US2990477A - Bridge gating circuit with floating bias source - Google Patents

Description

June 27, 1961 R, M, MaGINTYRE 2,990,477

BRIDGE GATING CIRCUIT WITH FLOATING BIAS SOURCE Filed Aug. '7, 1956 a CONSTANT CURRENT fz BIAS 1 SOURCE 3 INPUT OUTPUT -r CIRCUIT CIRCUIT INPUT SIGNAL l8 OUTPUT SIGNAL United States Patent i f 2,990,477 BRIDGE GATING CIRCUIT WITH BIAS SOURCE "mesne assignments, to Thompson Ramo Wooldridge Inc.', Cleveland, hio,'a corporation of Ohio Filed Aug. 7, 1956, Ser. No. 602,550

4 Claims. (Cl. 307-885) The present invention relates to an electronic gating circuit and, more particularly, to a bridge gating circuit for selectively passing an applied voltage signal.

A conventional bridge gating circuit includes a plural-- ity of unidirectional current-conductive devices, together with a bias source for supplying a current to the unidirectional devices to bias them in the conductive region, and

a means for turning the bias current either on or off. When the bias current is on, the bridge circuit presentsa lowimpedance and is capable of passing with relatively little attenuation a signal voltage which may be applied to it. When the bias current is turned off, the application of a signal. voltageenconnters a substantially infinite back-impedance presented by one or more of the unidrrectional current-conductive devices.

In such a conventional bridge gating circuit, the bias source and the means for turning the bias current on or oft may be a single circuit, as shown, for example, in U.S. Patent No. 2,577,015, for Switching System, issued December. 4,1951, to W. R. Johnson. Another conventional arrangement is to provide a bias source and an on-oif control. device as two separate circuits coupled substantially in parallel to the bridge gating circuit in such a way that the operation of the on-off control device is capable of overriding the action of the bias source. Arrangements of this type are described in an article entitled Accurate Linear Bi-Directional Diode Gates, by J. Millman and T. H. Puckett, published in Proceeding of. the IRE, J-anuary 19, 1955, at pages 29-37.

FLOATING f Robert M. Maclntyre, Los Angeles, Califi, assig'n'onby 1 the impedances The performance of a gating circuit may be indicated by means of factors known as gain, error voltage output, and transfer function. The gain is a factor less than one which represents the portion of the input voltage actually transferred to the output circuit. The error voltage out 5 put exists because-of the fact that within the gating circuit the values of operating voltages or of circuit ele--. ments either depart from their established design values in the first instance, or'else drift fromtheir initial values during'operation of the circuit. The transferfunction refleets the over-'allperforrnance of the gating circuit including both gain and error vo1-tage output, and is defined as the ratio of the voltage developed across the output circuit to the voltage supplied bythe input circuit.

It is, of course, desirable that the value of the transfer Q function should be maintained at unity. Since the circuit gain is always less than unity the transfer function may achieveithe value .ofunity only if the error voltage output happens toprovide exactly the right compensating effect. -It will be readily appreciated that because of vari ous considerations it is highly undesirable to rely upon the error voltage output for such a purpose. 'Rather, it is desirable to maintain the value of the transfer function atunity by having the gain be equal to unity andthe.

It is, therefore, an object of the present invention to provide an improved form of electronic gating circuit for selectively passing an appliedsignal voltage in response to, one value of a bi-valued control voltage.

Another object ofthe invention is to provide an improved bridge gating circuit for selectively passing an applied signal voltage with the minimum amount of attenuation. z. i

Still another object of the invention is toprovide a bridge gating circuit for selectively passing an applied sig-" nal voltage, the effectiveness ofrthe transfer of the signal voltage being substantially equal to the ratio of the impedance of an associated output circuit to the sum of of the output circuit and an associated input circuit.

According to the present invention, it is recognized that" prior art bridge gating'circuits have caused attenuation of a gated signal voltage for two separate reasons: First, an attenuation resulting from the impedance presented.

by the unidirectional current-conductive devices of the bridge circuit when biased in their conductive state; and,

second, an attenuation resulting froma diversion of a portion of the signal current through the bias source which.

provides the bias current for, the bridge circuit. The

present invention also recognizes that whereas the first ofthcse attenuation factors appears to be necessary and. unavoidable, the second factor can be entirely eliminated, providing a very substantial improvement in the over-all.

performance of the bridge gating circuit.

