Bistable Multivibrator using Transistors

 In the world of electronics, bistable multivibrators stand as fundamental building blocks, and their operation relies on the ingenious properties of transistors. These multivibrators serve as the cornerstone for memory circuits, flip-flops, and various applications in digital electronics. Understanding their functionality unveils the magic behind the intricate dance of electrons within transistor circuits.

What is a Bistable Multivibrator?

At its core, a bistable multivibrator is a two-state electronic circuit. It can exist in one of two stable states indefinitely, hence the term "bistable." This stability relies on the feedback loop within the circuit, sustaining its state until an external trigger prompts a change.

For the circuit operation explanation herein, refer to the simplest bistable multivibrator which is the fixed bias bistable multivibrator without commutator capacitors circuit diagram. 

A bistable multivibrator, featuring two stable states, operates using two devices, Q1 and Q2, ensuring that when one device is ON, the other remains OFF. Initially, when Q1 is OFF, the collector voltage of Q1 stands at VC1 = VCC, while Q2 ON maintains a collector voltage close to 0V (VC2 ≈ 0V). This represents the initial stable state of the multivibrator. To transition between these stable states and toggle Q1 to ON and Q2 to OFF, an external trigger is applied. Upon triggering, Q1 switches ON (VC1 ≈ 0V), while Q2 switches OFF (VC2 = VCC). The states of Q1 and Q2 flip only with the application of another trigger. In this binary representation, where VCC signifies "1" and 0V signifies "0", the levels retain their respective values until triggered, making this circuit a one-bit memory element within digital systems. Arrays of such circuits form registers, storing sequences of binary digits (0s and 1s), serving as fundamental memory units in digital computers. This versatile circuit goes by various names like binary, flip-flop, scale-of-two, and Eccles–Jordan circuit. When the ON device reaches saturation, it's termed a saturating binary; alternatively, when the ON device remains in the active region, it's known as a non-saturating binary. An emitter-coupled binary takes on the name Schmitt trigger. Besides functioning as a binary system, it also serves as an amplitude comparator and a squaring circuit.

Types of Transistor based Bistable Multivibrator Circuit

A bistable multivibrator circuit can be designed using (BJT or MOSFET) transistors in two ways: 

(1) Fixed bias BJT Bistable Multivibrator

(2) Self bias BJT Bistable Multivibrator

Fixed Biased Bistable Mutivibrator

The following shows circuit diagram of fixed biased bistable multivibrator implemented using two BJT(Bi-Polar Junction Transistor).

fixed bias bistable multivibrator circuit diagram

This configuration earns the title "fixed-bias bistable multivibrator" due to its utilization of two distinct DC sources(+Vcc and -Vbb), pair of equal value resistors for biasing the transistors. Comprising two inverters, the setup involves interconnecting the output of one with the input of the other. Imagine, for instance, that initially Q1 is inactive while Q2 operates in a saturated ON state. At this point, the voltage at the first collector registers as VCC (equivalent to 1 in binary), while the second collector voltage stands at VCE(sat) (binary equivalent: 0). Upon applying a negative trigger at the base of the active device (Q2), Q2 transitions to an OFF state, causing its collector voltage to elevate to VCC. Consequently, Q1 switches to an ON state, leading its collector voltage to drop to VCE(sat). This prompts a verification process to confirm the actual states of Q1 (OFF) and Q2 (ON, in saturation).

Self Biased Bistable Multivibrator

The following is circuit diagram of self biased bistable multivibrator.

self bias bistable multivibrator circuit diagram

In a self-bias bistable multivibrator, eliminating the negative VBB source is achieved by adding an emitter resistor (RE) in the emitter lead. RE creates a voltage drop used to establish the necessary additional voltage. This resistor, RE, in the self-bias bistable multivibrator, plays a crucial role in stabilizing currents and voltages. Specifically, in this configuration, the ON transistor is intentionally driven into saturation, defining it as a saturating bistable multivibrator. This design boasts a significant advantage: minimal power dissipation occurs in both ON and OFF states, enabling the use of transistors with lower power dissipation capabilities. However, its primary drawback lies in a longer storage time, leading to decreased switching speed.

Self bias is also a popular name for biasing BJT, MOSFET transistor as explained in self biased BJT amplifier and self bias depletion MOSFET.


1. Transistors:

A typical bistable multivibrator employs two transistors - usually of the NPN or PNP type - arranged in a feedback loop. Each transistor acts as a switch for the other, causing state changes.

2. Capacitors and Resistors:

Capacitors and resistors support the transistor configuration, regulating the timing of the switching process and stabilizing the circuit.

Commutator Capacitors or Condensers

Commutator capacitor is used across R1 to minimize transition duration. In a bistable multivibrator, where two cross-coupling resistances, R1 and R1, are present, capacitors C1 and C1 are required across these resistances. This setup facilitates the swift transfer of conduction from one device to the other immediately after triggering. Consequently, the bistable multivibrator is adjusted as depicted in the circuit diagram below.

Triggering Mechanism

External signals, often in the form of pulses or voltage variations, trigger the transition between the stable states. These signals can be manual or automatic, depending on the intended application.

There are two ways to trigger a bistable multivibrator:

a. Unsymmetric triggering

b. Symmetric triggering

a. Unsymmetric triggering

In this triggering approach, a single trigger pulse sourced from one origin is directed to a specific point within the circuit. The subsequent trigger pulse, sourced differently, is then applied to a distinct location within the circuit as depicted in the following unsymmetric triggering of bistable multivibrator circuit diagram.

unsymmetrical triggering of bistable multivibrator circuit diagram

  Initially, the trigger was noted to be applied at the base of the ON device. However, due to the presence of commutating capacitors in this circuit, the trigger pulse doesn't directly connect to the base of Q2. Instead, it's directed to the collector of Q1 through a capacitor. Given the capacitor's tendency to behave as a short circuit during abrupt voltage changes, the negative pulse administered at the collector of Q1 effectively couples with the base of Q2.

b. Symmetric triggering

 In symmetric triggering, utilizing consecutive trigger pulses sourced from the same origin and applied at identical points within the circuit prompts the multivibrator to transition between its stable states. This particular triggering method finds common application in counters, depicted in the following symmetric triggering of bistable multivibrator circuit diagram.

symmetric triggering of bistable multivibrator circuit diagram

Applications and Significance

Bistable is one of the multivibrator, others are astable multivibrator and monostable multivibrator. Like bistable, monostable multivibrator can be designed using transistor see monostable multivibrator using transistors for this.

Bistable multivibrators find extensive use in digital electronics, serving as the backbone for various applications:

  • Flip-Flops and Memory Units: Used in computer memory circuits to store binary information.

  • Clocking Systems: Employed in timing and synchronization circuits.

  • Control Mechanisms: Applied in control systems, counters, and frequency dividers.


The intricate interplay between transistors, capacitors, and resistors in a bistable multivibrator showcases the elegance of electronic circuitry. Its ability to maintain stable states until triggered marks it as a fundamental component in digital electronics, contributing significantly to the advancement of technology.

By comprehending the mechanisms underlying bistable multivibrators using transistors, one gains insight into the bedrock of digital logic and the backbone of modern computing.

In the realm of electronics, where innovation thrives on understanding the fundamentals, bistable multivibrators stand tall as a testament to the brilliance of transistor-based circuits.This post aims to provide an overview of the concept, operation, and significance of bistable multivibrators using transistors, serving as a guide to anyone intrigued by the intricate workings of electronic circuits.

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