ER

(B)

-100-80

Windings 1 and 4 act as control windingsfor the switches, and the output, a squarewave, appears on winding 5. To followthis operation in detail, as~ume thattransistor A is conducting, which placesthe input voltage Vin across winding 2,positive at the top and negative at thebottom. Voltages with correspondingpolarities will be induced in all of theother windings. This induced voltage inwinding 1 makes the base of A negativewith respect to its emitter, and A conductswith very little voltage drop between theemi tter and collector. The vol tage in-duced in winding 4 is positive at the topand thus holds transistor B cutoff, sinceits base is positive with respect to boththe emitter and collector. Transistor Acontinues to conduct until the flux reachesthe saturation value +cf>m.

Upon saturation, several effects occursimultaneously; the current rises sharplyto a value determined by the base driveof the transistor, the induced voltage inthe transformer windings disappears, andthe voltage across the transistor rises.The loss of the base drive causes transistorA to open the circuit to winding 2 andcauses the flux to fall back slightly to theretentivity value which induces a smallvol tage of the opposite polarity in thewindings. This places a negative voltageon the base of the transistor B which be-gins to conduct, causing the flux to de-crease still further, and the cycle israpidly reversed until transistor B isconducting heavily and transistor A isopen. The input voltage is now con-nected across winding 3, positive at thebottom and negative at the top. Theflux decreases linearly until the core sat-urates in the opposite direction and asimilar reversal takes place to completethe cycle.

The output voltage is a square wavewhere the duration of each half-cycle is

COLLECTOR CURRENT (rno)-40

(A)

-20

Een~~ -25...------P~--+---+----;>

wC) -20H----+---+-~--------4....

-Jo> -15........---+---+----j1+--~~_+_---r----r--___,r_-__r--_._1a::

~~ -t0N---+---+----I1+----I+-~:-t-t----+--=-=--t----r---r-1-J-Joo -511-----+---J----I4----++-----+f--~~-+-I~~---_+_I

Fig. 1. (A) Col-lector character-istics of PNPtransistor. (B)Common emitterconnection show-

ing load circuit

Basic Circuit of Converter

Two junction transistors used as con-trolled switches and a transformer with asquare hysteresis loop core material formthe basic circuit of the voltage-to-fre-quency converter illustrated in Fig. 2..A. typical -i curve for such a transformerconstructed of Hipernik V is also illus-trated in Fig. 3. The two transistors al-ternately conduct and switch the inputvoltage Vin across windings 2 and 3,causing the flux in the core to be cycledbetween positive and negative saturation.

current required to saturate the transistordepend s great1y on the gain of the tran-sistor and the amount of load current.The effective transistor impedance can bechanged from over a megohm at cutoff toa few ohms at saturation by control of thebase voltage and current; this can becompared to the operation of a relaywhere the emitter-collector circuit isanalogous to the contacts and the base-emitter circuit is analogous to the ener-gizing coil. For the purpose of this paper,the above information can be summarizedby two simple rules. The PNP transistorswitch will conduct if the base is negativewith respect to either emitter or collectorand will block if the base is positive withrespect to both emitter and collector.

G. H. ROYERASSOCIATE MEMBER AlEE

Paper 55-73, recommended by the AlEE MagneticAmplifiers Committee and approved by the AlEECommittee on Technical Operations for presenta-tion at the AlEE Winter General Meeting, NewYork, N. Y., January 31-February 4,1955. Manu-script submitted October 25, 1954; made availablefor printing December 6, 1954.G. H. ROYER is with the Westinghouse ElectricCorporation, Pittsburgh, Pa,

