US3084265A - Threshold trigger having regenerative feedback but maintaining conduction only during presence of input signal - Google Patents

Description

Apnl 2, 1963 A. CLELAND 3,084,265

THRESHOLD TRIGGER HAVING REGENERATIVE FEEDBACK BUT I MAINTAINING CONDUCTION ONLY DURING PRESENCE OF INPUT SIGNAL Filed June a, 1959 All Ll Alllllllll l i S """v' "V" "I Y K R Q g INVENTOR Aaron Zyle CZelazzd l1 W m, LAW MM v ATTORN 3,084,265 THRESHGLD TREGGER HAViNG REGENERATEVE FEEBBACK BUT MAENTAENHNG CUNBUCTHQN GNLY DURENG PRESENCE F INPUT Si-GNAL Am'on Lyle Cieland, Manvilie, N..i., assignor to Aircraft Radio Corporation, Boonton, Ni, a corporation oi New Jersey Filed June 8, 1959, Ser. No. 818,594 8 (Jlairns. (Cl. EFL-S35) This invention relates to a threshold trigger circuit of the type which is rendered operative upon receipt of a signal and continues in operation as long as the signal persists. More specifically, the invention concerns a threshold trigger circuit utilizing transistors which is rendered operative upon receipt of a signal to actuate a switch or other load and continues in operative condition until the signal terminates at which time the trigger circuit becomes inoperative.

A major object of this invention is to provide a threshold trigger circuit which draws very little current from a power supply while in its ready condition awaiting receipt of a signal to become operative.

Another object of the invention is to provide a simple threshold trigger circuit which is light in weight and particularly adapted for use in aircraft to respond to the receipt of a radio signal for operating an indicator lamp during the period which the radio signal persists.

In the attainment of these objects an important feature of the invention resides in the arrangement of a pair of transistors of opposite conductivity types which are normally biased to cut oil but directly connected together so when one transistor is rendered conductive it applies potentials to the other transistor opposing the biasing potentials; and including a regenerative feedback circuit eliective when the other transistor is conducting for applying potentials to the one transistor in opposition to biasing potentials. Signals supplied to the base of the first transistor cause the trigger circuit to become conductive so the output of either transistor may be utilized to operate a switch such as a switching transistor.

Where an alternating current signal is applied to the base or the first transistor, a capacitor is connected to the coupling circuit and electrodes of the first transistor so it becomes charged as the first transistor conducts during alternate half-cycles of the alternating signal and thereby applies a potential to the second transistor sufiicient to render that transistor conductive whereupon the regenerative feedback circuit becomes efiected. It is essential that the regenerative feedback circuit apply potentials to the first transistor which oppose the normal biasing potentials and thereby render the first transistor more conductive, but which are insufficient to continue conduction of the first transistor in the absence of a signal applied to its base.

These and other objects, advantages and features of the invention will become more apparent from the following specification taken in conjunction with the accompanying drawing which illustrates a threshold trigger circuit embodying the invention in its preferred form.

As shown in the drawing the threshold trigger circuit includes a first transistor 11 connected in circuit with a potential source so it is normally biased to cut off. Toward that end, this transistor 11, which is of the NPN type has its emitter 12 connected through resistor 13 to ground and its collector 14 connected through a load resistor 15 to a source 16 of positive potential which in this case may be about 10 volts. Base 17 of the transistor 11 is connected to an intermediate point 18 of a voltage divider formed by resistors 19 and 21 extending between potential source 22, of about 10 volts, and

mag.

ground. Resistors l9 and 21 are so selected that base 17 is biased slightly above ground but insufficient to render the transistor 11 conductive when emitter 12 is near ground potential and collector 14 has a positive potential of about 10 volts.

