US2916637A

US2916637A - Multivibrator circuits with improved power-frequency capacity

Info

Publication number: US2916637A
Application number: US527355A
Authority: US
Inventors: Cravens L Wanlass
Original assignee: Thompson Ramo Wooldridge Inc
Current assignee: Northrop Grumman Space and Mission Systems Corp
Priority date: 1955-08-09
Filing date: 1955-08-09
Publication date: 1959-12-08
Anticipated expiration: 1976-12-08

Description

1959 c. WANLASS 2,916,637

MULTIVIBRATORCIRCUITS WITH IMPROVED POWER-FREQUENCY CAPACITY Filed Aug. 9; 1955 2 Sheets-Sheet 1 some cf flTf/VT/IL v ,4 n z oar/ 07 I 0071 07. arms: 577766 me 0/ Z2? eezaeg/vcz v INVENTOR. 660% 4 M4MI56 Dec. 8, 1959 c. L. WANLASS MULTIVIBRATOR CIRCUITS WITH IMPROVED POWER-FREQUENCY CAPACITY 2 Sheets-Sheet 2 r6 fE Filed Aug. 9, 1955 United States Patent MULTIVIBRATOR CIRCUITS WITH HWPROVED POWER-FREQUENCY CAPACITY Cravens L. Wanlass, Whittien CalilL, assignor, by mesne assignments, to Thompson Ramo Wooldridge Inc., Cleveland, Ohio, a corporation of Ohio Application August 9, 1955, Serial No. 527,355 13 Claims. (Cl. 30788.)

This invention relates to multivibrator circuits having improved power-frequency capacityand, more particularly, to transistor flip-flop circuits wherein an enhanced power capacity is possible for a given frequency of operation.

The expression power-frequency capacity as utilized herein is intended to indicate a mutual relationship between power and frequency where the increase in one necessitates a decrease in the other if the circuit is being operated at its maximum capability. Thus a low powerfrequency capacity may still allow a medium or high frequency'rate of operation where the power requirements are slight; and similarly a high power-frequency capability may still require some limitation upon the frequency of operation where a large amount of power is needed.

It has become conventional practice in the art to increase the power-frequency capacity of a multivibrator circuit by passing the multivibrator output signals through additional cathode follower stages. This arrangement then provides a power amplification through the cathode follower and also makes it possible to achieve a relatively low output impedance. However, the cathode follower stages are in no way incorporated into themultivibrator circuit itself and consequently the operation of the multivibrator is not changed. Thus no increased efliciency results in the multivibrator operation and the same amount of triggering power is still required. The onlyresult then is the ability of the multivibrator circuit to, control the supply of more power through the cathode follower stages. a

While the conventional approach offers some increase in power-frequency capacity, it will be shown herein that the same additional amplification or cathode follower stages may be incorporated into the multivibrator circuit itself in an efficient manner. The resultthen is that'not only is the power-frequency capacity of the multivibrator enhanced but the required triggering power is greatly reduced. -Thus'whi1e the conventional technique may improve the overall system power-frequency capacity by, a small factor, the present invention'may allow a considerable overall improvement of-a higher order of magnitude due to the double action thereof. J

In accordance. with the present invention the additional stages for power amplification are incorporated into the multivibrator cross-coupling circuit so that not only is the usual result of output power amplification accomplished but also a cross-coupling power amplification results. This cross-coupling does not interfere with the circuit stability but yet allows a 'c'onsiderable increase in trigger sensitivity. The term trigger sensitivity is employed here as a relative term indicating the amount of power required to cause the multivibrator to change states. The greater the trigger sensitivity, the less the required trigger power. I

- In particular, the invention is employed in a multivibrator circuit which comprisesfirst and second amplitiers. having first and second output circuits cross-coupled 2,916,637 te d nee. 8,1959.

tosecond and first load impedances, respectively. The amplifiers have first and second input circuits coupled to respective input impedances. The improvement of the invention then comprises the inclusion of third and fourth output amplifiers as a power amplification means completing the coupling path between the load impedances and the input irnpedances.

In a particular arrangement wherein transistors are employed, the additional amplifiers are connected with their base electrodes coupled to the junction of associated load resistors and the cross-coupled collector electrodes of the usual cross-coupled transistors. Operating potential then is applied to both load resistors andto the collector electrodes of the additional power amplifier transistors. The emitters of these transistors thenare coupled to the feedback impedances.