The use of junction transistors as con-trolled switches has been described pre-viously,1,2 but a few of the main pointswill be reviewed here. Fig. 1(A) showsthe collector characteristics of a PNPtransistor in the common emitter con-nection, as illustrated in Fig. l(B). Forthe load line shown, two operating pointsare of interest: cutoff, point F, where thetransistor sustains almost the entire sup-ply voltage E with very little leakage cur-rent and conduction, point 5, where alarge enough base drive is applied tosaturate the transistor and reduce thevoltage drop Vee to its lowest possiblevalue. Cutoff occurs when the base volt-age is slightly positive giving a smallpositive current in the base. Typicalvalues of cutoff current for small 200-megawatt 2N74 fused junction transistorsare from 5 to 50 microamperes at a supplyvol tage of 45 volts and a base voltage of+0.5 to 5 volts. In the conducting statevoltage drops of 0.1 to 0.5 volt are ob-tained with 50 to 150 milliamperes' loadcurrent; however, the amount of base

Transistor Switches

TH E combination of transistors andtransformers constructed of squarehysteresis loop material has producedmuch useful circuitry outside of the realmof magnetic amplifiers per see In thispaper a highly efficient and extremelyprecise converter for transforming directto alternating current, yielding a square-wave output whose frequency is propor-tional to the d-e input voltage, will be de-scribed. From among the many uses ofthis converter, the following applicationswill be discussed: 1. a transducer for tele-metering systems, and 2. an integratorwhich has found application as an ana-logue to digital converter.

A Switching Transistor D-C to A-CConverter Having an Output FrequencyProportional to the D-C Input Voltage

322 Royer-Switching Transistor D-C to A-C Converter JULY 1955

5OUTPUT

determined by the time necessary for theflux in the core to change from -cf>m to+m. The frequency of these squarewaves can be related to the parameters ofthe core and the input voltage. If N 2designates the number of turns on eachone of the two main windings 2 and 3and the voltage drop across the transistorsand winding resistance is assumed to benegligible, the input voltage Vin is ap-plied directly across a winding of N 2turns

dcPV' =N,,-ill ~ dt

Integrating this relation over one half-cycle

Therefore

f=~4N2cPm

From this expression it is evident that,for a given transformer, the output fre-quency is proportional to the d-e inputvoltage. Such a characteristic is wellsuited for telemetering applications wherethe output frequency transmitted betweena measuring point and a metering stationis an indication of a voltage measurandat the originating point. As the tran-sistors are used as switches or on-off de-vices, and their conduction resistance isonly a few ohms, their characteristics donot rnaterially affect the proportionality.The saturation flux of Hipernik V changesabout 0.07 per cent per degree centi-grade and, thus, the constant of propor-tionality will vary correspondingly withtemperature.

Bias Cireuit

To insure good switching operation, theminimum voltage at the transistor basesshould be about 1 volt. If a 20-to-1operating range is desired, the voltage in-

Fig. 2

INPUT TO INTEGRATOR

OUTPUT OF INTEGRATOR

Fig. 5. Voltage wave forms using the con-verter as an integrator .

output polarity each time the volt-secondarea under the input curve from the timeof the previous reversal reaches the valuenecessary to move the core flux from onesaturation point to the other. The out-put voltage follows the same wave shapeas the input, except that each output pulsehas the same volt-second area and everyother pulse is the mirror image of the in-put.

Rewriting Vin = N(d/dt) and integrat-ing over a complete .half-cycle of fluxchange

f cPmVindt=N d4J= 2N4JmcPmApplications

a wide ambient temperature range; the. circuit shown has been operated from

- 60 to +40 degrees centigrade, and to+75 degrees centigrade with selectedtransistors. Some laboratory models ofthis converter have operated over an in-put voltage range of 100 to 1. For a con-stant core temperature, converters of thisconfiguration have been operated over a20-to-1 range with a frequency outputproportional to the input voltage towithin a deviation of only 0.2 per cent offull-range frequency.