Thus, the present invention in its general form provides 1 a bridge gating circuit to which associated signal input and output circuits may be coupled, a floating bias source for supplying bias current to the unidirectional current-conductive devices of the bridge circuit, and an on-ofi control means for selectively interrupting the flow of bias current in order to control the passage of a sig nal voltage from the input circuit to the outputcircuit. The floating bias source is characterized by the fact that I it is electrically isolated from the input and output'cir Y cui-ts, except for the necessary interconnection via-the bridge circuit. Signal current is not drawn into the bias paththrough source because there is no; longer any return which it may flow back to the input circuit.

of the present invention than by prior art circuits.

' vides an ingenious manner of mechanizing the'floa'ting conventionalbridge gating" circuits are "presented" and" bias sourcefor an application where the gate is to 1 beturned on for periods of less than a predetermined length. The bias source provided for an application of 11 this type includes a capacitor forstoring a bias voltage,

' 'at least one resistor connected in series with the capacitor to produce a substantially constant bias current, the ca- 1 pacitor and resistor being coupled across the bridge I gating circuit; and a charging circuit including at :least one unidirectional current-conductivedevice as a serial; element thereof for replenishing the charge on the capacitorwhcnthe gating circuit is'in the 011" conditionfeatures of new invention. Which are believed to be novel and patentable are pointed out in the claims1v which form fa'pait or this specification For a, better urideistanding'ofthe invention, reference is made in PatentedJune 27, 1961* Many advantagesare provided byv the present inven- 1 tion as a direct result of the use of a floating bias sources Attenuation; of the signal voltage is'minimized, resulting in-a substantial increase in the, circuit gain, Also," the total error voltage output is considerably reduced and the toluances oncircuit componentv values are re laxed. vvlurthennore, in some applications, a smaller. number of circuit elements are required by the. circuits the following description to the accompanying drawings, wherein like parts are indicated by like reference numerals in which:

FIGURE 1 is a partial-schematic and partial-block diagram illustrating the general form of the invention;

FIGURE 2. is a schematic diagram specifically illus hating a' simplified version of the invention; I

FIGURE 3 is a schematic diagram specifically illustrating a preferred form of the constant current bias source of FIGURE 1 and the manner of its interconnection to the bridge circuit and the on-ofi control circuit; and

-FIGURE 4 is a schematic diagram specifically illustrating another form of the constant current bias source. ..Referring now to the drawing, there is illustrated in FIG. 1 an electronic voltage gate comprising a' bridge circuit 10, an on-off control circuit 20, a constant current bias source 30, an input circuit 40 and an output circuit 50. Bridge circuit includes a pair of bias control terminals 11 and 13, terminal 11 being biased positive with respect to terminal 13 When the bridge is to conduct, and being biased negative with respect to terminal 13 when the bridge is notto conduct. A pair of unilaterally conducting devices 15 and 16 provide a first conducting branch of the bridge circuit, poled to pass current from terminal 11 to terminal 13, and a pair of similar devices 17 and 18 provide a second conducting branch. A pair of signal terminals 12 and 14 are provided at the junction points between devices 15, 16, and 17, 18, respectively. Input circuit 40 is connected between signal terminal 14 and a lead 55 while output circuit 50 is connected between signal terminal 12 and lead 55.. On-ofi control device 20 is coupled to the bias control terminals 11, 13, via a unilaterally conducting device 11, a first output lead of the control circuit being connected to the cathode of device 21 whose" anode is connected to terminal 11, a second output lead ofcontrol circuit 20 being connected to terminal 13. Bias source 30 is connected directly across terminals 11 and 13.

.In operation, the four unilaterally conducting devices of bridge circuit 10 are normally maintained in a con dugtive state by bias current flowing, in the conventional sense, out of bias source 30 to terminal 11, thence through the bridge circuit, out of terminal 13 and back to the bias source. Terminal 11 istherefore normally at a potential which is positive with respect to that of terminal 13'. The upper output leadof on-off'control circuit 20 is normally positive with respect to the lower output lead, the voltage difference being substantially greater than the normal potential difference between terminals .11 and 13-, hence device 21 is normally backe biased. Because of the.very low forward impedance of devices 15-18 a signal voltage generated by' input circuit 40 normally encounters a very low impedance between signal terminals 12 and 14, and a very large proportion of the signal voltage appears across outpu circuit 50. g 1

The normal application of signal'voltage from input circuit 40 through bridge circuit 10 to output circuit 50 may be selectively interrupted by the operation of the on-otfcontrol circuit 20. When the upper lead of con trol circuit 20 assumes a potential which is substantially more negative than that of its lower output lead; device 21 becomes forward-biased, and hence conductive. Terminal 11 assumes a negative potent al with respect to terminal 13 and hence all of devices 1518-becom'e back-biased and non-conductive, while the constant current bias source 30 continues to supply a constant" output current which has now been diverted from bridge circuit 10 to flow instead through on-oif control circuit 2.0.

