US2894130A - Switching circuits - Google Patents

Description

July 7, 1959 G. J. MARTIN SWITCHING CIRCUITS Filed June 24. 1955 OPEN FIG. 4

CLOSED -2OOV INVENTOR.

J; MARTIN GUNTHER ATTORNEY United States Patent SWITCHING CIRCUITS Gunther J. Marlin, Ridgefield, Conn., assignor, by mesne assignments, to Schlumberger Well Surveying Corporation, Houston, Tex., a corporation of Texas Application June 24, 1955, Serial No. 517,771

11 Claims. (Cl. 250--27) This invention relates to electrical switching circuits and, more particularly, to diode-switch circuits for unipolar signals.

In computing and other applications where information is rapidly handled by electronic circuitry, electronic switching circuits have in large measure replaced inherently slower mechanical switching devices. Commonly employed in electronic switches are unilaterally conductive diodes, their widespread employment in part being attributable to their relatively low cost and in part to their relative insensitivity to the amplitude of a signal by which switching is efiected. Diode-switch circuits commonly are classed either as series diode switches or as shunt diode switches by an analogy of their operation to the operation of mechanical series and shunt switches.

Typical diode-switch circuits are disclosed in Waveforms by Chance et al, which is volume 19 of the Radiation Laboratory Series published by the McGraw-Hill Company, Inc. At page 370 in Fig. 10.6 is shown a series diode-switch, while a shunt diode-switch for bidirectional operation is shown in Fig. 10.9 at page 373. Where thermionic vacuum tube diodes are employed in these circuits to secure a very high back resistance, these circuits become subject to cathode heater drifts which shift the DC. level of the output signal relative to the input signal. While the phenomenon of cathode heater drift is rather fully explained in section 3.14 of the abovecited text and particularly at page 64 thereof, the phenomenon can be described briefly as a consequence of changes in the total thermionic emission from the cathode due primarily to variations in heater voltage and to ageing of emissive material. For purposes of analyzing this phenomenon, the diode may be considered as equivalent to a resistor representing the efiective resistance of the diode and, in series therewith, a small battery poled in the direction of easy conduction and having a potential variable with variations in thermionic emission. While variations in elfective resistance particularly affect the drift of series diode-switches, shunt diode-switches have a drift primarily dependent upon the battery action. In general, it may be said that a higher freedom from drift is achievable with shunt diode-switches. Nonetheless, the absence of any compensation for cathode heater drifts in a shunt diode switch directly limits the achievable linearity and, hence, is a direct drawback to the shunt diode-switches disclosed in the above-cited reference.

Previous shunt diode-switches, as exemplified by those illustrated in the above-cited reference, have the further drawback that complete grounding of the signal is difiicult to achieve. In Fig. 10.11 of the text is shown a shunt diode-switch Where grounding is achieved by balancing the junction of similarly poled diodes between equal valued potentials of opposite polarity. To clamp the junction point at ground, a second pair of diodes are connected in similarly poled relation in parallel with the first pair of diodes and with their junction directly connectedto ground. While this balanced circuit is satisfactory for bipolar signals, for unipolar signals the necessity of oppositely poled switching pulses is a disadvantage.

More generally, shunt diode-switches of the prior art are subject to one or more of the following disadvantages: (l) non-linear transmission of the signal, (2) incomplete attenuation of the signal (3) zero drift, and (4) coupling of the switching pulse to the signal output.

Accordingly, it is an object of the present invention to provide new and improved diode-switch circuits which are substantially free of each of the above-enumerated disadvantages.

Another object of this invention is to provide new and improved diode-switch circuits capable of high speed operation while reliably alfording a high. degree of accuracy.

Another object of the present invention is to provide new and improved diode-switch circuits for effectively switching unipolar signals on or off for intervals of any selected duration.

Still another object of this invention is to provide new and improved diode-switch circuits which may be actuated by a single switching pulse which, if desired, may be of extended duration.

Yet another object of this invention is to provide new and improved switching circuits having a relatively high input impedance.

These and other objects of the invention are attained, in accordance with the invention, by connecting switching diodes in shunt relation at spaced points along a resistive circuit having input and output terminals. The switching diodes have like electrodes connected to the resistive circuit at the spaced points and have their other electrodes each connected through a separate resistor to a point having a DC. potential tending to produce forward conduction through the diodes. By selectively driving the switching diodes to a high back resistance state a unipolar signal applied at the input terminal is selectively passed to the output terminal.