.The additional transistor amplifiers then not only provide an emitter-follower power amplification but also considerably enhance the trigger sensitivity since efiect-ively a low impedance feedback source is provided each time the transistor flip-flop is to be actuated into a changed state, In particular, where NPN transistors are employed as the additional power amplifiers, the turning off of one transistor causes a rise in potential which in turn causes one of the output power amplifiers to provide a cross-coupled low impedance driving source for the other transistor. s. .j I. 1 Thus in this manner the amount of power required to turn one of the cross-coupled transistorsoflf or to. turn one of the cross-coupled transistors on is greatly reduced through the output power amplifier which becomes operative to feedback power.

In this manner then the output power amplifiersare employed in accordance with the present invention in both a transition-aiding and output-signal-amplification capacity. In other words, the same output stages operate during the trigger pulse period in reducing the amount of power required for the triggering of thejvarious multivibrators and also increase the amount of power available under the control of the multivibrator.

While the invention may be employed with various types of cross-coupled amplifier multivibrators, it is especially useful with transistor flip-flops of the type described in my copending application for Multivibrator Circuits Employing Voltage Breakdown Devices, Serial No. 513,426, filed June 6, 1955.

When the output power amplifier arrangement is utilized with the multivibrator circuit described in this copending application, the low impedance output circuit of the emitter follower stage is coupled to the Zener diode input circuit for the amplifier. As is more fully explained in the copending application the multivibrator stable states are maintained in the voltage breakdown condition by one of the Zener input diodes. -When an input diode is broken down, it provides actuating cur rent for the amplifier to which it is coupled whereas the Zener diode coupled to the other amplifier does not break 'down and presents an effective open circuit.

The employment of the present invention with Zener diode type of multivibrator described in the abovementioned copending application provides several improvements. In the first place, a low impedance power source is provided for changing the state of the multivibrator and thus for causing a more rapid breakdown of the Zener diode. In addition to this the present invention makes it possible for the 'multivibrator output signal swings to rise to substantially the source potential for the high level output signal and then to fall to. substantially the same low level as previously available. i

7 Several variations in the basic circuit of the invention are contemplated Where increased trigger pulse crosscoupling isjdesired a bypass capacitor is normallyplaced the in parallel with the input impedance. This cross-coupling trigger response may be further enhanced by the inclusion of a diode which is arranged to pass a triggering pulse where the associated power amplifier is cut off. Inparticular where NPN transistor power amplifiers are employed the diode is connected to each power amplifier with its cathode connected thereto and its anode connected to the bypass capacitor in parallel with the input impedance. In this manner then when a cross-coupled transistor associated with a power amplifier is caused to conduct, and the cross-coupled load impedance causes a drop in the signal level at the base electrode of the power amplifier, the diode transmits a negative signal to the base electrode of the other cross-coupled transistor tending to cut it off rapidly.

A further advantage which is possible through the practice of the present invention is that transistor multivibrators may be devised wherein a single type of transistor is employed. Thus it is possible in accordance with the present invention to devise a transistor flip-flop utilizing two NPN cross-coupled transisfis and two NPN output transistors for driving a load. In this case an additional diode may be employed to prevent the passage of negative sampling or clock pulse signals back through the output NPN transistor into the input circuit of the transistor flip-flop.

Another arrangement which will prevent the passing of negative pulses back through an NPN output transistor is the utilization of a storage capacitor and buffer resistor at the base electrode of the output transistor to provide the output signal. This arrangement, however, does not provide the frequency response of the arrangement where the diode is employed.

In view of the fact that the various transistor multivibrator circuits which are described herein have been previously described in their unmodified form in the abovementioned copending application Serial No. 513,426, the description herein will be primarily concerned with the improvement of the invention.

Accordingly, it is an object of the present invention to provide an improved multivibrator circuit wherein output power amplifiers are employed to enhance both the power-frequency capacity and the trigger pulse sensitivity.

Another object is to provide means for improving the frequency response of a multivibrator wherein Zener diodes are employed as breakdown'elements coupled to the input circuits of cross-coupled transistors.

Yet another object is to provide means for increasing the maximum output signal swing in a multivibrator circuit employing voltage breakdown elements as the input impedances for cross-coupled amplifiers.

