wound with 300 turns on the primary and 100 turns on the secondary. The diode in the secondary circuit was a GE (General Electric Company) 1N538.

Core Integration Circuit

The core used for SR was a Magnetics, Inc., 50,026-2A (1 X V/2 X 3/8 inch) with various windings. The turns actually used in recording the data were 2,620. The resis-tor R was 5,100 ohms. In all tests, a ca-pacitor of 4 microfarads was used in series with SR and R.

Comparison Circuit

The reference amplifier was identical to the test amplifier. The auxiliary bridge rectifiers paralleling the two 100-ohm loads were made using GE IN538 silicon diodes. The bleeder resistors were 10,000 ohms, and the detection resistor 10,000 ohms. The blocking diode was a GE IN538.

References

1. P R O P O S E D S T A N D A R D T E S T C O D E S F O R M A G -N E T I C A M P L I F I E R S , AIEE C o m m i t t e e R e p o r t .

AIEE Transactions, p t , I . (Communication and Electronics), vo l . 78 , S e p t . 1959, p p . 4 5 3 - 5 6 .

2. R E C O M M E N D E D S Y M B O L S F O R M A G N E T I C A M -P L I F I E R P A P E R S — A R E P O R T , AIEE C o m m i t t e e R e -port. Ibid., N o v . 1959, p . 520 , N o t e C l .

3 . A T R A N S I E N T A N A L Y S E R F O R M A G N E T I C A M P L I F I E R S , E . J . Smith . Ibid. vol . 72 , S e p t . 1953, p p . 4 6 1 - 6 5 .

4. T H E C Y C L I C I N T E G R A T O R — A D E V I C E F O R M E A S U R I N G T H E F R E Q U E N C Y R E S P O N S E O F M A G -N E T I C A M P L I F I E R S , T . Dunnegan , J r . , J . D . Harn-den, J r . Ibid., vol . 7 3 , S e p t . 1954, p p . 3 5 8 - 6 5 .

5 . OSCILLOGRAPHIC TECHNIQUES FOR THE EVALU-ATION O F M A G N E T I C A M P L I F I E R R E S P O N S E , D . L. Critchlow. Ibid., vo l . 74 , N o v . 1955, p p . 607 -10 .

Self-Regulation in Magnetic-Transistor

Amplifiers

C. E. HARDIES ASSOCIATE MEMBER AIEE

MAGNETIC amplifiers incorporating transistors have received consider-

able emphasis in the literature. Many circuits have been devised using mag-netic amplifiers and transistors in combin-ation to achieve many different character-istics. This paper describes a character-istic available with this combination of de-vices that has not been heretofore de-scribed. By utilization of the circuitry to be described, an amplifier can be designed that has a self-regulated output for varia-tions in supply excitation. This charac-teristic is inherent to the circuitry; not accomplished by the addition of compensating networks. While self-reg-ulation is the primary advantage to be gained by use of the circuitry, no sacri-fice is made in gain, temperature stability, or frequency stability from that of a con-ventional magnetic amplifier. In addi-tion, since the transistor is utilized only as a switch (transistors have very low saturated resistances as compared with magnetic amplifiers) the regulation of the device with .load is very good.

The basic method to be discussed for achieving self-regulation for variations in supply excitation used in the present device has been treated in all-magnetic

P a p e r 59-1059, r e c o m m e n d e d b y t h e A I E E M a g -ne t ic Amplifiers C o m m i t t e e a n d a p p r o v e d b y t h e A I E E Techn ica l O p e r a t i o n s D e p a r t m e n t for p r e s e n t a t i o n a t t h e A I E E - I R E N o n l i n e a r M a g -net ics a n d M a g n e t i c Amplifiers Confe rence , W a s h i n g t o n , D . C , S e p t e m b e r 2 3 - 2 5 , 1959. M a n u s c r i p t s u b m i t t e d J u n e 24, 1959; m a d e ava i l -able for p r i n t i n g J u l y 16, 1959.

C. E . H A R D I E S is wi th M a g n e t i c s , I n c . , B u t l e r , P a . R . L. V A N A L L E N , f o r m e r l y w i t h M a g n e t i c s , Inc . , is n o w wi th N a t i o n a l A e r o n a u t i c s S p a c e A d -m i n i s t r a t i o n , W a s h i n g t o n , D . C.