In a preferred embodiment of the invention, a compensating diode in series with a resistor is connected at least to that one of the spaced points along the resistive circuit which is nearest the output terminal. The series resistor is preferably identical to the resistor for the corresponding switching diode and is connected to the same potential point. To couple a selector pulse of any desired duration to the switching diodes, coupling diodes are employed having like electrodes which are connected to the junction of the switching diodes and their respective resistors.

Other objects of the invention, together with its various advantages, will become apparent from the following detailed description of representative embodiments thereof, taken in conjunction with the accompanying drawing, in which:

Fig. 1 is a circuit diagram of a diode-switch circuit constructed in accordance with the invention;

Fig. 2 is a graph of typical high vacuum diode charactenstics.

Fig. 3 is a circuit diagram illustrating a modified version of diode-switch circuit in accordance with the invention; and

Fig. 4 is a circuit diagram for another version of diodeswitch circuit in accordance with the invention.

In the figures, like reference numerals are employed to designate similar parts.

In Fig. 1 there is shown the circuit of a shunt diode switch arranged for switching a unipolar signal V, which may be either fixed or variable with time. While. provision may be made for switching signals of either positive or negative polarity, the switching circuit of Fig. l is a ranged for signals of positive polarity. In dependence upon the polarity of a selector pulse e an output signal.

Y is derived from the switching circuit either equal to the input signal V, or at ground potential.

To achieve this effective switching action, there extends between an input terminal and an output terminal 11 a resistive circuit 12 whichmay comprise three series resistors14, 16, preferably each of a high value. For precise switching'of a signal to be applied to a high input impedance device such as an operational amplifier (not shown) resistors 14 and 15 may have a value of 0.5 megohm, while resistor 16 has a value of 1.0 megohm. By virtue of'the resistors 14-16, their junction points 17 18 areresistively separated from one another and from the input and output terminals 10, 11. The resistors 1416 thus provide a high resistance current path between the input terminal 10 and output terminal 11.

For effective switching of current flowing through the resistance circuit between terminal 10 and terminal 11, junction points 17, 18 therealong are selectively shunted to a reference potential point 19, such as ground. A first shunt circuit is provided between junction point 17 and ground by a switching diode 20 and a resistor 21 in series therewith. So that the reference potential at point 19 relative to the potential applied at input terminal 10 will tend to produce forward conduction through diode 20, this diode has its anode connected at junction point 17 and its cathode connected to resistor 21. A second shunt circuit is provided between junction point 18 and ground 19 by a switching diode 22 and a series resistor 23. In similarity to switching diode 20, the switching diode 22 has its anode connected to junction point 18 and cathode connected to the resistor 23, whereby the positive potential atinput terminal 10 relative to ground tends to produce forward conduction between the anode and cathode. intermediate the anode of diode 22 and junction 18, however, is a balancingpotentiometer 24 having its wiper conneoted to the junction point 18.

In order that'the diodes 20, 22 may offer a very high back resistance at low voltages, they are preferably of the high vacuum thermionic type, such as the 6AL5 type, employing an indirectly heated cathode. Typical diode characteristics in a plot of current against potential are shown in Fig. 2. An increase in thermionic emission produces a vertical shift in the curve. Without compensation the output potential V drifts negatively in proportion to the increase in emission. Likewise, the output is subject to a positive drift with decreasing emission. Even with a carefully regulated heater suply potential, cathode heater drift, in the absence of compensation, is appreciable. Therrnionic emission may also vary with aging and ambient termperature.

To compensate for changing diode characteristics including cathode heater drift, there is connected between junction point 18 and ground 19 a compensating shunt circuit comprising high-vacuum compensating diode 25 and a series resistor 26. The compensating diode 25 is arranged in oppositely poled relation to the switching diode 22, having its cathode connected to the terminal of the potentiometer 24 opposite from the terminal to which the anode of diode 22 is connected. In order that the resistances through the shunt circuits connected at junction '18 may readily be equalized by the balancing potentiometer 24, the resistance of resistor 26 is preferably equal to that of resistor 23. Thus, resistors 23, 26 may have a value of 4.7 kilohms, or approximately half a preferred value of 10 kilohms for the resistor 21.