From the output of transistor 11 at collector l4 and across load resistor 15 a coupling lead 23 is connected to base 24 of a second transistor 25, of the PNP type, which is likewise normally biased to cut oh by having its emitter 26 connected through emitter follower resistor 27 to a source 28 of positive potential of about 10 volts and its collector 29 connected through load resistor 31 to ground. If transistor 11 is rendered conductive, as by adjusting the potential of its base 17, the potential of base 24 is lowered with respect to emitter 26 sufiiciently to render transistor conductive. When this occurs, the voltage across emitter follower resistor 27 is applied through coupling resistor 33 in a regenerative feedback circuit to emitter 12 of transistor 11. In this way, the potential of emitter 12 is lowered to render the transistor 11 more conductive. However, the regenerative feedback potential applied to emitter 12 is insufficient to render transistor 11 conductive in the absence of a signal of a threashold level applied to base 17 in addition to the biasing potential from the voltage divider of resistors 19 and 21. Emitter 12 is normally biased slightly above ground by its connector between resistors 13 and 33 which are connected in series with resistor 27 between source 28 and ground.

Should a DC. potential of the desired threshold level be applied to point 18 and base 17, transistor 11 would become conductive upon the increase of the potential of base 17. When transistor 11 thereby begins to conduct, the voltage at collector 14 is reduced by reason of the current drawn through load resistor 15 so the coupling lead 23 lowers the potential of base 24 thereby rendering transistor 25 conductive. This causes a flow of current in emitter follower resistor 27 which lowers the potential of emitter 26 and this lowered potential is supplied through the regenerative feedback circuit to the emitter 12 of the transistor 11 thereby rendering the transistor 11 more conductive. This, in turn, further reduces the potential on base 23 so continued regenerative action increases conduction of transistor 25 to its saturation point. In this way, as long as the signal ap plied at point 18 and base 17 persists, both transistors continue to conduct and the output which appears across load resistor 31 in the collector circuit of transistor 25 may be utilized to operate a switching transistor 35 which in turn may control a signal light 36 if desired. As soon as the signal applied at point 18 is removed or falls below the threshold level, transistor 11 returns to its non-conductive state and the potential of base 24 is raised to cut off transistor 25 so the signal light 35 will be extinguished.

Since in the usual case an alternating current signal is available in a radio receiver for purposes of indicating the presence of such a signal, the drawing shows such a signal source at 36 coupled through a condenser 37 to the point 13 and the base 17. When an alternating current signal is thus applied to base 17, transistor ll. acts as a rectifier and is rendered conductive on alternate haltcycles of the signal from source 36 which raise the potential of base 17. This conduction of transistor 11 on alternate half-cycles develops a direct current potential across capacitor 38 connected between emitter 12 and collector M. A charge on capacitor 33 lowers the potential of collector l4 and coupling lead 23 to lower the potential of base 24. This charge on the capacitor 33 is also effective through resistor 33 to raise the potential on emitter 26. Changes in potentials applied to the electrodes of transistor 25 as a result of charging condenser osages 38; render transistor 25 conductive to cause current to flow through resistor 27 and, accordingly, a drop in potential at the emitter connection of resistor 27. This potential at emitter 26 is fed back to emitter 12 of transistor 11 to cause this transistor to conduct and thus initiate regenerative action to render transistor 2'5 heavily conductive. Through this regenerative feedback transistor 25 will remain conductive as long as an adequate signal is applied to base 17 of transistor 11. When transistor 11 is fully conductive, capacitor 38 is discharged through emitter 12 and collector 14 and the regenerative feedback connection will maintain conduction of both transistors until the signal applied to the base 17 of transistor 11 falls below the threshold level.

By way of example, the following is a tabulation of values which may be used for the components of the circuit although it will be apparent that many variations are possible without departing from the invention:

Inasmuch as during normal standby operation tran sistors' 11 and 25 are both non-conductive, the current drain on the system supplyin the circuitis at a minimum, Whenever a signal is applied to base 17 of the threshold level, both transistors become conductive and continue in a state of conduction as long as the signal on base 17 persists at threshold level. Immediately following the termination of the signal at base 17 both transistors return to their non-conductive state. It is unnecessary that the current in the two transistors be equal and in fact whatever load is placed on transistor 25 has no material effect on the operation of transistor 11.

As the circuit shown in the accompanying drawing and described in the foregoing specification is but one pre ferred embodiment of the invention, it is intended that it be construed in an illustrative sense as many modifications will become apparent without departing the scope of the invention as defined in the appended claims.

