit (64, 62) connected to said complement output current node (12) and having a reference side thereof connected to said voltage reference level VR1, said second differential amplifier circuit (64, 62) drawing an amount of current from said complement output current node (12) when its voltage level is too high with respect to said voltage reference level VR1, thereby adjusting the UP voltage level of said complement output current node (12), whereby the UP and the DOWN logic levels can be accurately and independently controlled in the logic circuit network. Preferred embodiments of the present invention appear in the dependant claims. Brief Description of the Drawings Fig. 1 is a schematic circuit diagram of one embodiment of the present invention. Fig. 2 is a schematic circuit diagram of one embodiment which may be utilized to generate the reference voltage VR1 for the circuit of Fig. 1. Detailed Description of the Preferred Embodiment The present invention relates to a logic circuit network for producing logic levels in response to an input signal, and includes a circuit for independently adjusting at least one of those logic levels. The present invention is designed to be used with a logic circuit with an output current node, a complement output current node, and at least one input line, with the logic circuit generating an output voltage level at the output current node which depends on the amount of current drawn through the output current node, and for generating a complement output voltage level at the complement output current node which depends on the amount of current drawn through the complement output current node. There are a wide variety of circuits which may be utilized to implement the above logic circuit. The present invention is not limited or restricted to any one of these circuit configurations. A specific logic circuit configuration is set forth in Fig. 1 in order to illustrate the features and operation of the present invention. However, the present invention is not limited to this particular configuration. Referring now to Fig. 1, a standard current switch emitter follower circuit is shown therein, including an output current node 10, a complement output current node 12, a first input line 14, and a second input line 16. The embodiment of Fig. 1 for implementing the above nodes comprises a voltage source 20, a first input transistor 22, with a first end connected to the output current node 10, a control input connected to the input line 14, and a second end, and a second input transistor 24 with a first end connected to the output current node 10, a control input connected to the second input line 16, and having a second end. A first resistance 26 is connected by way of a resistor 25 to the voltage source 20, and at its other end to the first end of the first and second input transistors 22 and 24. A reference transistor 28 is disposed with its first end connected to a complement output current node 12, with its control input connected to a reference voltage VR0, and having a second end. A second resistance 30 is connected by way of resistor 25 to the voltage source 10, and at its other end to the first end of the reference transistor 28. A logic circuit current source 32 is connected to the second ends of the first and second input transistors 22 and 24 and to the second end of the reference transistor 28. In the embodiment shown in Fig. 1, this logic circuit current source 32 comprises a current source transistor 34 with a first end thereof connected to the second ends of the input transistors 14 and 16 and the reference transistor 28, with a control input thereof connected to a current source reference voltage Vcs, and with a second end thereof connected through a resistor 36 to a second voltage source VEE. It can be seen from a review of the configuration of Fig. 1 that the input transistors 22 and 24 are connected in a differential amplifier configuration with the reference transistor 28. In one embodiment, the present logic circuit may include a first voltage level shifting circuit 40 for shifting the output voltage level at the output current node 10, and a second voltage level shifting circuit 46 for shifting the complement output voltage level at the complement output current node 12. In the circuit shown in Fig. 1, the above-described transistors have been implemented by NPN bipolar transistors. Accordingly, input signals are applied to the bases of the input transistors 22 and 24. The collectors of the input transistors 22 and 24 are connected to the resistor 26, while the emitters thereof are connected to the collector of the common current source transistor 34. Likewise, the collector of the reference transistor 28 is connected to the resistor 30, while its emitter is connected to the collector of the common current source, transistor 34. The base of the reference transistor 28 is connected to a convenient logic level voltage reference which may be ground potential. Note that the use of a VR0 reference potential of ground permits the use of lower power, lower current