1

age limitation on Vin If NIlN 2 is madetoo large, the base current referred towinding 2 may be greater than the maxi-mum saturation current, and there will benothing left over to magnetize the core sothat this lower limit cannot be extendedindefinitely. As the switching voltagebecomes too low to saturate the tran-sistors properly, they can no longer supplythe magnetization current for the trans-former. Under these conditions, theoperation takes place along a minor hys-teresis loop. Thus instead of acting as asaturating core, it behaves as a linear in-ductance in parallel with its distributedcapacity. The device then changes itsmode of oscillation and operates as aquasi-linear oscillator whose frequencyincreases as the input voltage decreases.Two precautions must therefore be ob-served if this device is to be used for in-strumentation purposes: The input volt-age must not change sign and, further-more, it should not fall below the criticalvalue at which the mode of oscillationchanges. Input voltages of as low as 100millivolts and up to 40 volts have beenapplied and, by the proper choice of cores,and number of turns on windings 2 and3, converters have operated at frequenciesfrom one half-cycle per second up to 12kilocycles.

Since the transistors are operating asswitching devices, they will operate over

The circuit described in the previoussection is accurate enough to warrant ap-plication of the device as a telemeteringtransmitter. The output square wave canbe transmitted directly, or filtered, ifelimination of the higher harmonics is de-sired. While the converter may be loadedto deliver as much as 1 watt of outputpower, the voltage-to-frequency linearityis sacrificed so that an output bufferstage may be necessary to maintain the0.2-per-cent accuracy. A further ad-vantage of a buffer stage would be thatit could act as a clipper, giving a constantoutput magnitude over the operatingrange instead of one which varied with thefrequency. This output can be trans-mitted over any desired link to an indi-cating station giving a representation ofthe voltage measured Vin

It was stated that the saturation fluxof Hipemik V changes about 0.07 per centper degree centigrade and that this wouldvary the constant of proportionality inf =K Vin If the core were enclosed in aconstant temperature box held to a varia-tion of 2.5 degrees centigrade, the pro-portionalitywouldchangebyonly0.2 percent. This temperature limit should notbe difficult to obtain in a small, inexpen-sive oven.

If a unidirectional input voltage waveform, shown in Fig. 5, is applied to theinput terminals, the unit will reverse the

Thus, twice the number of turns timesm determines the volt-second area of theinput which is required for 'each outputpulse. This is then an analogue-to-digitalconverter in that the digital output(number of pulses) is the representationof the analogue input (integral of the ap-plied unidirectional voltage). A count ofthe number of output pulses during afixed time period would give the integralof the applied voltage over that time.

Conclusions

In this paper a simple and reliable cir-cuit has been illustrated to convert directto alternating current. Several of its ap-plications-precision telemetering and in-tegrating-have been described. Littleor no effect on the operation is notedwhen transistors are replaced or are in-terchanged. It was emphasized thatjunction transistors used as switchesmake useful circuit components whichwill operate over widely varying ambientconditions.

References

1. FUTURE ApPLICATION OF TRANSISTORS TOCONTROL MACHINE TOOLS, R. Louis Bright.Proceedings, 18th Annual Machine Tool Electrifica-tion Forum, Buffalo, New York, Apr. 1954.2. JUNCTION TRANSISTORS USED AS SWITCHBS,R.Louis Bright. AlEE Transactions, vol. 74, pt. I,Mar. 1955, pp. 111-21.

DiscussionGeorge C. Uchrin (Signal Corps EngineeringLaboratories, Fort Monmouth, N. J.): Acircuit similar to the one described in thepaper was devised in January 1954 by W. O.Taylor and the writer." The basic circuitand mode of operation are the same. It wasfound that grounded base as well asgrounded emitter could be used. Itwasalso found that almost any type of corematerial could be used and still obtain goodwave form.

The interest of the Laboratories in thiscircuit is its use in low-voltage to high-volt-age d-e transformation for the operation ofvarious .types of communications equip-ment. Power converters have been builtwhich were capable of delivering approxi-mately 60 watts output from a 24-volt d-esource.

REFERENCE

1. A NEW SELF-EXCITED SQUARE-WAVE TRAN-SISTOR POWER OSCILLATOR, George C. Uchrin,Wilfred O. Taylor. Proceedings, Institute of RadioEngineers, New York, N. Y., vol. 43, Jan. 1955p.99.