I The diode-switch circuit, as described thus far, operates to maintain the output signal V at reference or ground potential with a minimum of Zero drift. This condition is hereafter referred to as the open state of the circuit. To switch the circuit to its closed" state, provision is'made for driving the switching diodes to a reverse "operating point "27 where a high back resistance is obtamed. Tothis end, a cathode follower driver comprising a triode 28 and acathode resistor 29 may have a selector signal ie,, applied at its control-grid. Via high- vacuum coupling diodes 30, 32, the selector signal derived from the cathode resistor 29 is applied to the cathodes of switching diodes 20, 22 respectively. With respect to ground, the coupling diodes 30, 32 are similarly poled in relation to switching diodes 20, 22. Thus, diodes 20 and 30 have their cathodes connected and, likewise, diodes 22 and 32 have their cathodes connected. Interposed between the cathodes of diodes 22 and 32, however, is a current limiting resistor 33 which may have value such as 2.2 kilohms. The wavefrom sketched in Fig. 1 illustrates positive and negative levels between which the selector signal e may appropriately vary as it appears across the cathode resistor 'In an operation of the diode switching circuit of Fig. 1, an input signal V of positive polarity is applied at the input terminal .10. When a positive selector signal e is coupled via diodes 30, 32 to the cathodes of switching diodes 20, 22 sufiicientto carry the switching diodes 20, 22 to reverse point 2'7 (Fig. 2) in their vacuum tube characteristics, they will offer an exceedingly high back resistance between the junction points 17, 18 and ground. Since the compensating diode 25, by its oppositely poled connection, likewise offers an exceedingly high back resistance between junction 18 and ground, current 'ispassed between input terminal 10 and output terminal 11 without any appreciable attenuation due to shunting. If the output terminal 11 is coupled to a highimpedance load relative to the resistances of resistors 14-16, such as to the input of a D0. operational amplifier, the output potential V is substantially equal to the input potential V The back resistance of the switching diodes 20, 22 further serves to isolate the selector signal e from the output terminal 11. When the polarity of the selector signal e is reversed to become negative, the coupling diodes 30, 32 are carried to a reverse operating point, effectively isolating the selector signal from the switching diodes 20, 22. Accordingly the switching diodes 20, 22 are returned to a state of forward conduction where their effective resistance may be on the order of 2,000 ohms.

Using 2,000 ohms as an approximate value for the effective resistance of switching diodes 20, 22 the attenuation aiforded by the first shunt path atvjunction point 17 is equal to the ratio of 12 kilohms to 0.5 megohm, or 0.024. The attentuation afforded by the second shunt path connected at junction point 18 is approximately equal to the ratio of 7 kilohms to 0.5 megohrn, or 0.014. The overall attenuation is a product of the first and second attenuations, or approximately the ratio of 1 to 3,000. If a signal V, of 30 volts is applied at input terminal 10, a signal of only approximately 10 millivolts will be obtained at the output terminal 11. While these values are merely exemplary, they serve to indicate the precision in switching which is afforded.

Noteworthy is the fact that the potential across switching diode 20 and particularly that across Switching diode 22 are just slightly above 0 volts as indicated at 33' in Fig. 2. Similarly, a potential slightly below 0 volts, as indicted at 34, is applied across compensating diode 25, the reversal in polarity taking account of the reversely poled connection of the compensating diode 25. By providing the switching diode 22 and the compensating diode 25 with substantially the same current-potential characteristic, as by employing diodes in the same en velope, the close proximity of the potentials: across these diodes 22, 25 to zero ensures that their effective resistances are substantially identical. Accordingly, any change in the effective within the diodes'22, 25 due to such effects as cathode heater drift or ageing maybe balanced out at the wiper of the balancing potentiometer 24. In other Words, by connecting compensating diode 25 in oppositely poled or ba'ck to-b'ack relation with the switching diode 22, the output potential V,, may 'be switched substantially to ground potential relatively free of any variations due to changes in the tube characteristics of the switching diode 22. Expressed another way, the Compensation afiorded by the compensating diode 25 minimizes zero drift of the switching circuit so that the switching circuit not only has the high speed assoc ated with diode circuitry but also has a switching precision closely approximating that of a mechanical make-andbreak, shunt type switch.