Still another object is to provide a multivibrator circuit having an improved power-frequency capacity as well as a higher trigger pulse sensitivity due to the dual action of output power amplifiers utilized in a unique manner.

A specific object is to provide an improvement in a multivibrator circuit wherein first and second amplifiers have first and second output circuits cross-coupled to second and first load impedances, respectively, the amplifiers having first and second input circuits coupled to first and second input impedances, respectively, the improvement residing in the utilization of third and fourth output amplifiers to enhance both the trigger response and the power supplying capacity.

Another specific object of the invention is to provide improvement in a multivibrator circuit wherein amplifiers A and Ai have output circuits coupled to impedances Z and Z respectively, the amplifiers having respective input circuits coupled to impedances Z, and Z respectively, the improvement making it possible to reduce the amount of power required to trigger the multivibrator circuit while at the same time providing an increased power output capacity for the multivibrator for a given frequency.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, and are not intended as a definition of the limits of the invention.

Fig. 1 shows a block diagram of a basic embodiment of the invention;

Fig. 2 illustrates two species of the invention where (a) illustrates the use of NPN transistors and (12) illustrates the use of PNP transistors;

Fig, 3 illustrates two arrangements employing the invention wherein additional amplifiers are utilized to supply an output load, the embodiment of Fig. 3a illustrating the use of a storage capacitor for an output load, and the embodiment of Fig. 3b providing means for drawing output current directly through the multivibrator;

Fig. 4 illustrates two other species of multivibrators employing the present invention illustrating other forms of output circuits; and

Fig. 5 provides a graphical comparison of the operating characteristics of multivibrators with and without the present invention.

Reference is now made to Fig. 1 wherein the basic embodiment of the invention is shown in block diagram form. As indicated in Fig. 1 the invention is employed in a multivibrator circuit wherein amplifiers A and A; have output circuits coupled to impedances 2 and Z respectively. The input circuits of amplifiers A and A; are coupled to impedances Z and Z respectively. It will be noted that impedances Z Z may alternatively be voltage breakdown devices D1 and D2. Thus the term impedance is intended to include the voltage breakdown impedance of a diode, such an impedance being also referred to as a Zener diode characteristic.

The improving feature of the invention then is the utilization of output amplifiers A and A to provide a power amplifying coupling between load impedances Z and Z and the input impedances 2 and Z respectively. Wihenthe Zener diodes D1 and D2 are employed this coupling then exists between Z and Zener diode D1, and Z and Zener diode D2.

This arrangement not only provides a power amplification to present signals to output stages 0 and 0 but also introduces a power amplifying cross-coupling loop between amlplifier A through the signal change across impedance Z which then controls the power supplying function of amplifier A coupled to input impedance Z In a similar manner the signal change in amplifier A; is power amplified through the action of impedance Z and power amplifier A being thus cross-coupled to the input circuit of amplifier AP.

The particular manner in which the invention operates to provide the improvement mentioned above may be better understood by considering several of the specific arrangements which are possible. Reference is thus made to Fig. 2 wherein two types of multivibrators employing the present invention are shown in schematic form. In Fig. 2a, in particular, the cross-coupled amplifier is found in the form of transistor circuits T and T; having emitter electrodes connected together and to ground and col lector electrodes cross-coupled to output transistors T and T respectively. The load impedances then are found in the form of resistors R and R? coupled respectively to the base-collector junction between transistors T and T and the junction between the base electrode of T and the collector electrode of T}.

In the embodiment of Fig. 2a the input impedance consists of several elements. The essential elements are the voltage breakdown devices D1 and D2 connected to the base electrodes of transistors TE and T respectively.

.Zener diode D2 through resistor R In'addition, these diodes are connected to input resistors Ri and R and the series connection of the Zener diode and the resistors are bypassed by capacitors C1 and C2, respectively.

In operation then transistors T and T not only provide an output power amplification in the conventional manner but also considerablyenhance the trigger sensitivity, thus reducing the trigger power required, due to their respective cross-coupling efiects. In particular consider the operation of transistor T when a negative triggering signal is applied to the base of transistor T which is assumed to be in a conducting state.