In the claims:

1. A threshold trigger circuit comprising a'pair of transistors of opposite conductivity types connected in circuit with a potential source to normally apply cut off bias potentials to electrodes of both transistors, coupling means from one transistor to the other transistor effective upon conduction of said one transistor for applying potentials opposing the bias potentials on the electrodes of said other transistor, means for applying a signal to die base of said one transistor for initiating conduction of said other transistor, and a regenerative feedback circuit between corresponding electrodes of said transistors efiective upon conduction of said other transistor for applying potentials opposing the bias potentials on the electrodes of said one transistor but insufficient to render said one transistor conductive in the absence of a signal applied to the base thereof.

2. A threshold trigger circuit comprising a pair of transistors of opposite conductivity types, each of said transistors having an input electrode, an output electrode and a third electrode, respectively, means for applying cut-oil biasing potentials to the electrodes of said transistors, means connecting the output electrode of one of said transistors to the input electrode of the other of said transistors, means for applying a signal to the input electrode of said one transistor for initiating conduction of said other transistor, and regenerative feedback means between said third electrodes eitective on conduction of said other transistor for applying potentials overcoming the cut-off bias potentials on the electrodes of said one transistor to render said one transistor conductive.

3. A threshold trigger circuit comprising a pair of transistors of opposite conductivity types, each of said transistors having an input electrode, an output electrode, and a third electrode, respectively, means for applying cut-off biasing potentials to the electrodes of said transistors, means connecting the output electrode of one of said transistors to the input electrode of the other of said transistors, means for applying a signal to'the input electrodes of said one transistor for initiating conduction of said other transistor, and a regenerative feedback circuit including a resistive element between said third electrodes effective on conduction of said other transistor for applying potentials overcoming the cutoff bias potentials on the electrodes of said one transistor to render said one transistor conductive.

4. A threshold trigger circuit comprising a pair of transistors of opposite conductivity types connected in circuit with a potential source to normally apply cut off bias potentials to electrodes of both transistors, coupling means from the collector of said one transistor to the base of said other transistor effective upon conduction of said one transistor for applying a potential to the base of the other transistor opposing the cut oil bias potentials applied to electrodes of said other'transistor, means for applying a signal to the base of said one transistor, and a regenerative feedback circuit between the emitters of said transistors effective upon conduction of said other transistor for applying a potential to the emitter of said one transistor in opposition to the cut oif bias potentials applied toelectrodes of said one transistor but insufiicient to render said one transistor conductive, in the absence of a signal applied to its base.

5. A threshold trigger circuit comprising a pair 05 transistors of opposite conductivity types connected in circuit with a potential source to normally apply cut of? bias potentials to both transistors, coupling means from one transistor to the other transistor effective upon conduction of said one transistor for applying a potential opposing the bias potentials on the electrodes of said other transistor, means for applying an alternating current signal to the base of said one transistor for rendering said one transistor conductive on alternate half-cycles of the signal, a capacitor connected between an electrode of said one transistor and said coupling means to be charged by said conduction of said, one transistor on alternate half-cycles of said signal for applying the potential of the charge to said other transistor in opposition to the bias potentials applied to electrodes of said other transistor, and a regenerative feedback circuit between electrodes of said transistors etr'ective'upon conduction of said other transistor for applying a potential opposing the biaspotentials on the electrodes of said one transistor but insutficient to. render said one transistor conductive in the absence of a signal.

6. A threshold trigger circuit comprising a first transistor of the NPN type having its electrodes connected to a potential source to normally apply cut elf bias potentials to electrodes of said first transistor and including a load resistor in the collector circuit, means for applying a signal to the base of said first transistor, a second transistor of the PNP type having its electrodes connected in circuit with a potential source to normally apply cut off bias potentials to electrodes of said second transistor and including a resistor connected between said potential source and the emitter of said second transistor, a coupling circuit between the collector of said first transistor and the base of said second transistor for applying a po-v tential to the base of said second transistor effective upon conduction of said first transistor for opposing the cut off bias potentials applied to electrodes of said second transistor, and a regenerative feedback circuit connected be tween the emitter of said second transistor and the emitter of said first transistor elfective upon conduction 05 said second transistor for applying a potential to the emitter of the first transistor opposing the cut otI bias potentials applied to electrodes of said first transistor but insufiicient to render said first transistor conductive in the absence of a signal applied to its base.

