Magnetic Amplifiers in Bistable Operation L. A. F I Ν Ζ I G . G. F Ε Τ Η M E M B E R A I E E S T U D E N T M E M B E R A I E E

BISTABLE M A G N E T I C A M P L I F I E R S are charac­terized by two widely different stable levels of output,

one close to quiescent and the other close to fully saturated conditions. This "flip-flop" operation is desirable in some applications and can be achieved intentionally by over-re­generative mmf of one sort or another, properly introduced to modify the characteristic of self-saturating amplifiers.

Predictions of bistable amplifier behavior are obtained reliably by simple procedures, if one takes as starting point the transfer characteristic of the self-saturating amplifier, either calculated carefully under consideration of core and rectifier properties or, even better, measured experi­mentally. Overly simplified approaches, on the other hand, are not meaningful.

The mechanism of transition from the one output level to the other deserves detailed consideration in view of the well-known sluggishness of the response of these bistable amplifiers. Methods that are familiar in the study of the transient response of common amplifiers with low-suppre-sion index to even harmonics are extended to this situation. The differential equation of the control circuit is written under recognition that its flux linkages are related linearly to the output voltage; integration is performed by proce­dures that are also familiar.

It is recognized that the ^-shaped steady-state transfer characteristic represents the locus of points that are actually transversed in the transition as evidenced by Fig. 1, in which the half-cyclic voltage VL is related to the half-cyclic mmf of control NJC. (Averaging networks at the oscilloscope ter­minals have been used to reduce blurring of the trace.) Attention must be given to the meaning of the slopes of the various portions of this characteristic, relevant for the differential equation at hand.

The differential equation is rewritten so as to express the rate of change in terms of the "ac tuat ing" part of the signal

^ ^ ^ ^ ^ ^ ^ ^ ^ voltage (that is, in terms of the difference between the con-trol voltage applied and the half-cyclic average of resistive

^ drop at the instant consid-% ered). Thus information con-

% cerning the speed at which I the transition is taking place 1 at various points for given

*mmmmm^*r I control voltages is obtained ~" N c i c by inspection of the steady-

state transfer characteristic. Practically, a large portion of

Fig. 1. Locus of po ints V L , t h e t o t a l transition time is NJo de scr ibed in a switch- spent passing around the first i n g process "corner" met in the switching

Τ S E C O N D S

Fig. 2. E x p e r i m e n t a l inves t iga t ion of the inf luence of | E a x | u p o n r e s p o n s e ; o v e r - r e g e n e r a t i v e current f eedback .

Curve 1—Time to 63.2 per cent of total swing; build-down process; Curve 2—Time to 86.5 per cent of total swing; build-down process; Curve 3— Time to 63.2 per cent of total swing, build-up process; Curve 4—Time to 86.5 per cent of total swing, build-up process

process. This is particularly important in amplifiers with a prescribed narrow band of instability.

Experimental results shown in Fig. 2 for a bistable center-tap amplifier indicate that times to 63.2 and 86.5 per cent of total swing are almost inversely proportional to the minimum actuating voltage Eax at the first corner met in the build-up or build-down process. Short-cuts based on "apparent control circuit inductance," i.e., linearization of the transfer characteristic in the shape of an inverted Z, are not appropriate because they radically modify those parts of the transfer characteristic which are of prime importance in terms of transient response. •

Digest of paper 55-539, "Magnetic Amplifiers in Bistable Operation," recommended by the AIEE Committee on Magnetic Amplifiers and approved by the AIEE Committee on Technical Operation for presentation at the AIEE Summer General Meeting, Swamp-scott, Mass., June 27-July 1, 1955. Scheduled for publication in AIEE Communication and Electronics, 1955.

L. A. Finzi and G. C Feth are with Carnegie Institute of Technology, Pittsburgh, Pa.

This work has been supported in part by the U. S. Office of Naval Research

1008 Finzi, Feth—Magnetic Amplifiers ELECTRICAL. ENGINEERING