US2647995A - Trigger circuit - Google Patents

Description

Patented Aug. 4, 953

TRIGGER CIRCUIT Arthur H. Dickinson,

to International Business Machines New York, N. Y., a corporation of New tion, York Greenwich, Conn., assignor Corpora- Application December 7, 1950, Serial No. 199,687

6 Claims.

This invention pertains in general to a trigger circuit and more specifically to a trigger circuit in which the usual electron tubes are replaced by a crystal diode.

A crystal diode, so-called due to the analogy of the functioning thereof to the motion of electrons in a vacuum-tube diode, may be briefly described as a rectifying element including a minute block of doped semi-conductor having a positive and negative resistance characteristic under certain operating conditions, such as germanium or silicon, which is plated with metal on one surface and connected with an extremely fine metallic whisker on the parallel surface. The positive resistance characteristic may be defined as one in which there is a change in current in the same sense for each change of potential while in the case of the negative resistance characteristic the current varies inversely with the voltage. A trigger circuit including the crystal diode may be formed with a current limiting element being placed in series with the metallic whisker with a suitable pulsing source for effecting the triggering action.

The principal object of the invention is to provide a trigger circuit in which the so-called triggering action is brought about by an induced change in the resistance characteristics of a crystal diode.

A further object of the invention is to provide a trig er circuit in which the status thereof is altered by the initiation of a change in the resistance characteristic of a crystal rectifier rather than by an initiation of a change in the potential of a control grid or plate of a vacuum tube.

Another object of the invention is to provide a trigger circuit in which the trigger action is brought about due to a change in the physical and electrical characteristics of a block of semiconductor material.

A still further object of the invention is to provide a trigger circuit including an element of semi-conductor material having two conditions of stable equilibrium.

A still further object of the invention is to provide a trigger circuit including a block of semiconductor material regeneratively operable in its transversal from either of two states of stable equilibrium to the other.

This invention will best be understood by reference to the accompanying drawing, in which:

Fig. 1 is a circuit diagram of the main embodiment of the invention and showing the arrangement of the crystal diode in the circuit.

Fig. 2 is a characteristic curve for the crystal 2 diode and showing the positive and negative resistivity features of the crystal diode. I

Fig. 3 is a diagram showing the characteristic waveforms of the circuit of Fig. 1 while in operation.

Fig. 4 is a modification of the invention in which a vacuum tube is serially connected with the crystal diode in place of the resistor of Fig. 1.

Fig. 5 is a dynamic characteristic curve of the circuit of Fig. 4.

Fig. 6 is a diagram showing the characteristic waveforms of the circuit of Fig. 4 during an operating condition.

Fig. '7 represents the operation of the trigger with reference to the usual plate current plate voltage characteristics of the vacuum tube of Fig. 4.

Referring now in detail to Fig. 1, there is shown therein a source of operating potential I0 of the magnitude of approximately volts and supplying energy to the novel trigger circuit through the positive conductor II and thenega-- tive conductor I2. There is coupled between the conductors I I and I2 the novel trigger circuit including the crystal diode I3 and the current limiting resistor I4. A source I5 of positive and negative pulses is coupled through the capacitor I6 to the junction of the diode I3 and the resistor I4. An output voltage for control purposes may be taken from the junction of the diode I3 and the resistor I4 and the line I2.

The crystal diode I3 includes a minute block of doped semi-conductor H, such as germanium or silicon, which is plated with a metallic base I8 on one surface and connected with an-extremely fine metallic whisker IS on the parallel surface. Now when the applied voltage of the base I8 with respect to the Whisker I9 is positive, as is the case of Fig. l, the diode exhibits relatively high resistance properties commonly termed high back resistance. Also when the applied voltage of the whisker I9 with respect to the base I8 is positive the diode exhibits relatively low resistance properties commonly referred to as low forward resistance. Thus the crystal diode I3 having the features of a high back resistance and a low forward resistance, which are essential characteristics of a rectifier, functions as a rectifying element. As described in Crystal Rectifiers by H. C. Torrey and C. A. Whitmer, Radiation Laboratory Series vol. 15 (McGraw-Hill 1948), the rectifying action of the crystal diode results from the properties of the surface layer of the semi-conductor I1 at the point contact with the whisker I9. In the region of this contact, there is an excess concentration of negative in Fig. 3. The trigger will remain in the charge which acts as a potential barrier to the flow of electrons from the semi-conductor or crystal into the whisker. When a negative potential is applied to the whisker, the height of this barrier is increased so that the number of electrons having sufficient energy to pass over the barrier is greatly diminished. But when a positive potential is applied to the Whisker, the number of electrons having sufficient energy to pass over the barrier is increased. If the relation between the current passed by the barrier and the voltage applied between the whisker and the base were linear, there would be no rectification since the resistance would be constant. However, this is not the case since this relation is exponential and therefore extremely non-linear thereby making rectification possible by approximating the characteristic curve of an ideal rectifier. A more complete description concerning the barrier theory of rectification may be found in the aforementioned textbook.