7. A threshold trigger circuit comprising a first transistor of the NPN type having its electrodes connected in circuit with a potential source to normally apply cut ofi bias potentials to electrodes of said first transistor, means for applying an alternating current signal to the base of said first transistor for rendering said first transistor conductive on alternate half-cycles of said signal, a second transistor of the PNP type connected in circuit with a potential source to normally apply cut otf bias potentials to electrodes of said second transistor, coupling means between the collector of said first transistor and the base of said second transistor eflective upon conduction of said first transistor for applying a potential to the base of said second transistor in opposition to the cut ofi bias potentials applied to electrodes of said second transistor, a regenerative feedback circuit between the emitter of said second transistor and the emitter of said first transistor etfective upon conduction of said second transistor for applying a potential to the emitter of said first transistor in opposition to the cut off bias potentials applied to electrodes of said first transistor and condenser means providing a low impedance alternating current path between the collector and emitter of said first transistor to be charged by conduction of said first transistor during alternate half-cycles of said signal for applying through said coupling means a potential to the base of said second transistor for rendering said second transistor conductive in opposition to bias potentials applied to electrodes of said second transistor.

8. A threshold trigger circuit comprising a pair of transisters 01'} opposite conductivity types, biasing means normally rendering each of said transistors non-conductive including a first voltage divider having intermediate points respectively connected to the emitter electrodes of the transistors and a second voltage divider having an intermediate point connected to the base electrode of one of said transistors, said biasing means establishing a threshold level for said one transistor, a direct connection between the output electrode of said one transistor and the input electrode of said other transistor, means for applying a signal to the base electrode of said one transistor, and a regenerative feedback circuit between the emitter electrodes of said transistors including a part of said first voltage divider between said intermediate points effective to render both said transistors conductive when the signal applied to the base electrode of said one transistor is above the threshold level.

References Cited in the file of this patent UNITED STATES PATENTS 2,759,104 Shellett Aug. 14, 1956 2,798,160 Bruck July 2, 1957 2,840,727 Guggi June 24, 1958 2,843,743 Hamilton July 15, 1958 2,843,762 Trent July 15, 1958 2,853,633 McVey Sept. 23, 1958 2,858,456 Royer Oct. 28, 1958 2,905,815 Goodrich Sept. 22, 1959 2,971,134 Cockrell Feb. 7, 1961 FOREIGN PATENTS 789,478 Great Britain Jan. 22, 1958 OTHER REFERENCES Transistor Circuit Handbook, Garner, 1957 edition, published by Educational Book Co. Publishing Division Coyne Electrical School.

Claims (1)

1. A THRESHOLD TRIGGER CIRCUIT COMPRISING A PAIR OF TRANSISTORS OF OPPOSITE CONDUCTIVITY TYPES CONNECTED IN CIRCUIT WITH A POTENTIAL SOURCE TO NORMALLY APPLY CUT OFF BIAS POTENTIALS TO ELECTRODES OF BOTH TRANSISTORS, COUPLING MEANS FROM ONE TRANSISTOR TO THE OTHER TRANSISTOR EFFECTIVE UPON CONDUCTION OF SAID ONE TRANSISTOR FOR APPLYING POTENTIALS OPPOSING THE BIAS POTENTIALS ON THE ELECTRODES OF SAID OTHER TRANSISTOR, MEANS FOR APPLYING A SIGNAL TO THE BASE OF SAID ONE TRANSISTOR FOR INITIATING CONDUCTION OF SAID OTHER TRANSISTOR, AND A REGENERATIVE FEEDBACK CIRCUIT BETWEEN CORRESPONDING ELECTRODES OF SAID TRANSISTORS EFFECTIVE UPON CONDUCTION OF SAID OTHER TRANSISTOR FOR APPLYING POTENTIALS OPPOSING THE BIAS POTENTIALS ON THE ELECTRODES OF SAID ONE TRANSISTOR BUT INSUFFICIENT TO RENDER SAID ONE TRANSISTOR CONDUCTIVE IN THE ABSENCE OF A SIGNAL APPLIED TO THE BASE THEREOF.

Images