Bistable Amplifiers

Positive feedback in a magnetic amplifier can be increased to more than 100 per cent by increasing turns in the feedback winding. Transfer curves may then become double-valued and give rise to abrupt load current changes with changing control current. Such amplifiers are called bistable. In Fig. 209, the effect of increasing feedback would be to decrease the slope of the feedback line. If the feedback were increased gradually, operation would remain stable until the feedback line had the same slope as the transfer curve. Then the load current would become some indefinite value along the transfer curve. If the feedback were increased further stable operation would be had at only one of two values of load current. Bistability is illustrated in Fig. 212(a). Here a transfer curve similar to those of Fig. 211 is shown except that it is with load voltage ordi-nates and expanded N_CI_C abscissas. The amount of feedback in excess of 100 per cent is drawn as line AB with slope less than that of the main part of the 100 per cent feedback transfer curve. Another line, CD, is drawn parallel to the line AB. These lines are tangent to the transfer curve at points A and C. With feedback > 100 per cent, let d-c control current be decreased from some negative value toward zero. Load voltage or current follows the transfer curve until it reaches point A; then it jumps to point B, and further increase of control current results in very little load voltage increase beyond point B. If control current is subsequently reduced, load voltage follows the top of the transfer curve until it reaches point C; then it drops abruptly to point D.

Bistable action is shown in Fig. 212(b) as a function of control NI/in., with points A, B, C, and D corresponding to those in Fig. 212(a). Line AB in Fig. 212(a) represents feedback ampere-turns N_fI_f in excess of 100 per cent, which are proportional to E_L. Line AB extended intersects the axis of abscissas at F', and CD extended intersects at G. Vertical lines erected at A' and F' intersect the transfer curve at A and F, respectively. F'A' represents ampere-turns N_fI_f when control ampere-turns N_CI_C are at point F. When decreasing negative N_CI_C reach value F, the load voltage jumps from A to B. Points F' and G are projected downward to Fig. 212(b). In this figure the output jumps to final value B, but the increase actually takes place along the dotted line. Decreasing additional feedback N_fI_f reduces the differential amount F'G of control N_cI_c and reduces the width of the bistable loop. Conversely, increasing N_fI_f widens the loop and provides a greater margin for variations in N_fI_f due to voltage, temperature, etc. Bistable amplifiers are used in protective and control circuits to turn relays or indicators on or off when control power varies between narrow limits and the inherent lock-in action is desirable.