At this time a relatively large amount of current is drawn through resistor R? depressing the potential applied to the base of transistor T and causing its emitter to be depressed. As a result the potential across Zener diode D1 is depressed and this diode does not enter its breakdown state, as is more fully explained in the abovementioned copending application Serial No. 513,426.

On the other hand, transistor TE is in a nonconducting state so that the potential at the base electrode of transistor T is high.- This transistor is thus conducting providing a low impedance current source for Zener diode D2 causing it to break down due to the high potential thereacross. Diode D2 thus passes ample input current to'the base of transistor T to maintain it in a highly conducting state.

' When a negative pulse is applied to transistor T tending to cut it off, the potential rises at the base of transistor T which is then driven into a power amplification state where it becomes a low impedance source coupled in series with Zener diode D1. Transistor T then drives the base of transistor T more positive and causes transistor T to conduct more heavily. Transistor T; is caused to conduct less and thus the base of transistor T continues to rise. This process continues until the base and emitter or transistor T are substantially at +13 potential.

Inthis manner the cross-coupling time constant is greatly reduced so that the trigger power required may be lessened.

In a similar manner when transistor T is highly conducting and Zener diode D1 is broken down, the amplitude of the negative triggering pulse required to be applied to its base is considerably lessened due to the power amplification action of output transistor T which drives Therefore, it will be recognized that not only do the output transistors provide a power amplification of the multivibrator output signals but they also reduce the amount of power required to trigger the multivibrator. In addition this arrangement increases the potential swing of the multivibrator output signals so that they may vary in this case from substantially ground to substantially +E.

Another species of transistor circuit is shown in Fig. 2(b) where PNP transistors are employed. I The operation of this circuit is similar to that of Fig. 2(a) except for the inversion of the various potentials involved. In the operation of the embodiment of Fig. 2(b), two diodes D1 and D2'are caused to break down when the output transistors reduce the potential applied thereto sufiiciently below ground.

From the description thus far it should be apparent that the arrangement of the invention makes it possible to achieve a considerable increase in the power-frequency capacity of a system employing a plurality of multivibrators where certain multivibrators provide actuating signals for others. The reason for this is that the double effect of the inclusion of the output amplifiers in the multivibrators in accordance with the invention increases each multivibrators capacity while at the same time decreasing the amount of trigger power required in the system.

As pointed out above, another important feature of the of allone-type. Thisfeature is illustrated in two specific circuits'in Fig. 3 where Fig. 3(a) is similar to that shown in Fig. 2(a), with the further inclusion of two

output circuits

10 and 20. These circuits include bulfer resistors R and R respectively; include second output transistors-T and T each transistor having its base electrode connected to the associated bufiier resistor; and in clude output storage capacitors C and C each having one end connected to the junction of the associated buffer resistor and transistor base, and the other end connected to ground.

' In operation each of the second output transistors receives a forward biasing signal through its associated buffer resistor when the first output transistor coupled thereto is in a highly conducting state.

It is also during the time that a first output transistor is conducting that the associated output storage capacitor is charged up to provide a signal representing the state of the multivibrator. Thus, in particular, when transistor T} is cut off, or in the state of low conduction, first output transistor T is driven into the high voltage and conduction state to supply the breakdown potential and current for Zener diode D2, as explained above.

At this time then charging current is provided through buffer resistor R to bring the signal level of capacitor C up to a high level, which may be a O-representing output signal. This high level signal then also causes the forward biasing of the second output transistor T Thus when a negative clock pulse or sampling signal is examples of clock pulse gating circuits, and also to copending application for Input Circuits and Matrices Employing Zener Diodes as Voltage Breakdown Gating Elements by C. L. Wanlass, filed July 15, 1955, Serial No. 522,242.

It will be noted then that when the transistor T 'i's actuated to a high conducting state and output transistor T is thereby in the low voltage state, second output transistor T is also in the low voltage state. .This output circuit characteristic is very satisfactory when the negative and gating technique of the just mentioned co-pending application is employed.

While the arrangement of Fig. 3a is satisfactory and allows the utilization of all NPN transistors, it is somewhat limited in frequency response due to the utilization of the buifer resistors and the storage capacitors. Thus in Fig. 3b an arrangement is illustrated wherein power may be drawn directly through the multivibrator. However, this arrangement raises the possibility of causing negative triggering signals to pass back through 'either of output transistors T or T when a clock pulse is applied. This problem may be remedied by adding diodes D and D22, each of which is connected between the base of the associated second output transistor and the associated one of bypass capacitors C1 and C2.