. While very'good compensation is achieved when a compensating diode is coupled only to the switching diode nearest the output terminal, a more perfect compensation may be obtained by coupling a compensating diode to each of the switching diodes in the diode switching circuit. An embodiment of the invention incorporating this very efiective compensation and further illustratingthe application of the invention to a switching of negative signals is shown in Fig. 3. To accommodate the negative polarity of the input signal V the switching diodes 20, 22 have their cathodes connected to junction points 17, 18 respectively, sothat the positive polarity of ground relative to the negative input signal V tends to produce forward conduction. A corresponding reversal in the connections of coupling diodes 30, 32 is made to mamtain a connection of like electrodes between diodes 20, 30 and diodes 22, 32.

i For effective compensation, the compensat ng diode 25 is connected in oppositely poled relation to diode 22 and thus has its anode connected to junction point 18. In this embodiment, the balancing potentiometer 24 is omitted, resistors 23, 26 being valued to achieve equal effective resistances in the circuits of diodes 22 and 25. To compensate for changing characteristics of sw tching diode 20, a high-vacuum compensating diode 35 is connected in oppositely poled relation to it, there being a resistor 36 connecting diode 35 in series. w1th ground.

In an operation of the diode-switch circu t of Fig. 3, a negative signal 'V, is applied to input terminal 19. By ,applicattion of a negative selector signal e the sw tching circuit may be closed in the sense of affording a conductive path between the input and output terminals free of appreciable grounding. Thus a'negative selector signal drives the anodes of the switching diodes 20, 22 negatively to a reverse potential with respect to the potentials applied at their cathodes. This condition is achieved by drawing current from ground through the respective resistors 21, 23, coupling diodes 30, 32 and cathode resistor 29 to the B-- supply for the cathode follower 28. Since the switching diodes are driven to a reverse operating point and the compensating diodes 25, 35 are connected to have a reverse operating point, all of the diodes 20, 22, 25, 35 afford a high back resistance through which no appreciable current is diverted.

The switch is opened in the sense of effectively grounding the output terminal 11 by applying a positive selector signal e to the coupling diodes 30, 32. Such a positive selector signal drives the coupling diodes 30, 32 to a reverse point, leaving the switching diodes 20, 22 in a condition of forward conduction. The attenuation afforded by the switching diodes 20, 22 may be identical with that obtained by circuitry as shown in Fig. 1. In the circuitry of Fig. 3, however, variations in switching diode characteristics are compensated in each of the shunt circuits. Hence, cathode heater drift and ageing in each of switching diodes 20 and 22 are compensated so as to have no appreciable effect at the output terminal 11.

The speedy and highly accurate switching circuit illustrated in Fig. 3 has been found well adapted to signal converting systems such as shown and described in copending application Serial No. 516,868, filed June 21, 1955, by G. Martin for Signal Converting Systems and assigned to the assignee hereof.

:While a higher degree of. attenuation may be achieved by driving the switching diodes well into a state of forward conduction, it has been found desirable to operate the switching diodes at a forward conduction point 33 verylclose to zero potential in order that the effective resistances of the switching diodes and the compensating diodes may be closely matched. Accordingly, where it is desired to obtain a higher degree of attenuation and yet retain the excellent compensation afforded by this invention, resort may be had to the diode-switch circuit of Fig. 4. In this circuit, there is added in advance of the resistor 14 a series resistor 37joined with resistor 14 at point 38. Exemplary values for the resistors 14, 15, 16 and 37 are, respectively, 0.75, 0.75, 1.0, and 0.5 megohms.

For an input signal of negative polarity, diodes 20, 22 and 25 of Fig. 4 are connected just as shown in Fig. 3, that is, with the switching diodes arranged for forward conduction and the compensating diode driven to a rereverse point by the negative potential of the input signal V relative to ground. The balancing potentiometer 24 is incorporated in the circuit of Fig. 4, as it was in Fig. 1.

To increase the eifective attenuation in the open condition of theswitching circuit, there is connected between ground and the junction point 38 a high-vacuum switching diode 40 connected, like switching diodes 20 and 22, with its cathode tied to the junction point. In series with diode 40 is a resistor 41. Whereas resistor 21 has, in a preferred version of the circuit, a resistance of 10 kilohms or about twice the resistance of resistor 23, the resistor 41 may have substantially three times the resistance of resistor 23, or 15 kilohms.