terminations for the signal lines. Moreover, there is no need for a high quality positive power supply or for all of the chips to operate at the same temperature when ground potential is utilized. The first voltage level shifting circuit 40 in this embodiment is implemented by an NPN transistor 42 with its base connected to the output current node 10, with its collector connected to the voltage source 20, and with its emitter connected through a resistor 44 to a voltage source VT. Likewise, the second voltage level shifting circuit 46 is implemented by an NPN transistor 48 with its base connected to the complement output current node 12, with its collector connected to the voltage source 20, and with its emitter connected through a resistor 50 to the voltage source VT. This is the standard emitter-follower configuration for voltage level shifting. The emitter for the transistor 42 is connected to an NOR output terminal 45, while the emitter for the transistor 48 is connected to the OR output terminal 52. In operation, when at least one of the input signals applied to the input terminals 14 and 16 is in the UP state, the corresponding transistor 22 or 28 is turned on, thereby causing most of the current which flows through the constant current source transistor 34 to flow through that respective input transistor 22 or 24 and through the resistor 26 up to the voltage source 20. This current flow through the resistors 25 and 26 causes a voltage drop at the output current node 10. The resulting voltage at the output current node 10 is a DOWN level voltage. The transistor 42 for the emitter-follower circuit 40 shifts this voltage level at the output current node 10 down by the base-emitter voltage drop VBE. Accordingly, the output at the output line 45 is a voltage level shifted DOWN signal representing the logical NOR Function. Because of the differential circuit operation of the transistors 22, 24, and 28, when most of the current from the current source transistor 34 shifts to the input transistors, the reference transistor 28 shuts off. Accordingly, no current flows through the resistor 30 and the voltage level at the complement output current node 12 rises to an UP voltage level. This UP voltage level is applied to the base of the emitter-follower circuit transistor 48, which acts to drop this voltage level by the base-to-emitter voltage drop VBE of the transistor 48. Accordingly, the signal at the output terminal 52 is a voltage shifted UP signal representing the logical OR Function. The above-described operation is reversed if the input signals at the input terminals 14 and 16 are both low so that the input transistors 22 and 24 are not conducting. In that case, the reference transistor 28 is conducting so that the voltage level at the complement output node 12 will be a DOWN level, while the voltage level at the output current level 10 through which no current is being drawn, is at an UP level. It can be seen that by controlling the common current source reference voltage VCS applied to the base of the transistor 34, the DOWN level may be controlled. This DOWN level control occurs because the regulation of the current through the common current source transistor 34 regulates the amount of current through either the resistor 26 or the resistor 30, and thus the voltage drop at the current node 10 or the current node 12, whichever node is drawing current therethrough. The present invention comprises the combination of the above-described logic circuit with a specially-designed circuit connected to a voltage reference level VR1, which specially-designed circuit draws an amount of current from whichever of the output current node 10 or the complement output current node 12 has a voltage level which is closest to a predetermined constant plus the voltage reference level VR1. The amount of current drawn by this specially-designed current drawing circuit is set to cause the closest voltage level V to approach this predetermined constant plus the voltage reference level VR1. In one embodiment of this current drawing means, a first differential amplifier circuit is connected to the output current node 10, and having with a reference side thereof connected to the voltage reference level VR1. Likewise, a second differential amplifier circuit is connected to the complement output current node 12 and has a reference side thereof connected to the voltage reference level VR1. In the embodiment of Fig. 1, this first differential amplifier circuit comprises a first transistor 60 with its control input connected to the shifted voltage level output 45 of the first voltage level shifting circuit 40, and with one end of this first transistor 60 connected to the output current node 10 and with the other end thereof connected to a common current source 70. This first differential amplifier further includes a reference transistor 62 with its control input connected to the reference voltage VR1, and with its first end connected to a voltage source, and with its second end connected to the common current source 