Considering diode D in particular it will be noted that the anode thereof is connected to the base of transistor T and the cathode is coupled to capacitor C1. Thus when a negative signal appears at the emitter of transistor T it is prevented from passing back through capacitor C1 to the base of transistor T} by diode D If this were not done, it will be recognized that this negative signal passing back to the transistor would cause it to trigger at the wrong time.

The reason for this is that output transistor T is forward biased so as to allow the passage of a negative signal back into the circuit when transistor T is highly conducting and transistor T is in a: low conduction 01 7 cut-off state. It is at this time, it will be recalled, that the base potential of output transistor T is high and that it provides a high forward biasing potential for second output transistor T The operation of diode D in preventing the passage of negative signals spuriously back through output transistor T and capacitor C2 to the base of transistor T should be understood from this example.

Another possible improvement is also illustrated in the circuit of Fig. 3b in the form of diodes D and D These diodes are provided to enhance the triggering rate and provide a coupling path for negative signals through the associated bypass capacitors to the base electrodes of the associated one of cross-coupled transistors T and T Thus in a particular case a negative signal applied to transistor T causes increased conduction in transistor T Negative current then passes through Zener diode D2 and increases the conduction of transistor T}. This then causes a negative potential drop at the base of transistor T Without diode D then this negative potential would have to be regenerated through the back impedance of transistor T With diode D in a circuit, however, the negative change is transmitted through the low impedance forward bias characteristic thereof and coupling capacitor C1 to the base electrode of transistor T Thus in this manner the triggering action is greatly enhanced.

While there is some advantage in utilizing the invention where the transistor types are all the same, as for example, all of the NPN type, the invention is not so limited and two schematic arrangements are shown in Fig. 4 where both NPN and PNP types of transistors are employed in the

output circuits

10 and 20. It will be noted that in the embodiment of Fig. 4a the power amplifier transistors T and T are PNP transistors as are the cross-coupled transistors T and T In this embodiment, then, each first output transistor provides a low impedance negative source for the associated Zener diode when the associated cross-coupled transistor is in a cut-off or low conduction state. Thus, in particular, when transistor T} is cut on, first output transistor T receives a forward biasing negative potential and therefore tends to drive the anode of Zener diode D2 towards '-E potential. This causes a breakdown and therefore provides actuating base current for cross-coupled tran sistor T Each of the output stages shown in Fig. 4 includes two transistors having their emitters connected together and to an output storage capacitor. The collector of one of the transistors is connected to ground and the other receives the source potential. It will be noted that the base electrodes of both output transistors are connected together and receive a controlling input signal though a buffer resistor connected to the emitter electrode of the associated first output transistor. Since all of these output circuits function in a similar manner output circuit 10 of Fig. 4b will be described as illustrative.

For the purpose of this illustration it will be considered that transistor T is in a high conduction state in that transistor Tg is cut off. This means that first output transistor T is highly conducting and Zener diode D1 is broken down. A relatively high negative potential signal therefore is applied to the base electrodes of both output transistors T and T This causes transistor T to conduct and negatively charge output storage capacitor The feature of the present invention is that it provides an increased power capacity in the output signal, while at the same time enhancing the trigger sensitivity in providing a low impedance potential source of substantially E for the associated Zener diode.

It is important to note that the embodiments of Fig. 4 may be operated with either positive or negative signals and that the power amplifier output transistors function as well in either case.

The improvement that the invention provides is illustrated in Fig. 5 where comparative output waveforms for one clock pulse period are shown. These waveforms indicate graphically how the output signals would look without, and with, the inclusion of the extra power amplifiers utilized in accordance with the invention. Two cases are considered in Fig. 5. In one an output storage capacitor of 3,000 micromicrofarads is assumed and the multivibrator is pulsed at three different frequencies, namely 300 kc., 400 kc., and 500 kc. The waveforms then illustrate the general form of the output signal which is developed across the storage capacitor when the multivibrator is first triggered into a high state, at the beginning of the clock pulse period, and then is triggered into a low state at the end of the period. It will be noted then that without the extra power amplifier the output signal barely reaches its peak amplitude for a pulse rate of 300 kc., that the pulse amplitude is attenuated somewhat for a 400 kc. pulse rate and that at 500 kc. the amplitude is down to approximately two-thirds value.