The functioning of the trigger circuit of Fig. 1 depends upon the feature of the crystal diode characteristic curve 20 (Fig. 2) of the existence of a negative-resistance region after the attainment of a maximum voltage as denoted by reference character 2Ia. The characteristic curve 20 of Fig. 2 which represents the locus of operating conditions to which the diode reverts is obtainable by static voltage and current measurements.

The insertion of the resistor I4 in series with the diode I3 results in a circuit which, as shown by the plurality of intersections of the load line 2I with the characteristic curve 20, may be switched from the off stable position to the on stable position, or vice versa, by the application of a suitable potential thereto from the pulse source I5. It is to be noted that the characteristic curve obtainable for different germanium or silicon crystal diodes varies with each crystal and also that the slope of the load line 2 I is dependent upon the magnitude of the resistor I4.

When the trigger is in an ofi position, as shown in Fig. 2, the voltage drop across the crystal diode I3 is larger than the voltage drop across the resistor I4, as shown by the respective waveforms VB and V14 in Fig. 3. When a negative pulse from the source I is applied to the junction of the resistor I4 and the diode I3, the voltage drop across the diode I3 increases until it reaches the maximum voltage 2Ia (Fig. 2) at which time there is a change in the resistance characteristics of the crystal I1 from a positive to a negative region resulting in an appreciable reduction in the potential drop across the diode I3 such as shown by the waveform V13 in Fig. 3. At the same time the potential drop across the resistor l4, as shown by V14, initially decreases and then increases and the current through the diode I3 and the resistor I4 rises to the higher of two values whereby the trigger assumes an on status.

The trigger will remain in an on position until a positive pulse is applied from the pulse source I5 to the junction of the resistor I4 and the diode I3 thereby causing the trigger to shift to the other stable position, the off position. When the positive pulse is applied to the circuit, the potential across the resistor I4 will initially increase and then decrease, while the voltage drop across the diode I3 will decrease a slight amount and then increase such that when the trigger assumes its off position the voltage drops across the resistor I4 and the diode I3 will be respectively at the lower and higher of two values such as shown lofi 7 4 position until a negative pulse is once again applied to the circuit.

The physical aspects of the germanium or silicon crystal I1 traversal from the positive and negative resistance region and back again are not fully understood. It is believed sufficient to state that when the trigger is initially in an off position and the voltage across the diode I3 is raised to the peak 2Ia, the excursion of the crystal in the negative resistance region is regenerative. When the trigger is in an on position and the current therethrough is reduced its excursion from a negative back to a positive resistance region is likewise regenerative.

Inasmuch as the pulse source I5 looks into a relatively low impedance in the form of the resistor I4 it is seen that power is required to drive the trigger circuit of Fig. 1. A circuit which may be triggered by pulses substantially reduced in power is shown in Fig. 4 wherein the resistor I4 of Fig. 1 is replaced by the vacuum tube 22.

In the circuit of Fig. 4 elements similar to elements in the circuit of Fig. 1 bear identical reference characters.

The electron discharge device or tube 22 is connected in series with the diode I3 with the anode of vacuum tube 22 coupled to the whisker I9 while the cathode thereof is directly coupled to the line I2. The control grid of the electron device 22 is coupled through the resistor 23 to the potentiometer 24 coupled across the lines I2 and 25. The lines I2 and 25 are connected to a bias power source not shown while, as in Fig. 1, the lines I I and I2 are coupled to a main power source which is not shown. The wiping arm of the potentiometer 24 is adjusted in order to bias the tube 22 such that it will be conductive for both of the stable positions of the trigger. The control grid of the tube 22 is also coupled through the capacitor I6 to a source I5 of positive and negative pulses.

Now assuming that the trigger is in an 011" position, it is to be noted from Fig. 5 that the resistance of the diode I3 is positive and relatively high, and that the current through the diode I3 and the tube 22 is at the lower of two values. Likewise it is to be noted from Fig. 6 that the voltage drop across the diode I3 is higher of two values and that the voltage drop across the tube 22 is at the lower of two values. Also the quiescent operating point when the trigger is off is that as indicated in Fig. '7.