R. L. VAN ALLEN ASSOCIATE MEMBER AIEE

devices called the * 'transactor"1 and the "shunt-coupled magnetic amplifier.,,2

In these devices, the output is taken essentially in parallel with the magnetic cores so that output exists during the time interval while the cores are changing flux. This parallel operation requires that current limiting impedances be placed in series with the gate windings to limit the gate current after firing. Actually, the principle of having output occur during the prefiring interval rather than in postfiring intervals, as with conventional magnetic amplifiers, is the only item common to these all-magnetic devices and the magnetic-transistor am-plifier. The self-regulating properties of this circuit result from the application of this principle.

Circuit Description

Fig. 1 is a schematic diagram of the single-ended magnetic-transistor ampli-

fier. It contains a basic center-tapped magnetic amplifier and a transistor, together with their associated power and bias supplies.

The polarity of the transistor bias, acting through i?&, is negative to the base of the transistor and sufficient in magnitude to hold the transistor in its "on" or short circuit condition under the most adverse conditions of load, excitation, and temperature. The polar-ity of the magnetic amplifier is such that its output is positive with respect to the base of the transistor. Thus, out-put is delivered via the switched transistor during the time intervals before the magnetic amplifier fires (prefiring), and and no output is delivered during the time intervals either core is saturated (postfiring). The transistor is held "off" or open circuited during postfiring in-tervals because of the positive voltage established at the base by the magnetic amplifier. The bias current which passed through the base during the prefiring intervals is shunted through the saturated core of the magnetic amplifier during postfiring intervals. This operation may be more clearly understood by comparing the voltage waveforms during these two intervals which are shown in Fig. 2.

Fig. 2 shows photographs of the volt-age waveforms measured between var-ious points in the circuit of Fig. 1.

Fig. 1 . Magnetic-transistor

amplifier single-ended d-c

V g / 120 volts 60 cps (cycles

per second)

N i : N 2 : N 3 = 1 2 0 : 6 : 3 0

U L2/ 50004-2A

Ngi, N g 2 = 1,200 turns

Nci, N c 2 = 1,000 turns

Q i , type 2 N 2 4 1 A transistor

Rb = 2,000 ohms

R2 = 150 ohms

1—\Jy s/vw—-̂ Q i R L b

ό

o : N t

JANUARY 1960 Hardies, Van Allen—Self-Regulation in Magnetic-Transistor Amplifiers 905

Fig. 2. Oscillograms of voltages from circuit of Fig. 1

A — V o l t a g e a to b B—Voltage across Rb C—Vol tage a to c D—Voltage across Ri

The signal level is such that the mag-netic amplifier fires at approximately 90 degrees. Each of the photographs show one full cycle of the circuit opera-tion. Fig. 2 (A) shows the power sup-ply voltage to the transistor and load. This voltage was measured across points a and b of Fig 1. Fig. 2 (B) shows the bias current to the transistor measured as the voltage across Rb. From this waveform, it can be seen that during post-firing intervals the drop across Rb

is increased by the output voltage of the magnetic amplifier, indicating that the bias current is shunted by the gating core.

Fig. 2 (C) is the output of the magnetic amplifier as measured across the base to emitter terminals. During the prefiring intervals, the base is a few tenths of a volt negative indicating that the transis-

tor is saturated or short circuited. Dur-ing the postfiring intervals this voltage is positive, effectively biasing the tran-sistor off.

Fig. 2 (D) is the waveform of the voltage measured across load resistor RL. As has been described, the output occurs during the prefiring intervals. This output has a constant volt-second value, determined by the reset of the magnetic cores. The output of the transistor occurs in the prefiring interval determined by the firing angle of the magnetic amplifier. Then, like the transactor and shunt-coupled amplifier, the output is self-regulating with variations in supply voltage. When the supply voltage is increased the instantaneous value of the output will increase, but the firing angle will occur earlier in time maintaining constant volt-seconds and thus regulating the output. Likewise, when the supply voltage is decreased, the firing angle will occur later in time with lower instantan-eous values also regulating the output.

Fig. 3, curve A shows a typical transfer characteristic for the amplifier shown schematically in Fig. 1. The magnetic-transistor device has zero output at zero control signal, because with the self-saturating magnetic amplifier full on, the transistor is biased off. This transfer characteristic was taken from a unit containing 50-50 nickel-iron mag-netic cores having a high squareness ratio.

