Explain astable multivibrator.

Q.) Explain astable multivibrator.

 Subject: Digital Circuit Design

An Astable Multivibrator, also known as a free-running multivibrator, is a type of oscillator circuit that generates a square wave output waveform without any external triggering input. It is called "astable" because it is not stable in any state but continually switches from one state to the other.

The basic operation of an astable multivibrator can be explained in the following steps:

  1. Initialization: Initially, let's assume that the output of the first transistor Q1 is in the LOW state (0), and the output of the second transistor Q2 is in the HIGH state (1).

  2. Charging of Capacitor C1: Since Q1 is OFF, the capacitor C1 starts charging towards Vcc through the resistor R1.

  3. Switching of Q1: When the voltage across the capacitor C1 becomes sufficient to turn ON the transistor Q1, it switches ON. This causes the output of Q1 to go HIGH.

  4. Discharging of Capacitor C1: Now, the capacitor C1 starts discharging through the transistor Q1, which is now ON.

  5. Switching of Q2: As the capacitor C1 discharges, the voltage across it decreases. When it becomes insufficient to keep Q2 ON, Q2 switches OFF. This causes the output of Q2 to go LOW.

  6. Charging of Capacitor C2: Now, since Q2 is OFF, the capacitor C2 starts charging towards Vcc through the resistor R2.

  7. Switching of Q2: When the voltage across the capacitor C2 becomes sufficient to turn ON the transistor Q2, it switches ON. This causes the output of Q2 to go HIGH.

  8. Discharging of Capacitor C2: Now, the capacitor C2 starts discharging through the transistor Q2, which is now ON.

  9. Switching of Q1: As the capacitor C2 discharges, the voltage across it decreases. When it becomes insufficient to keep Q1 ON, Q1 switches OFF. This causes the output of Q1 to go LOW.

  10. Repeat: The cycle then repeats from step 2.

The frequency of oscillation of an astable multivibrator can be calculated using the following formula:

f = 1 / T = 1 / (0.693 * (R1*C1 + R2*C2))

Where:

  • f is the frequency of oscillation
  • T is the time period of oscillation
  • R1 and R2 are the resistances
  • C1 and C2 are the capacitances

The duty cycle of an astable multivibrator can be calculated using the following formula:

D = T1 / T = R1*C1 / (R1*C1 + R2*C2)

Where:

  • D is the duty cycle
  • T1 is the time period for which the output is HIGH
  • T is the total time period of oscillation

The main features of an astable multivibrator are:

Feature Description
Frequency of Oscillation Determined by the RC time constants of the circuit
Duty Cycle Can be varied by changing the values of R and C
Output Waveform Square wave
Stability Not stable in any state
Triggering No external triggering required
Applications Used in timing circuits, oscillators, and pulse generation

In conclusion, an astable multivibrator is a simple and versatile electronic circuit that can be used to generate a variety of signals for various applications.