A bistable multivibrator is a fundamental digital electronic circuit that has two stable states. The circuit remains in either of these states indefinitely until an external triggering signal forces it to change its state. Because of this property, the bistable multivibrator is widely used as a memory element in digital systems.
In digital electronics, the bistable multivibrator forms the basic building block of flip-flops, registers, and counters. It stores one bit of information, either logic 0 or logic 1.
Table of Contents
Basic Concept
A bistable circuit has two stable equilibrium states:
- State 1 → Output \(Q = 1\), \( \overline{Q} = 0 \)
- State 2 → Output \(Q = 0\), \( \overline{Q} = 1 \)
The circuit can remain in any one of these states indefinitely without any input. A triggering pulse applied at the input causes the circuit to switch from one state to the other.
Thus, the bistable multivibrator acts as a binary storage device.
Construction of Bistable Multivibrator
A bistable multivibrator can be implemented using:
- Two cross-coupled transistors
- Two logic gates (NOR or NAND gates)
- Operational amplifiers
The most common implementation uses two cross-coupled transistors where the collector of one transistor is connected to the base of the other transistor through a resistor network.
Working Principle
Consider a transistor-based bistable multivibrator consisting of two transistors
\(T_1\) and \(T_2\).
- When \(T_1\) is ON, it goes into saturation.
- The collector voltage of \(T_1\) becomes low.
- This low voltage drives the base of \(T_2\) low, turning \(T_2\) OFF.
Thus the circuit remains stable in this state:
\[
T_1 \text{ ON}, \quad T_2 \text{ OFF}
\]
When an external triggering pulse is applied:
- \(T_1\) switches OFF
- \(T_2\) switches ON
The circuit now moves to the second stable state:
\[
T_1 \text{ OFF}, \quad T_2 \text{ ON}
\]
The circuit will remain in this new state until another trigger is applied.
Truth Table
| Trigger Input | Next State Q | Next State \( \overline{Q} \) |
|---|---|---|
| 0 | Previous State | Previous State |
| 1 | Toggle | Complement |
Characteristics
- Two stable output states
- Stores one bit of information
- Requires external triggering for switching
- Produces complementary outputs \(Q\) and \( \overline{Q} \)
Applications
Bistable multivibrators are widely used in digital electronics systems.
- Flip-flops
- Binary counters
- Shift registers
- Memory storage circuits
- Frequency division circuits
- Switching circuits
How a Bistable Multivibrator acts as Frequency Divider?
A bistable multivibrator has two stable states and changes its state only when a triggering pulse is applied at the input. Each time a trigger pulse arrives, the circuit switches from one state to the other. This process is known as
toggling.
Assume the output of the bistable multivibrator is initially:
\[
Q = 0
\]
When the first triggering pulse arrives, the circuit changes its state to:
\[
Q = 1
\]
When the second triggering pulse arrives, the circuit toggles again and the output becomes:
\[
Q = 0
\]
Thus, the output changes state only after every triggering pulse. To complete one full output cycle (0 → 1 → 0), the circuit requires two input pulses.
If the input triggering frequency is \( f_{in} \), then the output frequency becomes:
\[
f_{out} = \frac{f_{in}}{2}
\]
This means the output signal frequency is half of the input frequency.
Because the bistable multivibrator divides the input frequency by two, it is commonly used as a frequency divider or divide-by-2 circuit. This principle is widely used in digital electronics in applications such as binary counters, clock dividers, and frequency scaling circuits.
Comparison with Other Multivibrators
| Type | Stable States | External Trigger | Application |
|---|---|---|---|
| Bistable | 2 | Required | Memory, Flip-flops |
| Monostable | 1 | Required | Timer circuits |
| Astable | 0 | Not required | Oscillators |
Advantages
- Simple circuit design
- Reliable switching between two states
- Stores digital information
- Widely used in digital electronics
Conclusion
The bistable multivibrator is an essential circuit in digital electronics. Its ability to maintain two stable states makes it an ideal building block for flip-flops and memory circuits. Almost all modern digital systems, including computers and communication devices, rely on bistable circuits for storing and processing binary information.