Design Considerations

The magnetic amplifier in the magnetic-transistor amplifier shown in Fig. 1 operates in the conventional manner with the base circuit of the transistor as its load. The base diode conducts bias current in its forward direction during prefiring intervals. During this time, the magnetic amplifier's load can can be considered as being made up of a resistor and battery in series. The battery voltage being the forward drop across the base diode. In post-firing intervals, the current which will flow in each of the gate windings will be essen-tially equal to the bias current. The gate windings current capacity can be determined directly from the required load current since the bias and load (collector) currents are related in the transistor by its gain. The following expression yields the current rating required in the gate windings as a func-tion of the load current of the entire magnetic-transistor device.

^ 2 2/3

0 0.2 0.4 0.6 0.8 1.0 1.2 14 AMPERE TURNS CONTROL

Fig. 3. Transfer characteristic for single-ended d-c magnetic transistor amplifier

A — W i t h L i , l_2 made from Orthonol B—With L i , L2 made from H y M u 8 0 C—Wi th L i , l_2 made from H y M u 8 0 and wi th

circuit off

where

Jg = average current in the gate windings of the magnetic amplifier

h = average bias current Tc = collector and average current through

the load RL ß = current gain of the transistor, grounded

emitter

The voltage output of the magnetic amplifier is dictated by the transistor base diode. The output voltage from the magnetic amplifier must be kept below the ratings for maximum reverse voltage on the base to emitter junction.

With these current and voltage re-quirements for the magnetic amplifier's load circuit, one can design a magnetic amplifier to suit the other requirements of the particular application such as gain, sensitivity, and response time ac-cording to conventional magnetic ampli-fier techniques.

The transistor used in the magnetic-transistor amplifier is selected so as to be able to withstand the required supply voltage when it is open-circuited and pass the load current when fully turned on. In cases where the maximum collector current is not specified by the manufacturer, this can be determined from the saturated resistance of the device and the allowed junction dissi-pation. In determining collector current, the transistor must be derated for the highest ambient temperature in which the device is to be used.

The value of the bias resistor is selected

906 Hardies, Van Allen—Self-Regulation in Magnetic-Transistor Amplifiers JANUARY I960

(ft) R » \ 4 KJy- νΛ/W f

Q i

-ΛΛΛΛτ-N,

H4-

Vn

• N i

Fig. 4. D-C, magnetic-transistor amplifier using sensitive core material

V g / 120 volts 60 cps Nu N 2 :N 3 = 120:3:30 Li, L2 50004-2D Ngi, Ng2 = 1,200 turns U, 50007-2A N = 600 turns Nci/ Nc2 = 1,000 turns Q i , type 2N241A transistor Rb = 2,000 ohms RL = 1 5 0 ohms Rd = 10,000 ohms

Fig. 5. A-C, magnetic-transistor amplifier

V g / 120 volts 60 cps N1 :N2 :N3 :N4 = 120:6:30:30 U L2/ 50004-2A Ngi, Ng2=1,200 turns Nci, Nc2 = 1,000 turns Q i , type 2N241A transistor Rb = 2,000 ohms Ri = 150 ohms

ό Vg

:N,

Φ

Vn

: N t

Fig. 6 (left.) Polarity reversible magnetic-transistor amplifier, all values same as the equiva-

lent shown in Fig. 1

Fig. 7 (right). Effect of sup-ply voltage variation

A—120-volt supply B—108-volt supply C—132-volt supply

0 0.8 0.9 i.O 1.1 1.2 1.3 1.4 AMPERE T U R N S CONTROL

1.5

JANUARY 1960 Hardies, Van Allen—Self-Regulation in Magnetic-Transistor Amplifiers 907

so that the bias current equals the maxi-mum average load current divided by the gain of the transistor.

Ec Ecß

where

Ec — average transistor supply voltage h = average bias current ß = current gain of the transistor Jh — average load current (maximum)

For cases where high sensitivity is required, the gain of the magnetic am-plifier portion of the circuit can be improved using any of the more sensitive core materials. Fig. 3, curve B shows the transfer characteristic of a unit using an 80% nickel-iron core material and the circuit if Fig. 1. Since the squareness ratio of the 80% nickel-iron is lower, there is an appreciable output at zero control current. This is to be expected since the transistor is biased "on" in absence of output from the magnetic amplifier. Self-saturating magnetic am-plifiers made with cores having a poor squareness ratio will exhibit reduced output because of the voltage absorbed in changing flux from Br to Bm. A more desirable transfer characteristic can be obtained as shown by curve C in Fig. 3 by the simple expedient of modi-fying the waveshape of the transistor supply voltage.