With the inclusion of the power amplifiers connected in accordance with the present invention, however, the output waveform is still substantially square even at 500 kc. so that pulse repetition rates between 500 kc. and l megacycle appear to be feasible.

This improvement is also illustrated with the assumption of a fixed frequency of operation, namely at 200 kc. and a variation in the load, or output storage capacitor. Thus it will be noted that without the improvement of the invention the output signal reaches its maximum amplitude for a load of 3,000 micromicrofarads, but is considerably reduced in amplitude with a load of 5,000 micromicrofarads. With the present invention, on the other hand, the amplitude of the output signal still reaches its maximum point with a 5,000 micromicrofarad load.

Thus in one case the invention allows an increased frequency with the same load, without reducing the reliability of multivibrator operation, and in the other case allows an increased load at the same frequency without impairing the reliability of the operation.

From the foregoing description it is apparent that the present invention provides an improved multivibrator circuit wherein additional power amplifiers are employed to enhance both the power-frequency capacity of the multivibrator and its trigger pulse sensitivity.

It has been pointed out that the invention is especially advantageous when it is employed in a multivibrator wherein Zener diodes are employed as voltage breakdown elements coupled to the input circuits of crosscoupled transistors. In this case the additional power amplifiers provide low impedance driving sources for the Zener diodes and allow a very rapid breakdown of these diodes during the transition period. Furthermore, it has been shown that the invention allows a greater output voltage swing to substantially the source potential.

While a few specific forms of the invention have been shown in detail, it will be understood that a wide variety of other types are contemplated and consequently it is recognized that those skilled in the art will develop a multitude of other species without departing from the spirit of the present invention.

What is claimed as new is:

l. A transistor flip-flop having a relatively high powerfrequency capacity, said flip-flop comprising first and second transistors, each having base, collector and emitter electrodes; first and second load resistors coupled to the collector electrodes of said second and first transistors, respectively; first and second input impedances connected between the base electrode of said first and second transistors, and said first and second load resistors respectively, each input impedance including a Zener diode; and an output transistor corresponding to each of said first and second transistors, said output transistor including base, collector and emitter electrodes, the base electrode of said output transistor being coupled to said cor- 9 responding load resistors and theemitter electrode of said output transistor being coupled to said corresponding input impedance.

2. The transistor flip-flop defined in claim 1 wherein said transistors are NPN transistors, a positive source potential being applied to said load resistors and to the collector electrodes of said output transistors.

3. The transistor flip-flop defined in claim 1 wherein said transistors are PNP transistors, a negative source potential being applied to said load resistors and to the collector electrodes of said output transistors.

4. In a transistor multivibrator wherein first and second transistors are crosscoupled to second and first load impedances, respectively, the base electrodes of said first and second transistors being coupled to respective Zener diodes and to associated bypass capacitors, the improvement comprising: first and second output transistors having base electrodes connected to said first and second load impedances, respectively; a first pair of diodes coupled to said bypass capacitors, respectively, coupling the emitter electrodes of said output transistor thereto, and providing a low impedance charging path therefor; and a second pair of diodes coupled between the base electrodes of said output transistors, respectively, and said bypass capacitors for providing a low impedance discharging path therefor.

5. A multivibrator circuit comprising: first andsecond amplifiers having respective relatively low impedance input circuits and relatively high impedance output circuits; third and fourth amplifiers having input circuits coupled to the output circuits of said first and second amplifiers, respectively, and having output circuits coupled to the input circuits of said second and first amplifiers, respectively, the operation of said multivibrator circuit causing said third and fourth amplifiers to assume either a first state during which a relatively high voltage output appears across the associated output circuit or a second state when a relatively low voltage appears across the associated output circuit; and first andv second voltage breakdown devices in the output circuits of said third and fourth amplifiers, respectively, said voltage breakdown devices each having a breakdown potential in the voltage region between the high and low voltage output condition of the associated one of said third and fourth amplifiers, said first and second voltage breakdown devices thereby passing relatively high current to the input circuits of said second and first amplifiers, respectively, in response to the occurrence of said first state in said third and fourth amplifiers, respectively, and said voltage breakdown devices passing substantially no current to the associated input circuit when receiving said relatively low voltage output.

