Bistable Multivibrator

Bistable Multivibrators are the multivibrators which depend upon the external triggers so as to switch between their two permissible stable states. These circuits are also referred to as Trigger Circuits or Eccles Jordon Circuits or Scale-of-2 Toggle Circuits or Binary or more popularly as Flip-Flops, forming the basic building blocks of sequential digital systems. These circuits can be designed in different ways, say for example, they can compose of transistors or Op-Amps or 555 timer ICs along with passive components, the resistors. Figure 1 shows such a circuit designed using two NPN bipolar junction transistors (BJTs) Q₁ and Q₂ and four resistors R_C1, R_C2, R₁ and R₂.

Initially, let us consider that the SPDT switch is position 1 which inturn grounds the base of the transistor Q₁. As a result, Q₁ will be OFF (cutoff region) while its collector will be held at V_CC, due to which the output at O₁ will go high. This inturn forward biases the BE junction of transistor Q₂, switching it ON (into saturation mode of operation). Due to this, the collector current flows through the collector resistor R_C2, shorting the collector terminal of Q₂ to ground. Thus, for this case, the output at O₂ terminal goes low.

This state of the circuit remains unchanged for an indefinite period of time, unless triggered externally. In this case, the act of changing the switch position from 1 to 2 acts like an external trigger for the circuit. When done so, the base of transistor Q₂ will be grounded, switching it OFF (cutoff region). This also causes the V_CC to appear at the collector terminal of Q₂, which inturn results in a high output at O₂ terminal. Further, at this state, Q₁ will switch ON (gets into saturation mode of operation) as it has its base connected to the collector terminal of Q₂ via R₂. Due to this, the collector terminal of Q₁ will be shorted to ground, causing the output at the terminal O₁ to go low. This state of the circuit is again maintained until triggered once again.

From the explanation presented, the following two points can be concluded on the nature of the bistable circuits.

1. Bistable circuits are not self-triggered as they rely on the user-provided trigger inputs so as to change their state.
2. In these circuits, the output wave-forms obtained at the terminal O₁ and O₂ are complementary to each other, always.

In addition, it is to be noted that triggering in the case of Bistable circuit is more conveniently experienced by electronic-means rather than mechanically. This kind of triggering can be either asymmetric or symmetric in nature. In the case of asymmetric triggering, the trigger for each of the transistor is obtained from separate sources and is independent of each other. Next, the symmetrical triggering can be any one of the following types: symmetrical base triggering or symmetrical collector triggering or symmetrical hybrid triggering.
These bistable circuits are used as memory storage devices, timing circuits, frequency dividers, electronic toggle switch, counting circuits, shift registers, clock pulse generators, relay controllers and even in the field of radar and communications.