Signals generatedby input circuit 40, therefore, do not the bias current have a substantially constant value, however, available types of unidirectional devices are not" vention as illustrated in FIG. 1, it will be seen that during normal operation when the bridge is conducting the on-oif control circuit 20 is effectively decoupled by de vice 21. Bias source 30 is' connected to the bridge cirshit but does not provide a path for signal current since the potentials of terminals 11 and 13 are identical insofar as alternating current is concerned. Therefore, signal current flowing from input circuit 40 to terminal 14 divides at that point, part of it flowing through devices 17 and 15 to terminal 12, the remainder flowing through devices 18 and 16 to terminal 12, the entire current then flowing into output circuit 50 and thence back to input circuit 40.

The nature of the floating bias arrangement provided by the invention may be conveniently explained with reference to the article by Millm'an and Puckett and, more specifically, to FIG. 10 on page 34 thereof, and to the accompanying discussion which indicates that current flowing from the signal input source through the bridge circuit has three possible return paths, a first one through the signal output circuit and second and third paths through the bias circuits. The present invention eliminates the second and third return paths for signal current. Thus, in FIG. 1 of the drawing in the present application, it will be noted that there is an absence 7 of any direct electrical connection between the lead' 55 and bias source 30. v

The significance and operation of the present invention will be more easily understood with reference to FIG. 2 in which like parts are given the same reference numerals as inFIG. 1. In FIG. 2 the constant current bias source 30 is illustrated as comprising a battery 31 connected in series with a resistor 32, where it is assumed that the resistance value of resistor 32. is very large compared to the impedance values in the bridge circuit. On-off control circuit 20 is illustrated as comprising a pair of batteries 23 and 24, connected in oppo site polarities, and a switch 25 for selectively placing one or the other of the batteries in the circuit. Input circuit 40 is illustrated as comprising a source of alter-' nating potential 41 connected in series witha resistor 42, while output circuit 50 is illustrated as a resistor 51.

The operation of the gating circuit of FIG. 2 may be described by the equation:

5, R s s-ir'l' 1.

where E represents the voltage appearing across output RL s L Reference is again made to the: Millman Bucket t article and, more specifically to: the" equations forgain G which are set forth in Table l on page 33 thereof.

It be readily observed thatthe gain equationfor the circuit of the'present invention is much. siniplerand contains far fewer factors. It may also be easily estab-:.

lished-that, for the same set of circuit values in the inputand output circuits and the gating circuit, the present invention provides a substantially higher value of gain than do any of the conventional ing circuits. 1

It will be shown that the present invention, while maintaining all of the advantages of prior art bridge gating I circuits, and While providing a higher value of gain, also eliminates entirely an important source of error voltage output. In the article by Millman and Puckett,

types'of bridge gata factor U(E) (see Table 1) indicates the fraction of the unbalance in bias voltages which appears across the output circuit. The unbalance in bias voltages is equal to the numerical difference between the supply voltages plus B and minus E, illustrated in FIG. 9 and FIG..'10-

of the Millman and Puckett article. it is important to note that where a high degree of ac- In this connection, v

curacy and of precision is required, the difficulty of pro- FIG. 3 does not show input circuit 40 or'outputcircuit 50, but does include a grounded lead or reference tercondiillustrating a pre-.

minal 55, and indicates that the input signal maybe applied between terminals 14 and 55, while the output signal may' be derived between terminals 12 and 55. The constant current bias source is not connected to ground terminal 55, but is electrically isolated from the input and output circuits except for the intercoupling via bridge circuit 10, thus providing the floating bias source of the present invention.