70. Likewise, the second differential amplifier comprises a first transistor 64 with its control input connected to the OR output terminal 52, with one end thereof connected to the complement current output node 12, and with the other end thereof connected to the common current source 70. The second differential amplifier further includes a reference transistor with its control input connected to the voltage reference VR1, and with its first end connected to a voltage source, and with its second end connected to the common current source 70. In a preferred embodiment, these first and second differential amplifier circuits share a common current source and share a common reference transistor 62 connected to the voltage reference level VR1. This common current source may simply be comprised of a resistor 72 connected to voltage source VT. In the embodiment shown in Fig. 1, the transistors 60, 62 and 64 are implemented by NPN transistors. Accordingly, the base of the transistor 60 is connected to the NOR output terminal 45, while its collector is connected to the output current node 10, and its emitter is connected to the common current source 70. Likewise, the transistor 64 has its base connected to the OR output terminal 52, while its collector is connected to the complement output current node 12, and its emitter is connected to the constant current source 70. Finally, the transistor 62 has its base connected to the voltage level VR1, its emitter connected to the common current source 70, and its collector connected to a suitable voltage source. The present circuit operates as follows: the UP voltage level is set by setting or adjusting the voltage level VR1 at the differential amplifier reference transistor 62. The differential amplifier connected to the output terminal which is at an UP level will act to force that output level to have the same voltage value as the voltage reference level VR1. For example, assume that the OR output terminal 52 is at an UP level. Since the base of the differential amplifier transistor 64 is connected to this output terminal 52, then the transistor 64 conducts and forms a differential amplifier with the reference transistor 62. This transistor 64 draws enough current through the resistors 25 and 30 to make the OR output terminal 52 equal to the desired UP level set at the base of the reference transistor 62. For example, assume that the OR output terminal 52 has a voltage level which is too high relative to the reference voltage level VR1. In this instance, since the voltage at the base of the transistor 64 is greater than the voltage at the base of the reference transistor 62, the transistor 64 draws more of the current from the constant current source 70 through the resistors 25 and 30. This increased current flow through the resistors 25 and 30 causes an increased voltage drop at the complement output current node 12. This lower voltage voltage level at the complement output current node 12 is shifted down in voltage by the VBE voltage drop of the emitter-follower transistor 48, and is applied to the base of the transistor 64 as the OR UP level. This lower base voltage at the transistor 64 decreases the amount of current flowing therethrough. This feedback operation between the emitter-follower circuit 46 and the transistor 64 continues until the voltage level at the OR output terminal 52 is equal to the VR1 voltage level at the reference transistor 62. Likewise, if the UP voltage level at the OR output terminal 52 becomes too low relative to the voltage reference VR1, then more current is drawn from the constant current source 70 through the reference transistor 62, and less current is drawn through the resistors 25 and 30 and the transistor 64. This reduction in current drawn through the resistor 25 and 30 causes the voltage level at the complement output current node 12 to rise which, in turn, causes the UP level at the OR output terminal 52 to rise also. It can thus be seen that this differential amplifier comprising the transistor 62 and the transistor 64 operates in a negative feedback mode. Note that when the OR output terminal 52 is at an UP level, then the NOR terminal 45 is at a DOWN level. Accordingly, the transistor 60 is not conducting. In the event that the NOR output terminal 45 is at an UP level, then the first differential amplifier formed by the transistors 60 and the reference transistor 62 in conjunction with the constant current source 70 begins to operate. This first differential amplifier likewise functions in a negative feedback mode to control the voltage level at the output current node 10 by either increasing or decreasing the amount of current drawn through the resistors 25 and 26 and through the transistor 60 until the UP voltage level at the output terminal 45 is equal to the reference voltage level VR1 on the reference transistor 62. During this first differential amplifier operation, the first transistor 64 of the second differential amplifier is held off by the DOWN voltage level at the OR output terminal 52. Referring now to Fig. 2, there