The circuit diagram of Fig. 4 is the same as Fig. 1 except that a small choke has been added in series with the transistor supply source. The size of this choke will be determined by the squareness ratio of the 80% nickel-iron cores in the magnetic amplifier. The choke should be made of a square hys-teresis loop material, such as Orthonol, since its function is to absorb a few millivolt-seconds from the first part of each half cycle. The volt-seconds ab-sorbed essentially remove the supply voltage from the transistor during the time that the magnetic amplifier changes the flux in its cores from Br to Bm. Resistor Rd is used to pass magnetizing current of the choke since the choke functions when the transistor is not operating.

Magnetic-transistor amplifiers can be designed using either germanium or silicon transistors. Since the voltage drop across the base diodes of silicon transistors is higher than that found in germanium transistors, the magnetic amplifier for use with silicon transistors must have a higher output voltage than one designed for use with germanium transistors. Since the maximum reverse voltage for silicon transistors is consider-

ably higher than for germanium, this presents no problem.

Circuit Modifications

There are additional magnetic-tran-sistor circuits using the same scheme of that shown in Fig. 1 which will also exhibit self-regulation. The circuit shown in Fig. 5 may be used to deliver an a-c output. The transistor in this circuit is connected across the d-c terminals of a full-wave rectifier bridge which is in series with the load and a-c supply source. When the transistor switch is open-circuited, the bridge presents a high impedance to the load and when short-circuited offers a low impedance.

The transistor is supplied with a bias to turn it on and the magnetic ampli-fier is used to gate the transistor switch as was explained previously.

Fig. 6 shows the circuit of a push-pull magnetic-transistor amplifier. This cir-cuit is made up of two single-ended units connected so that a given direction of signal increases the output of one of the single-ended units and decreases the output of the other. This circuit has d-c output and can be taken differentially to obtain reversible polarity d-c output. The circuit also exhibits self-regulation, having the same properties of that shown in Fig. 1.

Experimental Results

Tests were conducted on magnetic-transistor amplifiers using both silicon and germanium transistors with 50% and 80% nickel-alloy core materials. Fig. 7 shows a typical transfer characteristic of the circuit in Fig. 1 as well as the effects on ± 10% variation in sup-ply voltage. It is of interest that regu-lation can be maintained past the knee in the region of maximum output. This is accomplished by designing the magnetic amplifier such that at full output and minimum supply voltage the core flux is operated from knee to knee. This flux excursion is the maximum volt-seconds of the core and is a constant; hence, supply voltages in excess of this minimum do not adversely affect the output.

The regulation of the circuit with variations in load is excellent when germanium transistors are used since the saturation resistance of these transis-tors is one ohm or less. The regulation with load using silicon transistors is higher since the saturation resistance of silicon transistors is much higher than

germanium. The variation in output was small over temperature ranges from — 45 to + 5 2 degrees centigrade. The effect of temperature, change was to translate the transfer curve by an amount predicted by the change in hysteresis loop width as a function of temperature, approximately 0.07% per degree centigrade.

A check of the speed of response showed that with low Nc

2/Rc ratios, the speed of response was less than 1 cycle. It is interesting that the circuit exhibits essentially the same response for both rise and fall times. This probably can be attributed to the limiting of circulating currents flowing in the postfiring in-terval.

Fig. 8 (A) shows the output of the magnetic amplifier alone, when termi-nated into a resistive load. The rise time of the voltage which occurs at the firing angle requires 200 microseconds. Fig. 8 (B) shows the output of the magnetic-transistor amplifier under the same condi-tions. The switching time of the transis-tor is 24 microseconds. This is the maxi-mum amount of time the transistor is in its class A region during each half-cycle. The fact that the transistor switches very much faster than the magnetic amplifier output in this combination device permits operation of this circuitry at much higher frequencies than are possible with only the magnetic amplifier.

