Astable Multivibrator circuit explained in detail | Using BJT

Summary

This video explains how an astable multivibrator circuit works using transistors to generate a square wave signal. The circuit uses resistors, capacitors, and NPN transistors to create a timed oscillation, ideal for applications like blinking LEDs. The video covers the charging and discharging of capacitors, the functioning of transistors as switches, and the relationship between resistor and capacitor values to control the frequency of the output. Practical steps are given to assemble the circuit, visualize the square wave, and calculate the correct resistor values for LED usage.

Takeaways

• 😀 A square wave is a common waveform generated inside a computer, often referred to as the clock signal, which synchronizes all operations.
• 😀 An astable multivibrator is a circuit that generates a square wave, and it's often used in various applications like blinking LEDs.
• 😀 A capacitor charges gradually across a voltage source, with the charging rate depending on the resistance in series with it.
• 😀 Capacitors retain their charge after disconnecting the power source, and they can discharge when a conductor shorts the leads.
• 😀 A resistor slows down the charging rate of a capacitor, with a higher resistance leading to a slower charging process.
• 😀 Transistors act as switches; when a base-emitter voltage of around 0.7V is applied, the transistor turns on, allowing current to flow through the collector-emitter path.
• 😀 Capacitors in an astable multivibrator circuit charge and discharge at different rates, with the voltage across them controlling the state of the transistors.
• 😀 In an astable multivibrator, two transistors switch on and off alternately, producing a square wave output that oscillates between 0V and the supply voltage (VCC).
• 😀 The frequency of the square wave generated by the multivibrator circuit depends on the values of the base resistors and capacitors.
• 😀 To calculate the appropriate collector resistor for a load, use the formula based on the supply voltage, load voltage, and current to ensure proper LED functionality.

Q & A

What is a square wave, and where is it commonly used?

A square wave is a signal that alternates between two voltage levels, typically high and low, at regular intervals. It is commonly used in computers as a clock signal to synchronize all internal operations.

What is an astable multivibrator?

An astable multivibrator is an electronic circuit that continuously switches between two unstable states, generating a square wave output without requiring any external triggering.

Why is it called 'astable'?

It is called 'astable' because the circuit has no stable state. The output continuously oscillates between high and low voltage levels without settling into one state.

What are the main components of a transistor-based astable multivibrator?

The circuit typically consists of two NPN transistors, two capacitors, and four resistors—two collector resistors and two base resistors.

What role does a capacitor play in the astable multivibrator circuit?

The capacitor controls the timing of the circuit by charging and discharging through resistors, creating the time delay that determines the frequency of oscillation.

How does a transistor function in this circuit?

In the astable multivibrator, the transistor acts as a switch. It turns on when the voltage between its base and emitter reaches approximately 0.7 volts, allowing current to flow from collector to emitter.

What happens when one transistor in the circuit turns on?

When one transistor turns on, its collector voltage drops to nearly zero, which affects the charging of the capacitor connected to the base of the opposite transistor, eventually turning the other transistor on and the first one off, creating oscillation.

How does the circuit produce a square wave output?

The circuit alternates between turning each transistor on and off. This switching action causes the collector voltage of each transistor to oscillate between 0 volts and the supply voltage, generating a square wave output.

What factors determine the frequency of the square wave output?

The frequency of the square wave depends on the resistance of the base resistors and the capacitance of the capacitors. Increasing either resistance or capacitance decreases the frequency, while decreasing them increases it.

How can you calculate the appropriate collector resistor value for driving an LED load?

You can calculate the collector resistor using Ohm’s law: Rc = (Vcc - Vload) / Iload. For example, with a 9V supply, a 3.2V LED voltage, and 20mA current, Rc = (9 - 3.2) / 0.02 = 290 ohms. The nearest standard value is 330 ohms.

What is the purpose of the base resistor in the circuit?

The base resistor limits the current flowing into the transistor’s base to prevent excessive base current and ensure proper switching behavior when controlling the transistor with higher voltages.

Why do the two transistors in the circuit not activate at the same time?

Because no two transistors are exactly identical, one transistor always switches slightly faster than the other, initiating the alternating switching process that leads to continuous oscillation.

How does changing the polarity of a capacitor affect transistor switching?

When one capacitor reverses its polarity during operation, it creates a negative voltage at the base of the opposite transistor, which quickly turns that transistor off, ensuring the circuit continues oscillating.

What does the waveform look like on an oscilloscope?

The waveform appears as a square wave alternating between 0 volts and the supply voltage, with both transistors producing signals that are 180 degrees out of phase with each other.

What practical applications can an astable multivibrator have?

Astable multivibrators are used in various applications such as LED flashers, pulse generators, timing circuits, and as basic clock signals in digital systems.