The trigger circuit of Fig. 4 is switched from an off to an on position when a positive pulse is fed from the source I5 to the control grid of the tube 22 causing the tube to be rendered more conductive. As a result of tube 22 being made more conductive, the current flow through the diode I3 and the tube 22 will increase causing ah initial increase in the potential drop V13 across the diode I3 as shown by the voltage peak 30 in the waveforms of Fig. 6 and decrease in the potential drop V22 across the tube 22 as shown by the voltage drop 3I in the waveforms of Fig. 6. After the voltage drop across the diode I3 has reached its peak value, the diode I3 will shift rapidly from a positive to a negative resistance characteristic causing the voltage drop across the diode to decrease to the value shown in Fig. 6. Likewise after the diode I3 shifts from a positive to a negative resistance characteristic, the potential drop across the tube 22 shall increase to the value shown in Fig. 6. It is to be noted from Fig. '7 that since the diode I3 is now operating in the negative resistance region that the load resistance for the tube 22 has been lowered causing the shift in the load line from load line 26 to load line 21.

Thus when the trigger assumes its on position, it is to be noted from Fig. 6 that the potential drop V22 across the tube 22 is at the higher of two values; that the voltage drop V13 across *the diode I3 is at the lower of two values; that from Fig. 5 the current is at the higher of two values; and that from Fig. 7 the grid bias of the tube 22 remains unchanged.

The trigger will remain in an on position until a negative pulse is applied to the grid of the tube 22 from the source of pulses l5. Upon such happening the current flow through the diode l3 and the tube 22 will greatly diminish, as represented by the intersection of the load line 21 with the horizontal axis in Fig. 7, and the voltage drop V22 across the tube will approximate the applied potential as shown by the horizontal axis in Fig. 7, and the voltage drop V22 across the tube will approximate the applied potential as shown by the peak voltage 28 in the waveform V22 of Fig. 6 while the potential drop across the crystal diode l3 will approach zero. In shifting from the on to the off position the resistance characteristic of the diode shall change from a negative to a positive characteristic resulting in the resistance of the diode being increased, causing a change in the slope of the load line from line 21 to line 29 (Fig. 7). In the off position the potential drop across the tube 22 is decreased to the lower of two values, while the potential drop across the diode will be increased to the higher of two values. The differenc in slope between the load lines 26 and 29 may be traced to the fact that the resistance of the tube 22 is materially lowered when the tube is made more conductive at the time the trigger is shifting from an off to an on position.

While there have been shown and described and pointed out the fundamental novel features of the invention as applied to a preferred embodiment, it will be understood that various omissions and substitutions and changes in the form and details of the device illustrated and in its operation may be made by those skilled in the art without departing from the spirit of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the following claims.

What I claim is:

1. A trigger circuit having two conditions of stable equilibrium comprising a block of semiconductor material havin positive and negative impedance characteristics, a metallic base coupled to one side of said block, a point contact means coupled to the side parallel to said one side of said block, current limiting means serially coupled to said point contact means, said material exhibiting a positive impedance characteristic while in one of said conditions and exhibiting a negative impedance characteristic While in the other of said conditions, and pulsing means coupled to the junction of said limiting means and said point means for changing the impedance characteristics of said material whereby said circuit shifts from one condition to the other condition of equilibrium.

2. A trigger circuit having two stable positions of equilibrium comprising a variable impedance including a semi-conductor element, an electron 6 discharge device serially coupled to said impedance, said impedance exhibiting a positive resistance characteristic in one of said positions and a negatve resistance characteristic in the other of said positions, means including a source of pulses coupled to said device for inducing a change in said characteristics whereby said circuit shifts from one position to the other.

3. A trigger circuit capable of being reversed from one state of equilibrium to the other comprising a crystal diode, an electron discharge device serially coupled to said diode, said diode having a positive resistance characteristic in one of said states and'a negative resistance characteristic in the other of said states, means including a source of pulses coupled to said device for inducing a change in said characteristics whereby said circuit shifts from one state to the other.

4. A trigger circuit having two stable operating positions comprising a two terminal variable im pedance device including a semi-conductor element, said device having a positive resistance characteristic in one of said positions and a negative resistance characteristic in the other of said positions, a low conducting electron discharge device serially coupled to said impedance device, means including a source of pulses coupled to said device for causing a change in said characteristics whereby said circuit shifts from one position to the other.

5. A trigger circuit operable to two sustained conditions of equilibrium comprising a. block of semi-conductor material having positive and negative impedance characteristics, a metallic base coupled to one side of said block, point contact means coupled to the opposite side of said block, a low conducting electron discharge device connected to said contact means, and means including a source of pulses coupled to said device for inducing a change in said characteristics whereby said circuit shifts from one condition to the other.

6'. A trigger circuit operable to two sustained conditions of equilibrium comprising a twoterminal variable impedance device, said device having positive and negative impedance characteristics, biased electron discharge means serially coupled to said device, and means including pulse means coupled to said electron means for causing a change in said characteristics whereby said circuit shifts from one of said conditions to the other.

ARTHUR H. DICKINSON.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,531,076 Moore Nov. 21, 1950 2,565,497 Harling Aug. 28, 1951 OTHER REFERENCES Amplifier RCA Review,

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