In FIG. 3 a flip-flop 26 is illustrated as a preferred type of on-oif control circuit, coupled to the bridge circuit via a pair of unilaterally conducting devices 21, 22. As is well known in the art, a flip-flop may be conveniently utilized to provide a pair of steady-state electrically controlled signals whose values are complementary, that is, one signal is high when the other signal is low, and vice-versa. It will be understood th-at'a source of energy, not shown, may be coupled to the fiip-. flop, and that some form of input circuit, not shown, may, be utilized for selectively changing the flip-flop from one to the other of its stable states. The flip-flop may, if desired, be connected at some suitable point to ground terminal 55, since it is not essential to the present invention that the input and output circuits be isolated from the onoff control device. The reason for this is that the on-ofi control device is isolated from the bridge circuit by the back-biasing of devices 21 and 22 when the bridge circuit is conductive.

The preferred form of constant current bias source 30 as illustrated in FIG. 3 includes a resistor 33 connected to bias control terminal 11 of the bridge circuit, a resistor 34 connected to the other end of resistor 33, an energy storage device such as a capacitor 35 connected to resistor 34, and a resistor 36 connected between capacitor 35 and terminal 13. Capacitor 35 has its upper plate norm-ally charged positively with respect to its lower plate and provides bias current to bridge circuit 10 through a series loop which includes resistors 33, 34, and 36. A unilaterally conducting device 61 has its anode connected to the lower output lead of flip-flop 26 and its cathode connected to the junction between resistors 33 and 34, while a unilaterally conducting device 62 has its cathode connected to the upper output lead of flip-flop 26 and its anode con. nected to the junction between capacitor 35 and resistor 36. Devices 61 and 62 are normally back-biased when the bridge circuit 10 is conductive. However, when flipflop 2 6' is selec'tively operated so that itsupperi output lead! becomes negativewith respect to its lower output:lead, 1:

the "bridge circuitis biased E-non;conductive, then current flows from flip-flop 26 through device 61,i"resistor=. 34, capacitor 35', and device 62,*and hence back to:fiip'=: flop 26, thus replenish-ingthe charge on capacitor 35; The purpose ofresistor 34is to limit thecharging' current to capacitor 35.: During the-times when the bridge circuit-x. is non-conductive and the flip-flop 'is supplying energy :11 through devices 61*and' 62 to replenish the chargeo'n: capacitor 35, the constant-current bias source 30 never-'. theless'continues to supply a substantially constant cur-:0-

rent to terminals 11 and 1'3.- Thus again, as in the circuit of FIG. 2, the constant current bias source is not turned 01f but merely has its output current by-p'assed througha' different path when the bridge circuit is non-conductive:

Since it is generally preferred not topermit the :value" of the bias current to vary substantially, thepefiodof time during which the-gate is permitted to conduct must belimitedwith due regardto the valuev of capacitor-35:: and to the.otftime available for replenishing the charge In an application in which the circuit of FIG. 3 was actually used, the gatewas operated inan on the'capacitor'.

regular cyclic-fashion, being turned on for approximately 150 microseconds, then off for approximately l50rnicro-' 5 seconds," and so on.

Although. other circuit values may be used, a satisfactory'set of values which have been successfully employed in the circuit ofFIG. 3 are listed as follows:

All unilaterally conductingtdevices are siliconldiodest Resistors 33, 36 ohms ,1,0 ,000 Resistor 34 V do 1,000 Capacitor 35 microfarads 10 Flip-flop 26 is able to supply 3 milliamperes ofcurrent 10 volts between its two output at a voltage difference of terminals.

73, a transformer secondary 74, a'unilaterally conducting devicej 1 having its cathode connected to the junction between resistor 71 and capacitor 72 and its anode connected to one end ofsecondary winding 74, and a resistor 76 connectedbetween theother end of secondary winding 1 74 and the other terminal of capacitor 72. .Resistor 7.6

serves to limitthe charging current to. capacitor;-72.. In operation, the application of energy in alternating form across transformer primary 73 serves to maintain the upper plate of capacitor 72 at a positive potential with respect to its lower plate, thus continuously supplying current to the bridge circuit.

The advantages provided by the present invention may now be conveniently summarized in view of the foregoing description of the various circuit modifications illustrated in the drawing. The floating bias source of the present invention produces a substantial improvement in the overall operation of the gate,

Not only is the value of the circuit gain substantially nearer to unity, but the total error voltage output is greatly reduced, while at the same time, limitations on the values of the various circuit parameters are relaxed. For a particular application in which the gating circuit is to be turned on for time periods of less than a predetermined length, the present invention has also provided a very useful and ingenious circuit wherein the bias current may be supplied from a capacitor when the gate is on, and the charge on the capacitor may be replenished when the gate is off.

