Multivibrators: Bistable

Definition

A bistable multivibrator is an electronic circuit that has two stable states and can be switched from one stable state to the other by an external trigger signal. It stays in either state until another trigger is applied.

In simple terms, it acts like a 1-bit memory device because it can store one binary state at a time:

• Logic 0
• Logic 1

Main Content

1. Basic Concept and States of Bistable Multivibrator

• A bistable multivibrator has exactly two stable output conditions, commonly called SET and RESET states.
• In one state, one transistor is ON and the other is OFF; when triggered, their conditions interchange and the circuit moves to the other stable state.

A bistable circuit is built using active components such as transistors, op-amps, or logic gates. The classic transistor bistable circuit uses two transistors cross-coupled through resistors. Due to positive feedback, once one transistor turns ON more strongly, it forces the other transistor OFF, and the circuit locks into that condition.

This positive feedback is the reason the circuit remains stable in one of the two positions. The circuit does not oscillate by itself because both stable states are valid and permanent until an external input changes the state.

Example:

• If transistor Q1 is ON and Q2 is OFF, the output may represent binary 1.
• If Q2 is ON and Q1 is OFF, the output may represent binary 0.

This makes the bistable multivibrator a foundational element in sequential logic circuits.

2. Circuit Construction and Triggering Mechanism

• The standard transistor bistable circuit consists of two transistors, collector resistors, base resistors, and cross-coupling resistors.
• A trigger pulse applied to one side causes the circuit to switch states by changing transistor conduction.

A simple bistable circuit can be visualized as follows:

        Vcc                          Vcc
         |                            |
        RC1                          RC2
         |                            |
        C1                           C2
         |                            |
        Q1                           Q2
       B |                           | B
         |----R----->|               |<----R----|
              cross-coupling network

In practice:

• When Q1 is conducting, its collector voltage drops.
• This low collector voltage is fed through the coupling network to keep Q2 OFF.
• A trigger at the base of Q2 can reverse the condition, turning Q2 ON and Q1 OFF.

Triggering can be of different types:

Set trigger

• : Forces the circuit into one chosen state.

Reset trigger

• : Forces the circuit into the opposite state.

Because switching depends on the trigger pulse, the bistable circuit is also called a flip-flop in digital electronics.

3. Operation, Characteristics, and Memory Action

• The bistable multivibrator stores one binary bit and changes state only when instructed.
• Its output remains fixed between triggers, making it ideal for storage and control functions.

The operation is based on regenerative feedback:

1. Suppose Q1 is ON and Q2 is OFF.
2. A trigger pulse is applied to the base of Q1 or Q2.
3. The circuit rapidly switches to the opposite condition.
4. The new state remains until another trigger arrives.

Important characteristics:

Two stable states

No free-running oscillation

High switching speed

Useful as a binary storage element

Can be clocked or directly triggered

Common forms of bistable circuits:

Transistor bistable multivibrator

SR latch

Flip-flops using NAND/NOR gates

IC-based bistable circuits

Example of application in memory:

• A bistable circuit can represent whether a switch is ON or OFF.
• In digital systems, it can hold a bit in registers, counters, and storage elements.

Working / Process

1. Initial Stable State

• One transistor is in saturation and the other is cut off.
• The circuit stays in this state due to positive feedback and cross-coupling.

2. Trigger Application

• A pulse is applied to the appropriate input.
• The trigger slightly changes the base current of the conducting transistor or activates the non-conducting one.

3. State Reversal

• The conducting transistor begins to turn OFF.
• The previously OFF transistor turns ON.
• The output flips to the opposite stable state and remains there until the next trigger.

Advantages / Applications

• Provides simple binary storage of one bit of data.
• Used in flip-flops, latches, registers, and counters in digital electronics.
• Offers fast switching and reliable state retention with very low power in stable conditions.

Bistable multivibrators are widely used in:

• Memory storage circuits
• Toggle switches
• Pulse counting circuits
• Frequency division
• Debouncing circuits for mechanical switches
• Digital logic systems
• Timing and control circuits

Summary

• A bistable multivibrator has two stable states and switches only when triggered.
• It is commonly used as a flip-flop or 1-bit memory element.
• It is important in digital electronics for storage, counting, and control.

Important terms to remember

• Bistable state
• Set
• Reset
• Flip-flop
• Cross-coupling
• Positive feedback
• Memory element