Bistable Operation

Author: Leonard Krugman

Fig. 6-22. (A) Basic bistable trigger circuit. (B) Idealized characteristics.

Figure 6-22 (A) illustrates the basic bistable circuit. The fundamental requirement for this type of operation is that the load line intersects the characteristic curve once in each of the three operating regions. This automatically establishes three operation points: one in the unstable negative-resistance region; one in the saturation region; and one in the cut-off region. The last two points are stable, hence, circuit operation is properly defined as bistable. The operation shown in Figure (6-22 (B) is as follows: When operation is at point P_lf the circuit is stable, since the current is low; this is referred to as the off-state. If a positive pulse is now applied to the emitter, operation enters the regenerative region at point A. The operation swings rapidly to the saturation region where, at point P₃, the circuit is again stabilized. Since the current at this point has considerable magnitude, this is referred to as on-state. To move operation back into the off-state requires a negative trigger pulse whose magnitude is at least equal to E₃. This pulse moves operation back into the unstable negative-resistance region at point B, where it rapidly swings back to the stable off-state point P₁.

The value of R_E is selected to provide the three necessary operating points. It is not critical and may vary considerably but, in general, it should be fairly low. Notice that the potential of the emitter battery E_e fixes the location of P₁, which in turn determines the required value of the trigger pulse E₁ A low battery voltage, then, causes sensitive operation, since the triggering can be accomplished with a small pulse. A large value of E_e results in less sensitive but more reliable operation, since the circuit is less likely to be triggered by noise or other unwanted circuit disturbances. The final choice of both E_e and R_E should be based on the most sensitive combination providing reliability.