A. G. Milnes (Carnegie Institute of Tech-nology, Pittsburgh, Pa.): The switchingtransistor oscillator described by Dr. Royeris of considerable interest as a supply sourcefor the operation of magnetic-amplifierequipment. The efficiency is high,. thesource impedance is suitably low, and thereliability is likely to be good when high-power conservatively rated .transistors areavailable. Such converters may thereforetend to replace rotating electric machinessupplying small amounts of 400-cycle-per-second power in various military applica-tions.

324 Royer-Switching Transistor D-C to A-C Converter JULY 1955

Fig. 6. Arrangementof switching-transistor

oscillators

i ---+lz iy ix

MAGNETIZING CURRENT

the oscillators increases their attractivenessas supply sources.

G. H. Royer: Many different types of ap-plications may be made of the basic d-eto a-c converter described in this paper.Mr. Uchrin's application was in the field ofd-e power supply circuitry where a low-voltage direct current is converted to asquare-wave alternating current trans-formed to a high-voltage level and rectified,yielding a good high-voltage d-e source re-quiring very little filtering because the recti-fied square wave has practically no ripple.

In such an application the only require-ments are that the output voltage wave formbe square to reduce the filtering required,and that the regulation of the transformerbe good in order to maintain good voltageregulation. However, Prof. Milnes' .dis-cussion relates to the use of the device tosupply a-c power to a magnetic amplifieror other a-c load, thus eliminating the neces-sity of operating at a frequency fixed by theavailable line power. In suchan applica-tion it is desirable to maintain the outputfrequency constant under varying loadingconditions.

The use of ordinary transformer steel assuggested by Mr. Uchrin has the obviousadvantage that it is less expensive thansquare-loop material; however, it wouldnot be satisfactory in Prof. Milnes' applica-tion because when such steel is used the fre-quency shifts considerably with a change inload. To illustrate the reason for this be-havior, consider the operation of a circuitsimilar to that shown in Fig. 2 combiningtransistors and ordinary transformer steel.For a fixed input voltage Vin the base drivevoltage to transistor A will be Vin(NI / N 2 ) .The characteristic of transistor A is suchthat it will have a very low voltage dropas the current increases up the value Ixdetermined by the transistor current gain.Beyond this current the transistor voltagedrop increases sharply for little further in-crease in current. A typical normal -i

Jl5z zIIIII!II1IIIIIII

:~ =======--=--=--=--=--=-s>:IIIIIIIIIIIII)IIII

IIII

W0::oU

X:::J....JLL

i

Fig. 7. Normal -icurve for transformer

steel

the possibility exists of no correcting actionbeing obtained if the phase displacement oftwo oscillators is exactly 180 degrees dif-ferent from that required, but in practicethe oscillators have sufficient slip to ensurethat this condition is unstable and presentsno difficulty.

A rather similar circuit has been devisedin which corrective action is applied to bothtransistors of the two oscillators and is nowunder examination. This uses no more com-ponents than the circuit illustrated althoughmore windings are required.

The necessary saturation flux swings orvoltage-time ratings of reactors L I and L z aresolely related to the rating of the main coresand so the phase-locking action is not dis-turbed by change in the input voltage to thesystem. If the voltage-time rating of reac-tor L I is made half of that of L 2 (with thetotal absorption of L I and L 2 adjusted tomatch the main core), the phase lag ob-tained is 120 degrees. In this way withthree oscillators it is possible to produce a3-phase supply system. The prospect ofobtaining 2-phase and 3-phase systems from

2.

IA

2B

If magnetic-amplifier equipments are sup-plied from independent transistor oscillatorsit is probable that design frequencies willincrease since performance for a given weightis thereby improved. This is true up to atleast 2,400 cycles per second, which happensto be a frequency for which appreciableexperience has been gained in \ magnetic-amplifier design and in the operation ofassociated apparatus such as motors, gyros,and torque relays.

