Галерея 3114846

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United States Patent 3,114,846 SELF-RESE'ITENG TUNNEL DlODE-TRANSISTfiR HYBPJD PULSE ClRCUlT Abraham 1. Pressman, Elkins Park, Pa., assignor to Radio Corporation of America, corporation of Delaware Filed Aug. 14, 1961, Ser. No. 131,128 6 Claims. (6i. 307-835) This invention relates to pulse circuits which employ the combination of a transistor and a negative resistance diode.
Pulse circuits are used extensively in information handling systems, digital computers for example, for performing various logical operations and related functions. It has been suggested that negative resistance diodes, tunnel diodes in particular, be used in such systems as active circuit elements because of their high speed switching and current gain capabilities. A tunnel diode can be used as the logical threshold element and a transistor may provide directionality of information flow, gain and inversion. Alternatively, standard gates may be used to perform logic and the combination of a tunnel diode and transistor may be used primarly as a directional amplifier.
It is desirable that the tunnel diode-transistor combination have gain in the forward direction when an input signal is applied and an equal gain when the input signal is terminated. Most prior art tunnel diode circuits fail in this respect. They are either bistable, requiring an auxiliary, high power reset signal to drive the circuit back to its initial state when the input signal is removed or, if the output does follow the inputs, the ratio of output current change to input current change is much less for one polarity input change than for the opposite polarity change.
It is an object of this invention to provide a tunnel diode-transistor pulse circuit which has improved operating characteristics.
It is another object of this invention to provide improved tunnel diode-transistor pulse circuits which are self-resetting.
It is still another object of the invention to provide improved self-resetting pulse circuits which make use of the high speed switching and current gain properties of a tunnel diode and the amplifying, directional and threshold properties of a transistor.
These and other objects of the present invention are accomplished by the combination of a tunnel diode and an inductor serially connected, in the order named, between the common electrode of, and the input electrode of, a transistor. A source of substantially constant current is connected at the common junction of the tunnel diode and the inductor. The magnitude of this current is such that a static load line is provided which intersects the volt-ampere characteristic of the tunnel diode in both regions of positive resistance. Input pulses are applied at the common junction aforementioned. The circuit is self-resetting when the L/R time constant of the inductor is greater than the RC time constant (provided by the internal resistance and intrinsic or distributed capacity) of the tunnel diode and less than the duration of the applied input pulses.
In the accompanying drawing, like reference characters refer to like components and:
FIG. 1 is a schematic diagram of a pulse circuit wherein the tunnel diode-transistor combination is used primarily to provide amplification;
FIG. 2 is a set of volt-ampere characteristics useful in describing the operation of the FIG. 1 circuit, and;
FIG. 3 is a schematic diagram of a pulse circuit wherein the tunnel diode-transistor combination performs logic,
inversion, amplification and isolation between the input and output terminals.
In the design of logic circuits there is a choice to be made regarding the function that each component performs. A transistor is capable of current gain, voltage gain, directionality and inversion, and also has threshold properties. A tunnel diode generally provides current gain at speeds higher than the transistor and has a sharp current threshold. The choice of functions performed by the elements narrows considerably when common base transistors are employed. The transistor in this configuration provides no current gain and therefore reliance is placed on the tunnel diode for the current amplification to permit reasonable fan-out. By fan-out is meant the number of output circuits or loads which can be driven by the transistor circuit. The choice that then remains is whether to use the tunnel diode as the logic threshold element or to use standard diode gates to perform the logic and tunnel diode-transistor combination primarily to provide amplification. In either case, it is desirable that the circuit be self-resetting, that is to say, that the circuit return to its initial operating state at the termination of the applied input pulse or pulses.
FIG. 1 is a schematic diagram of a circuit which is self-resetting and in which the tunnel diode is employed primarily as a current amplifying device, although it will be apparent from a later discussion that the tunnel diode threshold is important to the operation of the circuit. Input diodes 10a, 16b and have their anodes connected in common to a current source comprising a resistor 12 and a source of voltage, designated +V Each liia lilo is connected to a different input terminal 14a 140, respectively. This combination functions logically as an and gate. The voltage at the common anode junction 16 is low, relatively speaking, whenever one or more of the inputs applied at the input terminals 14a 14c is low. The voltage at the junction 16 is high, relatively speaking, only when all of the inputs are high. This type of operation will be recognized as that of an and gate. The output of the and gate is connected to the anode of a diode 20a. The cathode of the diode 20a is connected to a junction point 22 at: the anode of a negative resistance diode 24. Diodes 26b and 20c also have their cathodes connected to the common junc tion point 22.. The anodes of these diodes 20b, 20c may be connected to other and gates of the type described. The diodes 28a Ztic may perform one of several logical operations, depending upon the parameters of the negative resistance diode 24 and the current source connected at the anode thereof. In particular, the diodes 20a 290 may function as an or gate, and will be so described hereinafter.
