Draw the circuit diagram explaining the working of stable multivibrator with waveforms.

Q.) Draw the circuit diagram explaining the working of stable multivibrator with waveforms.

 Subject: electronic devices and circuits

A bistable multivibrator is an electronic circuit that has two stable states. It can be used to store binary information, representing a binary '0' or '1'. The bistable multivibrator can be implemented using various components, but a common way to build it is with two cross-coupled transistors or logic gates. Here, I will explain the working of a bistable multivibrator using transistors.

Circuit Diagram

Here is a basic circuit diagram of a bistable multivibrator using NPN transistors:

  Vcc
   │
   ├────┐
   │    │
  ┌┴─┐ ┌┴─┐
  │Q1│ │Q2│
  └┬─┘ └┬─┘
   │    │
   ├────┤
   │    │
  ┌┴─┐ ┌┴─┐
  │R1│ │R2│
  └┬─┘ └┬─┘
   │    │
   ├────┤
   │    │
  ┌┴─┐ ┌┴─┐
  │R3│ │R4│
  └┬─┘ └┬─┘
   │    │
   ├────┤
   │    │
  ┌┴─┐ ┌┴─┐
  │Q3│ │Q4│
  └┬─┘ └┬─┘
   │    │
   ├────┤
   │    │
   └────┘
   GND

Components

  • Q1 and Q2: NPN transistors
  • R1 and R2: Resistors for the collector of Q1 and Q2
  • R3 and R4: Resistors for the base of Q1 and Q2
  • Vcc: Power supply

Working Principle

  1. Initial Condition: Assume that Q1 is ON (saturated) and Q2 is OFF (cut-off). The collector of Q1 is at a low voltage (close to GND), and the collector of Q2 is at a high voltage (close to Vcc).

  2. Stable State: In this state, the base of Q2 is held low because it is connected to the collector of Q1. Since Q2 is off, its collector voltage is high, which keeps Q1 on. This is a stable state because each transistor keeps the other in its current state.

  3. Switching: To change the state, an external pulse can be applied to the base of Q2 to turn it on. When Q2 turns on, its collector voltage drops, which reduces the base voltage of Q1, turning it off. Now Q2 is on and Q1 is off, which is the other stable state.

  4. New Stable State: With Q2 on and Q1 off, the base of Q1 is now held low by the collector of Q2, and the collector of Q1 is high, which keeps Q2 on. This is the new stable state.

Waveforms

The waveforms for the bistable multivibrator would show two distinct levels, representing the two stable states. Here is a representation of the waveforms:

Q1 (Collector)   ---___---___---___
                   ON  OFF  ON  OFF

Q2 (Collector)   ___---___---___---
                  OFF  ON  OFF  ON

The transitions occur when an external pulse is applied to the base of either transistor to switch its state. The duration of the ON and OFF states is determined by the external pulses, not by the circuit itself.

Example

Let's say we have a bistable multivibrator with the following component values:

  • R1 and R2 = 10kΩ
  • R3 and R4 = 1kΩ
  • Vcc = 5V

In the stable state with Q1 on and Q2 off, the voltage at the collector of Q1 would be close to 0V, and the voltage at the collector of Q2 would be close to 5V. When an external pulse is applied to the base of Q2, it turns on, and the collector voltage of Q2 drops to near 0V, which turns Q1 off. Now the collector of Q1 is close to 5V, and this state will remain stable until another pulse is applied to switch the states again.

The bistable multivibrator is a fundamental building block in digital electronics, often used in flip-flops and memory circuits.