It is, of course, to be understood that the present invention is not limited to the use of semi-conductor diodes as the unilaterally conducting devices, but extends as well to any other type of unilaterally conducting devices' which are now available, or' may become so in the future. Also} the advantage of the floating bias source is not necessarily limited to a bridge circuit utilizing four unilaterally conducting devices, but may be applicable as well to a bridge circuit in which only two of the arms contain such devices. Moreover, although the embodi ment' of FIG. 3 shows the input andoutput signals as having a common, grounded terminaLiWith the'bias. cir-' cuit as'ungrounded, it should'be clear that thebias circuit could be grounded if the common terminal were not, it only being necessary that the bias source not be directly coupled to the input and output circuits. Although the conducting state of the bridge circuit has been referred to in the description as being the normal state, it is to be understood that this is merely a matter of definition, and that the opposite definition could have been used with the bridge" circuit being normally non-conducting and being made to conduct by selective operation of the on-ofi control circuit. Many other forms and circuit modifications, in addition to' those specifically shown and described hereing fall within the scope of the invention and will be readily apparent to those skilled in the art.

What I claim as new and desire to secure by Letters Patent'of the United States is:

1. An electronic gating circuit comprising, in combination: a bridge circuit including first, second, third, and fourth diodes, the cathode of said first diode being connected to the anode of said second diode, the cathode ofsaid third diode being connected to the anode of said fourth diode, the anodes of said first and third diodes being connected together, the cathodes of said second and fourth diodes being connectedtogether; a constant current source coupled between the 'junctionor said'firs't and third diodes'and the junction of said second and fourth diodes; a two-terminal input circuit, one terminal thereof being connected to the junction of said third and fourth diodes; a two-terminal output circuit, one terminal thereof being connected to (the junction of 'said first and second diodes; means to connect the other terminal of said output circuit to the other terminals of said input circuit only; and a current conducting path including switch means operative to selectively direct the current from said constant current source through said bridge circuit when a signal from said input circuit is to be passed to said output circuit and for directing the current from said constant current source through said current conducting path when a signal from said input circuit is to be inhibited from passing to said output circuit.

2. A current source selectively operable for supplying a substantially constant current to an output circuit for second output-terminals; a" first resistor, a capacitoryand a second resistor coupled serially together in the" order named between said first and second output terminals; an on oif control circuit having first and second output leads, Isaid first output lead being normally biased'negatively with respect to said second output lead; first and second unilaterally conductive devices, each having an anode and a cathode, said first device having its anode connected to said second output lead and its' cathode con nected to the junction between said first resistor and said capacitor, said second device having its anode connected to the junction between said capacitor and said second resistor and its cathode connected to said first output lead, whereby said capacitor normally acquires a charge having a predetermined polarity; a third unilaterally conductive device connected to one of the output leads of said on-off control circuit, providing a series circuit connected across said first and second output terminals so that current normally flows from said first output terminal to said first output lead, and from said second output lead to said second output terminal; and means coupled to said on-off control circuit far selectively biasing said first output lead positively with respect to said second output lead. during said time period.

3. The gating circuit of claim 1, wherein said constant current source includes a capacitor; and means for charging the capacitor of said constant current source when the signal from said input circuit is being inhibited from passing to said output circuit.

4. The gating circuit of claim 3, wherein said constant current source further includes at least one resistor connected in series with the capacitor; and said means for charging the capacitor of said constant current source includes means for connecting the capacitor to the switch means of said current conducting path during the charge interval.

References Cited in the file of this patent UNITED STATES PATENTS 2,098,370 Bartels Nov. 9, 1937 2,505,688 Picking Apr. 25, 1950 2,563,406 Goldberg Aug. 7, 1951 2,625,662 Gaynor et a1 Jan. 13, 1953 2,782,307 Von Sivers et al Feb. 19, 1957 2,817,757 Durbin Dec. 24, 1957 2,866,103 Blake et a1 Dec. 23, 1958 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 2 99O 47'I June .27 1961 Robert M MacIntyre It is hereby certified that error appears in the above numbered pat-- ent requiring correction and that the said Letters Patent should readies" corrected below.

Column 2 line 5O for 'toluances" read tolerances column 8, line 25 f "fa m 0 read for Signed and sealed this 1st dayyof May 1962.,

(SEAL) I Attest:

ERNEST SWIDER DAVID LADD -Attesting Officer Commissioner of Patents

Images