9.1 Introduction – Pulse and Digital Circuits


A bistable multivibrator has two stable states and this circuit also employs two devices, Q1 and Q2. If one device is ON, the other device is required to be OFF. If initially Q1 is OFF, the voltage at the collector of Q1 is VC1 = VCC; and when Q2 is ON, the voltage at this collector VC2 ≈ 0 V. This is the initial stable state for the bistable multivibrator. The circuit of flipped from one stable state to other (i.e driving into the other stable state in which Q1 is ON and Q2 is OFF) by an external trigger. On the application of a trigger, Q1 switches ON (VC1 ≈ 0 V) and Q2 switches OFF (VC2 = VCC) and the states of Q1 and Q2 are flipped only when another trigger is applied. If VCC is taken to represent “1” in binary and 0 V represents “0”. So “1” level remains as “1” and “0” level remains as a “0” till the application of a trigger signal. Hence, this type of circuit is used as a one-bit memory element in digital circuits. An array of such circuits can be used to write or store a string of binary digits (0 s or 1 s), called a register. This becomes the basic memory unit in digital computers. This circuit is also known by many names such as binary, flip-flop, scale-of-two circuit and Eccles–Jordan circuit. If the ON device is driven to saturation, the binary is called a saturating binary. If, on the other hand, the ON device is held in the active region, the binary is called a non-saturating binary. An emitter-coupled binary is called a Schmitt trigger. This circuit, in addition to operating as a binary, can also be used as an amplitude comparator and as a squaring circuit.