Let’s discuss a basic bistable circuit composed of two cross-coupled inverters (Inv1 and Inv2) and explain how it operates with reference to voltage transfer characteristics (VTCs).
Basic Bistable Circuit
The bistable circuit consists of two inverters, Inv1 and Inv2, connected in a cross-coupled configuration, as shown in Figure 1.
- Inv1: Input (Vin1) and output (Vout1).
- Inv2: Input (Vin2) and output (Vout2).
Voltage Transfer Characteristics (VTC)
- The VTC of the first inverter, Inv1, is depicted in Figure 1(a).
- The VTC of the second inverter, Inv2, is shown in Figure 1(b). Note that the axes for input and output voltages are interchanged.
Operating Points in Bistable Circuit
When two inverters are interconnected, Vin2 = Vout1 and Vin1 = Vout2. The operating points of the circuit are determined by combining the two VTCs, resulting in three operating points: A, B, and C, as shown in Figure 1(c).
Operating at Point A
Suppose the operating point is at A, where Vin1 = 0. This implies that Vout1 = 1, which equals Vin2. Consequently, Vout2 = 0, matching Vin1. The circuit remains stable at this operating point, and small deviations from it do not affect the outputs.
Operating at Point C:
Similarly, if the operating point is at C with Vin1 = 1, it follows that Vout1 = 0 (matching Vin2) and Vout2 = 1 (matching Vin1). Again, the circuit remains stable at this point, and small deviations have no impact on the outputs.
Operating at Point B:
Now, consider operating at B. If Vin1 is slightly reduced due to noise (ΔV), it causes Vout1 to increase and Vin2 to decrease, ultimately leading Vout2 to decrease. This process continues until Vin1 reaches 0 and Vout1 reaches VDD. Conversely, if Vin1 increases slightly, it reduces Vout1 until it reaches 0 and Vin1 reaches VDD. Therefore, at point B, even small voltage level deviations will shift the operating point to either A or C.
As a result, the circuit possesses two stable operating points, A and C, while B is an unstable operating point. Because the circuit has two stable operating points and can maintain two stable states (either 0 or VDD), it is termed a bistable circuit.