To drive each of the switching diodes to a reverse operating point, there is provided a triode 44 which amplilies the selector signal applied across grid-cathode circuit to drive the coupling diodes 30, 32 as well as a coupling diode 45 of the semiconductor or crystal type. Whereas the semiconductor diode 45 has its anode connected directly to the anode of switching diode 40, the anodes of coupling diodes 30, 32 are connected through current limiting resistors 47, 33, respectively, to the anodes of switching diodes 20, 22. Exemplary values for the re sisters 33 and 47 are 7.5 and 4.7 kilohms respectively.

In an operation of the diode switching circuit of Fig. 4, a negative signal V is applied at terminal 10 which, in a closed condition of the switching, circuit, is coupled without appreciable diversion to the output terminal 11. To obtain this closed condition of the switchingcircuit, a negative selector signal is derived from the amplifying triode 44 for application via coupling diodes 30, 32, and 45 to the respective anodes of switching diodes 20, 22 and 44%. Each of the three switching diodes are thereby driven to a reverse operating point where they exhibit a high back resistance fores'talling both a diversion of the input signal from the output terminal 11 and also a coupling of the selector signal to the output terminal 11. When the selector signal derived fromthe triode 44 is positive, on the other hand, each of the coupling diodes 30, 32 and 45 exhibits a high back resistance. In consequence, forward conduction through each of the switching diodes results in a virtual clamping of the output terminal 11 at ground potential. The compensating diode 25 serves to minimize zero drift in the manner described in conjunction with Fig. 1, the only difference lying in the reversal of polarity to accommodate the negative polarity of the input signal V Further modifications may be made in the form and design of diode-switch circuits in accordance with this invention without departing from its principles. Thus, while each of the diodes in Figs. 1-3, except diode 45, has been illustrated as a high vacuum thermionic discharge device, semiconductor or other asymmetrically conductive devices may be substituted, particularly for the diodes more remote from the output terminal 11. The use of high vacuum diodes is, however, preferred as the higher back resistance afforded by such diodes at low operating voltages enhances the completeness of the switching action and decoupling of the selector signal from the output terminal,

7 addit q a shu t pat sma e provided, a it ',,..i a i.ade andset es es stor o e with a ar dr n as swi ch n di t a h add t e a ll l pat s y. d ir h ve. and a e! wi h hem. pen t shunt na snszludins p s t po e compe n od It ma be las tha res s o 14 n some applications ma be an in rn s ur resi an e. and t a resistor 1Q m aa mav e i s asle in the load- Th series sistanse circ t m t en ssn st Qt t r 15 n The invention, therefore, is not to be limited to the specific embodiments shown and described but is of a scope definedin 'the appended claims.

1. In a signal switching circuit, the combination comprising a plurality of resistors forming a series circuit havinga signal input terminal and a signal output terminal, at'leasttwo switching diodes and a series resistor for each, similar first electrodes of said switching diodes being connected at spaced points along said circuit intermediate said terminals, said resistors for said switching diodes connectingthe second electrode of each switching diode to a point of fixed reference potential, a direct current signal source having one terminal connected to said input terminal and the other .trerriinal connected to said point to. produce forward conduction in said switching diodes, and means for selectively driving said switching diodes to a reverse point in their characteristic.

2. In a signal switching circuit, the combination coinprising a resistive. circuit having a signal input terminal and a signal output terminal, at least two switching diodes and a seriesresistor for each, similar first electrodes of said switching diodes being connected at points along said resistive circuit which are resistively separated from each other, said resistors connecting the second electrode of each switching diode to, a point of fixed reference potential, a direct current signal source having one terminal connected to said input terminal and the other terminal connected to said point to produce forward conduction in said switching diodes, and means coupled with the second electrode of each switching diode for selectively driving said switchingv diodes to a reverse point in their operating characteristic.

3. In a signal switching circuit, the combination comprising a resistance circuit having an input terminal and an output terminal, at least two switching diodes and a series resistor for each, similar first electrodes of said switching diodes being connected at points along said resistance circuit which are resistively separated from each other and from said input terminal, said resistors connecting the second electrode of each switching diode to a point, a constant direct current potential source having one terminal connected to said input terminal and the other terminal connected to said point to produce forward conduction said switching diodes, and means coupled in parallel with said series resistors for selectively driving said switching diodes simultaneously to a reverse point in their characteristic.