The anode of the negative resistance diode 24, which is preferably a tunnel diode, is connected by way of a resistor 26 to a source of voltage, designated +V The combination of the latter voltage source and the resistor 26 serves as a source of substantially constant current I An inductor 30 is connected between the common junction point 22 and the emitter electrode 32 of a grounded base, PNP transistor 34, whereby the tunnel diode 24 is effectively connected in parallel with the emitter-base diode of the transistor 34. The collector electrode 36 is normally reverse-biased with respect to the grounded base electrode 38 by connecting the collector electrode 36 to a source of biasing potential, designated V,,, by way of a resistor 40. The other terminal of the biasing source, which may be a battery (not shown), is connected to circuit ground. The output of the transistor 34 is illustrated as being supplied to the cathode of a diode 10d, which may be part of an and gate of the type described previously.
An or gate may be defined as one having two or more inputs and one output, in which the output is energized whenever any one or more of the inputs is energized. In FIG. 1, the input to the diode Ztla may be considered as being energized whenever the output of the and gate at the comon point 16 is high, relatively speaking. That is to say, the input to the diode 20a is energized whenever the and gate is energized by high inputs at all of the input terminals 14a 140. In like manner, the inputs to the other diodes 26b, 200 are energized when the voltages at the anodes thereof are high. A current AI is supplied to the common junction point 22 through the diode 20a whenever the input to the diode 20a is energized. It is desired that an output be provided at the collector electrode 36 of the transistor 34 in response to this input.
Consider now the operation of the tunnel diode 24- transistor 34 combination. The volt-ampere characteristic of the tunnel diode 24 is indicated by the solid curve 50 in FIG. 2. The volt-ampere characteristic of the transistor, looking into the emitter 32, is indicated by the dashed curve 52. These curves are static operating characteristics. The static voltage across the tunnel diode 24 must be the same as the static voltage across the emitterbase diode of the transistor 34. A combined operating characteristic of the tunnel diode 24 and the transistor 34 may be constructed therefore by adding the current through the diode 24 and into the emitter 32 of the transistor 34 at the same voltage. The combined characteristic is indicated in FIG. 2 by the solid curve 54, and includes the points a, b, c, d.
The tunnel diode 24 characteristic 50 has two regions ab and ef of positive resistance. These regions are joined by a region be of negative resistance. The substantially constant current 1,, supplied at the common junction 22 is chosen in value so that the load line 62 intersects both regions of positive resistance. The point 60 of intersection of the load line 62 with the characteristic 50 is a point of stable operation of the tunnel diode 24. The voltage across the diode 24 in this operating state is lower than the threshold of the transistor 34, and all of the current I supplied by the constant current source then flows into the tunnel diode 24. No current flows into the emitter electrode 32 to the collector 36, and the output voltage at the collector electrode 36 is low, relatively speaking, whereupon the diode d is energized. An additional increment of current AI is supplied at the common junction 22 whenever the input to one of the diodes 20a 20c is energized. This current increment raises the load line to the position indicated by the dashed line 62a. The current increment AI initially flows into the tunnel diode 24 because of the action of the inductor 39. The tunnel diode current increases momentarily to a value close to I,,+AI. When the diode 24 current exceeds the peak current I the diode 24 is switched rapidly through the negative resistance region to a condition of high voltage. The inductor 30, in tending to prevent a change in current through it, enhances the high speed of switching of the tunnel diode 24 in response to the current AI.
The dynamic operating condition of the tunnel diode 24 during the switching transient is not easily illustrated, and no attempt is made to illustrate this condition in FIG. 2. Sufiice it to say the current flows through the inductor 30 and into the emitter 32 of the transistor 34 when the tunnel diode 24 switches to the high voltage state and the threshold of the transistor 34 is exceeded. Assuming that the duration of the input current pulse is greater than the L/R time constant of the inductor 30, the tunnel diode 24-transistor 34 combination stabilizes at the point 66 of intersection of the load line 62a and the combined operating characteristic 54 during the period of the applied input pulse. The current flowing into the tunnel diode 24 is then l and the current flowing into the emitter electrode 32 is I The emitter current 1., flows almost in its entirety to the collector 36 and raises the 4 collector voltage to a high value. As may be seen in FIG. 2, the change in emitter current I is much greater than the change in input current Al; the ration I /AI is approximately the current amplification factor of the tunnel diode-transistor combination.
Assume that the input current pulse AI terminates after the tunnel diode 2-4-transistor 34 combination is stabilized at the operating point 66. The current supplied at the common junction point 22 then decreases to a value I In the absence of the inductor 3d, the tunnel diode 24 would remain in a stable state of high voltage and the operating point of the combination would be the point 68 of intersection of the combined characteristic 54 with the load line 62. The inductor 30, however, tends to oppose any change in current flowing into the emitter electrode 32. That is to say, the inductor 30 tends to maintain an emitter 32 current of l This value of current is greater than the difference between the constant current I, and the valley current I of the tunnel diode 24. Accordingly, the diode 24 switches rapidly through its negative resistance region to the low voltage state when the tunnel diode 24 current falls below I and the circuit stabilizes at the operating point 649 at the termination of the switching transient.