The transistor functions more nearly as a perfect switch than does the magnetic amplifier, and is less frequency sensitive than are the windings of the saturated cores. Therefore, the output of the magnetic-transistor amplifier is changed

Fig. 8. Comparison of waveforms of magnetic amplifier alone (A) ; and output of magnetic-

transistor amplifier (B)

908 Hardies, Van Allen—Self-Regulation in Magnetic-Transistor Amplifiers JANUARY 1960

less by variations in supply frequency than the magnetic amplifier alone.

Conclusions

Magnetic amplifiers and transistors can be combined in a hybrid amplifier to produce a variety of advantages, foremost of which is self-regulation for variations in supply voltage.

In addition to this self-regulation for

THE PROBLEM of digital to analog conversion occurs frequently in sys-

tems using digital computers. Where power gain is needed, where a high degree of accuracy must be maintained, and where speed of response is not a limiting factor, magnetic amplifiers have special utility in decoding of digital information. Based on the saturable reactor with multi-

variations in supply voltage, the cir-cuitry provides high gain, frequency and temperature stability, and excellent load characteristics.

The design of the magnetic-transistor amplifier is simple since the magnetic amplifier of the circuit is a conventional self-saturating circuit.

The use of this circuitry has the additional advantage of being suitable for use at higher supply frequencies since

pie control windings proportioned in a binary scale, a magnetic amplifier circuit has been devised to convert a 10-digit parallel binary-coded input to an analog voltage ranging from 0 to 102.3 volts average with an accuracy of better than one part in a thousand. With the 10-digit input, three saturable reactors are used for the basic conversion to analog and

the transistor is switched at approxi-mately 10 times the speed the magnetic amplifier output normally is switched.

References

1. T H E TRANSACTOR: A SELF-SATURATED TRANSFORMER, PART I, Allen S. Rosenstein. AIEE Transactions, pt. I (Communication and Electronics), vol. 77, May 1958, pp. 129-42. 2. SHUNT-COUPLED MAGNETIC AMPLIFIER CIR-CUITS, R. M. Hubbard. Ibid., vol. 78, May 1959, pp. 124-31.

a subsequent stage of self-balancing mag-netic amplifier provides additional gain to drive a servomagnetic amplifier. In the over-all conversion circuit, the inherent feedback, signal mixing, and power gain capabilities of magnetic amplifiers appear to result in a simpler and more reliable circuit than could be achieved with other techniques.1 This paper discusses the application of magnetic amplifiers in a digital to analog conversion.

The Saturable Reactor as a Basic Converter

Binary to analog conversion can be considered as a problem in signal weight-ing and addition. In the binary system only two symbols are used, 0 and 1. These symbols represent the coefficients of integral powers of the base 2. For ex-ample, the binary number 01011 repre-sents 0 X 2 4 + 1 X 2 3 + 0 X 2 2 + 1 X 2 1 + 1 X 21, or the decimal number 11. To con-vert a binary number to its analog equiv-alent, each coefficient is multiplied by the base 2 raised to the power associated with the digit and the quantities so ob-tained are summed. The conversion factors of the five binary digits in the example, going from the least significant digit (2°) to the most significant digit (24), would be 1, 2, 4, 8, and 16. In an electrical system, the signals repre-senting the binary "ones" are weighted, or multiplied by factors, in accord with these conversion factors and summed to obtain the analog equivalent.

Five binary digits can be converted to analog by a saturable reactor with five signal windings. With the simple satu-rable reactor, good linearity is obtained

Paper 59-1044, recommended by the AIEE Mag-netic Amplifiers Committee and approved by the AIEE Technical Operations Department for pres-entation at the AIEE-IRE Nonliner Magnetics and Magnetic Amplifiers Conference, Washington, D. C , September 23-25, 1959. Manuscript sub-mitted June 23, 1959; made available for printing July 16, 1959. I. DANYLCHUK and D. KATZ are with Bell Tele-phone Laboratories, Inc., Whippany, N. J.

BIAS WINDING

BINARY SIGNAL CONTROL INPUTS WINDINGS

5 0

CONTROL AMPERE-TURNS -

(B) COMMON O

(A)

Fig. 1 ( A ) . Saturable reactor for 5-digit binary-to-analog conversion. (B). Shift of co-ordinate axes of transfer characteristic

Magnetic Amplifier Binary-to-Analog

Conversion

I. DANYLCHUK D. KATZ ASSOCIATE MEMBER AIEE ASSOCIATE MEMBER AIEE

JANUARY 1960 Danylchuk, Katz—Magnetic Amplifier Binary-to-Analog Conversion 909