4. In a signal switching circuit, a resistive circuit having an input terminal forconnection to a source of direct current potential and an output terminal, at least two switching diode-s and a series resistor for each, similar first electrodes of said switching diodes being connected at spaced points along said resistive circuit intermediate said terminals, said resistors connecting the second electrode of each switching diode to a point at a reference direct current potential tending to produce forward conduction in said switching diodes, and means including a coupling diode for each of said switching diodes having like electrodes connected to the second electrodes of said switching. diodes for coupling a selector signal to said switching diodes selectively to drive said switching diodes to a reverse point in their operatng characteristic.

A signal switching circuit as defined in claim 4, wherein the. other electrodes of said coupling diodes are connected in common to a source of said selector signal.

6. In a signal switching circuit, the combination corn-f prisinga resistive circuit having an input termin2 1 l v jer connection to a source of direct current potential and an output terminal, at least two switching diodesanda cries resistor for each, similar first electrodes of said switching diodes being connected at spaced points along said re; sistive circuit intermediate said terminals, said resistors connecting the second electrode of each switching diode to a point having a direct current potential tending to produce forward conduction in said switching diodes, a cornpensating diode and a resistor in series therewith connected between the spaced point nearest said output ter minal and said direct current potential point with said compensating diode connected in oppositely poled relation to said switching diode, and means for selectively driving said switching diodes to a reverse point in their characteristic.

7. In a signal switching circuit, the combination comprising a resistancecircuit having a signal input terminal for connection to a source of direct current potential and a signal output terminal, at least two switching diodes and a series resistor for each, similar first electrodes of said switching diodes being connected at points along said circuit resistively separated from each other and from said input terminal, said resistors connecting the sec ond electrode of each switching diode to a point of fixed direct current reference potential tending'to produce a forward conduction in said switching diodes, for at least one of said switching diodes a compensating diode connected therewith in oppositely poled relation and having in series a resistor substantially equal in value to the resistor associated with the respective switch ing diode, said compensating diode and its resistor being connected between said reference potential point and the same point along said circuit, as is the respective switching diode, and means for selectively driving said switching diodes to a reverse point in their characteristic;

8. In a signal switching circuit, the combination comprising at least three resistors of high value forming a series resistance circuit having a signal input terminal and a signal output terminal, at least two switching diodes and foreach .a resistor having a value substantially less than any of said three resistors, similar electrodes of said switching diodes being connected at junction points of said series resistors, the resistors for said switching diodes connecting the second electrode of each switching diode to a point of fixed reference potential, a direct current signal source having one terminal connected to said input ten minal and the other terminal connected to said" point to produce forward conduction in said switching diodes whereby to attenuate transmission of a signal between said input and output terminals, and means for selectively driving said switching diodes to a reverse point in their characteristic substantially to remove such attenuation.

9. A signal switching circuit as recited in claim 8-, wherein said series circuit resistors are of increasing resistance towards said output terminal and the resistors for said switching diodes are of decreasing resistance towards said output terminal. V

10. In a signal switching circuit, the combination comprising a plurality of resistors forming a series circuit having an input terminal for connection to a source of direct current potential and an output terminal, at least two switching diodes and a resistor for each, similarfist electrodes of said switching diodes being connected at junction points of said series circuit resistors, the resistors for said switching diodes connecting the second electrode of each switching diode to a point at a reference direct current potential tending to produce forward conduction in said switching diodes, andmeans including a coupling diode for each of said switching diodes insimilarly poled relation with respect to said reference potential point; for simultaneously coupling a selector signal of; reversible polarity selectively to said switching diodes in accordance with such polarity.

11. A signal switching circuit as recited in claim 10, wherein said coupling diodes have a like electrode connected in common, and a resistor is connected between 5 the switching diode nearest the output terminal and the corresponding coupling diode.

References Cited in the file of this patent UNITED STATES PATENTS Lewis Dec. 26, 1950 Hindall May 20, 1952 Mierlo Nov. 18, 1952 Smith May 29, 1956

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