is shown a circuit which may be utilized to generate the voltage reference VR1 applied to the differential amplifier reference transistor 62. This circuit generates a voltage reference level VR1 which is a prescribed amount above the logic circuit reference level VR0 applied to the reference transistor 28. As noted previously, this reference voltage VR0 may conveniently be set to ground potential and may be supplied from off-chip. In the circuit of Fig. 2, an NPN transistor 80 is disposed with its emitter connected to the logic circuit reference potential VR0 and with its collector connected to the differential amplifier reference voltage terminal VR1. The voltage source 20 is connected through a resistor 82 to the base terminal of the transistor 80, and is also connected to the collector of the transistor 80 through a Schottky barrier diode 84. The differential amplifier reference potential VR1 thus differs from the logic circuit reference potential VR0 by the base-emitter voltage drop VBE for the transistor 80 minus the voltage drop for the Schottky barrier diode, i.e., VBE-VSBD. The Schottky barrier diode 84 and the transistor 80 can be chosen so that the voltage drop VBE for the transistor and the voltage drop VSBD for the diode 84 have very similar temperature coefficients so that VBE-VSBD are very stable, with a lower temperature coefficient than either the Schottky barrier diode or the transistor by itself. The voltage VR1 may be adjusted simply by adjusting the junction sizes of the Schottky barrier diode 84 and the transistor 80. It has been found through simulation that ringing is not a problem when current in the differential amplifier transistors 60, 62 and 64 is small compared to the current in the current switch (the transistors 22, 24, and 28). If ringing does occur in this circuit, it can be obviated simply by adding a resistor between the emitter and the constant current source 70 for each of the differential amplifier transistors 60, 62, and 64. These added resistors result in a lowered gain for the differential amplifier which prevents the ringing problem. It can thus be seen that the appropriate differential amplifier circuit draws an amount of current from whichever of the output current node 10 or the complement output current node 12 has a voltage level V which is UP, i.e., which is closest to a predetermined constant plus a voltage reference level VR1. Typically, the UP voltage level at the current nodes is equal to the voltage reference level VR1 plus the voltage drop VBE for the emitter-follower transistor 42 or 48. Accordingly, the predetermined constant will be equal to VBE for transistor 42 or 48. It should be noted that with the present circuit, there is no longer a need for stringent control of the voltage source VCC 20 because it is now possible to control both the UP and the DOWN logic levels. This feature is especially advantageous in high power environments where the regulation of the VCC power supply on-chip is very difficult. The present circuit provides higher speed switching than can be obtained with standard Schottky diode UP level control. This is because the Schottky diodes with their predetermined capacitance have been eliminated from the collector switching circuits of the input transistors 22 and 24 and the reference transistor 28. It has been determined through simulation that approximately 20% less switching delay is obtained with the present design. Additionally, with the present circuit the UP level can be independently controlled. The minimum voltage swing between the UP and the DOWN level is not restricted to the Schottky diode forward voltage drop or greater. Accordingly, smaller voltage swings between the UP and the DOWN levels can be utilized with a resulting smaller switching delay. It should be noted that the present circuit is especially suited for adjusting the UP level in response to VBE changes caused by temperature variation. Additionally, the present circuit compensates for UP level variations caused by fan-out increases. In particular, it is known that current drawn from the output of the logic circuit increases as fan-out increases, thereby tending to lower the UP level. The present invention compensates for this UP level variation caused by the fan-out. The present circuit is especially suited for logic chips where the reference voltage VR0 for the logic chip is supplied from off-chip. The invention is intended for use as a logic circuit in high performance large and very large scale integrated circuits. The present circuit permits the operation of these chips at well-controlled small-signal swings and at signal levels which permit fast switching speeds and low delay sensitivity to wire capacitance. Although the present invention has been described in the context of adjusting and/or controlling the UP level for a logic circuit, there are a variety of ways of defining an UP level. For example, if PNP transistors are utilized to implement the present invention, then the UP level may be the most negative level relative to the DOWN level.

