Question

Explain the operation of an astable multivibrator circuit using transistors or ICs.

Answer 1

An astable multivibrator circuit is a type of oscillator that generates a continuous square wave output with no stable states. It is also known as a free-running multivibrator or a self-triggering oscillator. The circuit can be implemented using transistors or integrated circuits (ICs) like the 555 timer, which is a popular choice for astable multivibrator applications due to its simplicity and versatility. Let's explain the operation of an astable multivibrator using both transistor and 555 IC configurations:

Transistor Astable Multivibrator:
The basic circuit of a transistor astable multivibrator consists of two transistors connected in a cross-coupled manner, creating a feedback loop that continuously switches the transistors on and off alternately.

Components needed:

Two transistors (NPN or PNP)
Four resistors (R1, R2, R3, R4)
Two capacitors (C1, C2)

Operation:

Assume both transistors are initially off (non-conducting).
Let's start with transistor Q1. When power is applied, C1 charges through R1. As C1 charges up, the base of Q1 starts receiving a positive voltage.
Once the voltage across C1 becomes sufficiently high, Q1 turns on. As Q1 turns on, the capacitor C1 discharges rapidly through Q1 and R3.
Now, Q2 comes into play. With Q1 on, its collector voltage drops, and C2 starts charging through R2.
When the voltage across C2 becomes sufficiently high, Q2 turns on, and C2 discharges rapidly through Q2 and R4.
The collector voltage of Q2 drops, turning off Q2, and the cycle repeats.
The circuit continuously oscillates between these two states, producing a square wave output at the collectors of both transistors.

The frequency of oscillation is determined by the values of resistors and capacitors in the circuit.

IC 555 Timer Astable Multivibrator:
The 555 timer IC can also be configured as an astable multivibrator by connecting it in the appropriate mode.

Components needed:

555 timer IC
Resistors (R1, R2)
Capacitor (C1)

Operation:

Connect the 555 timer IC in astable mode by connecting pins 2 and 6 (trigger and threshold) together, and grounding pin 4 (reset).
Connect the capacitor (C1) between pins 1 (ground) and 2 (trigger) of the IC.
Connect resistor R1 between Vcc and pin 2 (trigger).
Connect resistor R2 between pins 2 (trigger) and 7 (discharge).
The output of the astable multivibrator is taken from pin 3 (output).

Operation:

When power is applied, the capacitor C1 starts charging through resistors R1 and R2.
Once the voltage across C1 reaches the threshold level (2/3 of Vcc), the output (pin 3) goes HIGH.
The capacitor C1 then starts discharging through resistor R2 and pin 7.
Once the voltage across C1 drops below the trigger level (1/3 of Vcc), the output (pin 3) goes LOW.
The cycle repeats, and the IC continuously generates a square wave output.

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

Frequency (f) = 1.44 / ((R1 + 2 * R2) * C1)

Both the transistor and IC 555 configurations of the astable multivibrator circuit serve as stable oscillators capable of generating continuous square wave outputs. The choice between the two configurations depends on the specific application and requirements.