6. The multivibrator circuit defined in claim 5 wherein said first and second amplifiers comprise first and second transistors having base, collector, and emitter electrodes, the base electrodes of said first and second transistors being included in said low impedance input circuits and the collector electrodes of said first and second transistors being included in said high impedance output circuits, and wherein said third and fourth amplifiers comprise third and fourth transistors having base, collector, and emitter electrodes, the base electrodes of said third and fourth transistors being included in said high impedance input circuits and the emitter electrodes of said third and fourth transistors being included in said low impedance output circuits.

7. The multivibrator circuit defined in claim 6 wherein said transistors are all of the NPN type and wherein said voltage breakdown devices are diodes having cathodes coupled to said third and fourth amplifiers, respectively, and anodes coupled to the input circuits of said second and first amplifiers, respectively.

8. A transistor multivibrator comprising: first and second transistors cross coupled to form a multivibrator; a

third transistor serving as a coupling between said first and second transistors and having an input circuit coupled to the output circuit of said first transistor and an output 'circuitcoupled to the input circuit of said second transistor, the operation of said multivibrator circuit causing said third amplifier to produce either a high-voltage output signal at its output circuit for one state of the multivibrator, or a low-voltage output signal for the other state of the multivibrator; a voltage breakdown device coupled in current series with the output circuit of said third amplifier and having a breakdown potential in the region between the high and low output signals of said third amplifier; a capacitor coupled across said voltage breakdown device; and coupling means connected between the input and output circuits of said third amplifier for permitting the rapid charging and discharging of said capacitor.

9. The multivibrator circuit defined in claim 8 Wherein said coupling means includes a first diode coupling said capacitor to the output circuit of said third amplifier to provide a low impedance charging path for said capacitor, and a second diode coupled between said capacitor and the input circuit of said third amplifier to provide a low impedance discharging path for said capacitor.

10. A combination comprising: first and second transistors cross coupled to form a flip-flop, each of said transistors having base, collector, and emitter electrodes; first and second load impedances coupled to the collector electrodes of said second and first transistors, respectively; first and second Zener diodes coupled to the base electrodes of said first and second transistors to control the current which may pass to the associated transistor; a pair of output transistors having base electrodes coupled to said first and second load impedances, respectively, the operation of said flip-flop causing said output transistors to assume either a first state when a relatively high voltage appears at the emitter electrode thereof or a second state when a relatively low voltage appears at the emitter electrode thereof; and means coupling the emitter electrodes of said output transistors to said first and second Zener diodes, respectively, said Zener diodes being thereby actuated to pass relatively high current to the base electrode of the associated one of said first and second transistors in response to the first state of the associated output transistor.

11. The combination defined in claim 10 wherein first and second capacitors are coupled across said first and second Zener diodes, respectively, and wherein means is further provided for charging and discharging said capacitors.

12. The combination defined in claim 11 wherein said means for charging and discharging said capacitors includes third and fourth diodes coupling said first and second capacitors to the base electrodes of said output transistors, respectively, for providing a discharging path for said capacitors; and fifth and sixth diodes coupled between said capacitors and the emitter electrodes of said output transistors, respectively, for providing a charging path for said capacitors.

13. The combination defined in claim 12 wherein said transistors are of the NPN type and there is further included a second pair of NPN output transistors having base and collector electrodes coupled respectively to the emitter and collector electrodes of said output transistors, said third and fourth diodes having cathode electrodes coupled to the base electrodes of said output transistors, respectively, and anode electrodes coupled to said capacitors, respectively, and said fifth and sixth diodes having cathode electrodes coupled to the junction of the anode electrodes of said third and fourth diodes and said capacitors, respectively, and anode electrodes coupled to the emitters of said output transistors, respectively.

(References on following page) Referances Cited in the file of this patent UNITED STATES PATENTS Canfora May 4, 1948 Weiner Apr. 4, 1950 5 Bergfors May 2, 1950 Moore Feb. 6, 1951 12 Iohnstone Mar. 20, 1951 Anderson et a1. Dec. 16, 1952 Shockley Oct. 13, 1953 Harris Mar. 30, 1954 Wallace June 22, 1954 Linville Jan. 14, 1958 Haugk et a1. June 24, 1958