Claims: A logic circuit network for producing logic levels in response to at least one input signal including: a current switch emitter follower circuit having an output current node (10), a complement output current node (12) and at least one input line (14), and means (32, VCS) for controlling the DOWN logic level in the logic circuit network, means (60, 62, 64) for controlling the UP logic level in the logic circuit network characterized in that said UP logic level controlling means (60, 62, 64) further includes: a first differential amplifier circuit (60, 62) connected to said output current node (10) and having a reference side thereof connected to a voltage reference level VR1, said first differential amplifier circuit (60, 62) drawing an amount of current from said output current node (10) when its voltage level is too high with respect to said voltage reference level VR1 thereby adjusting the UP voltage level of said output current node (10), and a second differential amplifier circuit (64, 62) connected to said complement output current node (12) and having a reference side thereof connected to said voltage reference level VR1, said second differential amplifier circuit (64, 62) drawing an amount of current from said complement output current node (12) when its voltage level is too high with respect to said voltage reference level VR1, thereby adjusting the UP voltage level of said complement output current node (12), whereby the UP and the DOWN logic levels can be accurately and independently controlled in the logic circuit network. Logic circuit network according to claim 1 characterized in that said first (60, 62) and second (64, 62) differential amplifier circuits share a common current source and a common reference side thereof connected to said voltage reference level VR1. Logic circuit network according to claim 1 characterized in that it further includes: a first voltage level shifting circuit (40) for shifting the output voltage level at said output current node (10) and for providing a shifted voltage to one input of said first differential amplifier circuit (60, 62), and a second voltage level shifting circuit (46) for shifting the output level at said complement output current node (12) and for providing a shifted voltage to one input of said second differential amplifier circuit (64, 62), whereby said first and said second differential circuit respectively control the UP voltage level of the output of said first and said second voltage level shifting circuit. Logic circuit network according to claim 3 characterized in that said first voltage level shifting circuit (40) comprises a a first emitter follower circuit connected to said output current node for shifting the voltage level at said output node, and said second voltage level shifting circuit (46) comprises a second emitter follower circuit connected to said complemented output current node for shifting the voltage level at said complemented output node. Logic circuit network according to claim 4 characterized in that: said first differential amplifier circuit (60, 62) includes a first transistor (60) having a control input connected to the shifted voltage level of said first voltage level shifting circuit (40), and with one end thereof connected to said output current node (10) and with the other end thereof connected to a current source (70); and said second differential amplifier circuit (64, 62) includes a first transistor (64) having a control input connected to the shifted voltage level of said second voltage level shifting circuit (46), and with one end thereof connected to said complement current node (12) and with the other end thereof connected to said current source (70). A logic circuit network according to claim 5 characterized in that it includes: a voltage source (20), at least one input transistor (22) with a first end connected to said output current node (10), a control input connected to said input line (14), and a second end; a first resistance (26) connected between said voltage source (20) and the first end of said at least one input transistor (22); a reference transistor (28) with a first end connected to said complement output current node (12), a control input connected to a second reference voltage VR0, and a second end; a second resistance (30) connected between said voltage source (20) and the first end of said reference transistor (28), and a logic circuit current source (32) connected to the second ends of said at least one input transistor (22) and said reference transistor (28). Logic circuit network according to claim 6 characterized in that it further includes means (82, 80, 84) connected to said second reference voltage VR0 for generating said reference voltage level VR1 to differ from said second voltage level VR0 by a constant amount. Logic circuit according to claim 7 characterized in that it includes: a third resistance (82) connected between said voltage source (20) and the control input of a fifth transistor (80) having its emitter terminal connected to said second voltage level VR0 and its collector end connected to said reference voltage level VR1, a schottky diode (84) connected between the base and the collector of said fifth transistor (80) for adjusting said reference voltage level VR1, the temperature coefficient of said schottky diode (84) compensating for said temperature coefficient of said fifth transistor (80).

Inventors: ASHTON, GERARD JOHN, CAVALIERE, JOSEPH RICHARD, CHENG, MING TOCK
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1987
Main classification: H03K19
IPC codes: H03K19