Two switching-transistor oscillators ofthe same frequency can be interlinked so asto provide outputs which differ in phase by90 degrees in a prescribed sequence. Onepossible arrangement is shown in Fig. 6.The transistor oscillators 1 and 2 are of thegeneral type considered by Dr. Royer andmay be operated from the same d-e sourcealthough for clarity in the diagram twoseparate batteries are shown. The capaci-tors across transistors I B and 2B are notconcerned with phase locking but may bedesirable in the basic circuits to suppressabnormal high-frequency oscillations whenthe circuits are first energized. The phase-locking circuits are those containing thesaturable reactors L 1 and L z and the rectifierbridges which apply voltages to the resistorsR1 and R2 I t will be seen that the L 1, R1circuit is energized by the sum of the oscilla-tor voltages and the L2, Rz circuit by thedifference of the voltages.

If the output voltages have a phase dif-ference of exactly 90 degrees, the sum anddifference wave forms have pulses of thesame mean voltage-time area. The reac-tors L 1 and L z are designed so that the fullnonsaturating flux changes occur in them forthis condition and so no appreciable volt-ages appear across R1 and R I If the phasedifference between VI and V2 is, however,e.g., less than 90 degrees ( V2 lagging on VI),the pulse width of VI+V2 exceeds thesaturation pulse width of L I and a residualpulse appears across R I . This voltageswitches off transistor lA and so causes re-versal of oscillator 1 earlier than normal andtherefore increases the phase difference to-wards. the required 90-degree value. Ifthe phase lag exceeds 90 degrees the correc-tion is provided by the phase-locking circuitL2, R2 acting on oscillator 2. Theoretically

JULY 1955 Royer-Switching Transistor D-C to A-C Converter 325

Magnetic-Amplifier Control of D-CMotors

curve for transformer steel is shown in Fig. 7.As the input voltage is applied to winding 2,the flux increases linearly until it reachescPx corresponding to the point at which themagnetizing current is equal to Ix. Herethe voltage drop across the transistors in-creases rapidly, the voltage across thetransformer falls, the induced base voltagefalls, and the current in the transistor is re-duced from Ix toward zero in the samemanner as described in the paper. Thesharp wave form is determined by thesharp current limitation and voltage in-crease of the transistor at the current Ix.Any output load current will be reflectedback into winding 2 and, will be a part ofthis total transistor current Ix. Thus onlythe difference between the reflected loadcurrent and the transistor maximum currentIx will be available to magnetize the core,

A. KUSKOMEMBER AlEE

A LT HOUGH considerable work hasbeen done on the operation of ad-justable-speed d-e motors with armaturepower supplied by thyratrons- and bymetallic rectifiers controlled with adjust-able transformers.v" there is little infor-mation available on the use of magneticamplifiers for this purpose. For thisreason, an analytical and experimentalstudy was made of the steady-state oper-ation of a d-e motor with armature powersupplied through metallic rectifiers andcontrolled by saturable reactors or mag-netic amplifiers. More specifically, thespeed-torque characteristics of the motorwere determined for the various controlmethods and, in particular, the effective-ness of simple feedback connections andthe use of external armature-circuit in-ductance was ascertained in improvingthe speed regulation.

This study has shown that, where thearmature power for a d-e motor is being

Paper 55-60, recommended by the AlEE MagneticAmplifiers Committee and approved by the AlEECommittee on Technical Operations for presenta-tion at the AlEE Winter General Meeting, NewYork, N. Y., January 3I-February 4,1955. Manu-script submitted October 7, 1954; made availablefor printing December 3, 1954.A. KUSKO is with the Massachusetts Institute ofTechnology, Cambridge, Mass., and J. G. NELSONis with the Minneapolis-Honeywell Regulator Com-pany, Minneapolis, Minn.The work described in this paper was initiated ona consulting basis for the Hevi Duty Electric Com-pany, Milwaukee, Wis., and was completed by Mr.Nelson as a thesis in partial fulfillment of therequirements of the degree of Master of Scienceat the Massachusetts Institute of Technology.

and switching will occur at some lowervalue of current and flux (111 , cPy). A fur-ther increase in load current will result in astill lower magnetizing component (Iz, cPz).It is evident from the frequency governingexpressionf= V in/(4N