The tunnel diode 24 has an intrinsic capacitance which may be thought of as being connected between the common junction point 22 and circuit ground. The charge stored therein must be discharged before the tunnel diode 24 reaches the low voltage operating point 60, and it is the capacitance primarily which determines the switching time of the tunnel diode 24. The L/R time constant of the inductor 36 must be greater than the RC time constant of the tunnel diode 24 in order to achieve the selfresetting action aforementioned. The emitter 32 current again becomes zero after the tunnel diode 24 stabilizes at the operating point 60, and the output voltage at the collector electrode 36 goes low.
Operation of the FIG. 1 circuit may be summarized as follows. The tunnel diode 24 ordinarily is biased in the low voltage region of its operating characteristic. This voltage is less than the threshold of the transistor 34. An input current pulse of magnitude greater than I I causes the tunnel diode 24 to switch to the high voltage state. The inductor 30 enhances the high speed switching of the tunnel diode 24. The transistor 34 conducts after the tunnel diode 24 is switched to the high voltage state, and a positive-going output signal appears at the collector electrode 36. A magnetic field is built up around the inductor 30 in response to current flow therethrough and energy is stored in the magnetic field. The inductor 30 tends to prevent a decrease in emitter 32 current when the input pulse terminates. The current through the tunnel diode 24 then decreases below the valley current, and the tunnel diode 24 switches back to the low voltage state.
It will be apparent that the gate circuit comprising the diodes 20a 200 may perform a different logic from that described above. For example, if energizing one diode Ztla 20c increases the current supplied to the common junction point 22 an amount less than l l and greater than /2 (l -I the diode gate would operate as a majority gate since the tunnel diode 24 does not switch until its peak current l is exceeded. In this sense, then, the tunnel diode 24 operates indirectly as a logic threshold device in the FIG. 1 circuit, although it is intended primarily as a current amplifier in the given application. The transistor 34 provides voltage gain and isolation between the input and the output terminals.
FIG. 3 is a schematic diagram of a logic circuit where in the tunnel diode 24 functions more nearly like a true logic threshold device and in which the conventional diode gate is eliminated. In FIG. 3, each of a plurality of input terminals 36a 88c is connected to a junction point 82 at the anode of the tunnel diode 24 by a separate resistor 84a 34c, respectively. Either voltage or current pulses may be applied at the input terminals. In the former case, the resistors 34a 84c are selected in value to convert the voltage input signals to current pulses at the junction point 82. A resistor 88 and a source of voltage, designated +V serve as a substantially constant current source to supply a current I at the junction point 32.
An inductor 90 is connected between the junction point 82 and the base electrode 94 of an NPN transistor 96. The transistor 96 is connected in the common emitter configuration by connecting its emitter electrode 98 to a point of reference potential, indicated schematically by the symbol for circuit ground. The cathode of the tunnel diode 24 also is connected to ground. The collector 1th is reverse-biased with respect to the base 94 by connecting the collector electrode 1% to a source of biasing potential, designated +V by Way of a resistor 102. One of a pair of output terminals 15M- is connected to the collector electrode N; the other of the output terminals 104 is grounded.
The operation of the tunnel diode 24-transistor 96 combination in the FIG. 3 circuit is substantially the same as the operation of the tunnel diode 24-transistor 34 combination in FIG. 1 and need not be escribed in detail. The operating characteristic of the transistor 96 looking into the base electrode 94 may not be the same as the input characteristic of the transistor 34 (FIG. 1), but the difference generally is important only in determining the dynamic operating point of the combination after the current through the inductor 94) has reached a steady value. In certain cases, and depending upon the transistor 5% parameters, it may be desirable or necessary to connect the emitter electrode 93 to a source of voltage other than ground in order that the volt-ampere characteristic of the transistor may intersect the volt-ampere characteristic of the tunnel diode 24 at such a point as to provide optimum operation, and, more particularly, to assure that the characteristic curves are so related that the current I is greater than I,. I (FIG. 2). Connecting the emitter electrode 98 (FIG. 3) or the base electrode 38 (FIG. 1) to a bias potential has the effect of shifting the transistor input characteristic 52 to the left or right, depending upon the polarity of the bias.
Consider the operation of the FIG. 3 circuit when the tunnel diode 24 performs the logical or function. The constant current source biases the tunnel diode 24 at the point so of operation indicated in FIG. 2. An input pulse 11% applied at any of the input terminals 80a 89c supplies an increment of current AI at the junction point 82. This increase in current is suificient to switch the tunnel diode 24 to the high volt
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