With a neat sketch, explain the working of an astable multivibrator. What factors does the frequency of astable waves depend.

Q.) With a neat sketch, explain the working of an astable multivibrator. What factors does the frequency of astable waves depend.

 Subject: Electronic Devices and Circuit

Astable Multivibrator Working Principle

An astable multivibrator, often built with a 555 timer IC or a pair of transistors, is a circuit that oscillates between two states without any external triggering. It generates a continuous output in the form of a square wave without any input, which is why it's called "astable" (as opposed to "monostable" which requires triggering to change states).

Basic Circuit Using 555 Timer IC

Here's a basic sketch of an astable multivibrator using a 555 timer IC:

            +Vcc
              |
              |
             ---
             | | R1
             | |
             ---
              |
              +------|>|----+
              |             |
              |            ---
              |            | | R2
              |            | |
              |            ---
              |             |
              |             |
             _|_           _|_
  Reset 1 --|   \         |   \ 3 Output
            |    )555     |    )
  Trigger 2 --|___/         |___/ 
              |             |
              |             |
              |             |
             ---           ---
             | |           | |
             | | C1        | | C2
             | |           | |
             ---           ---
              |             |
              |             |
             GND           GND

Working Steps

  1. Initial Condition: Assume that the capacitor C1 is initially discharged, which makes the trigger pin (pin 2) of the 555 timer low. This sets the output (pin 3) high.

  2. Charging Phase: The output high state allows current to flow through resistor R1 into the capacitor C1, charging it. The voltage across C1 increases.

  3. Threshold Level: Once the voltage across C1 reaches 2/3 of the supply voltage (Vcc), the threshold pin (pin 6) is triggered, which in turn flips the output to low.

  4. Discharging Phase: The output low state connects the discharge pin (pin 7) to ground, allowing C1 to discharge through resistor R2.

  5. Trigger Level: When the voltage across C1 drops below 1/3 of Vcc, the trigger pin is activated again, setting the output high and starting the cycle over.

This process repeats indefinitely, creating a square wave at the output.

Factors Affecting the Frequency of Astable Waves

The frequency of the oscillation in an astable multivibrator depends on the time constant of the charging and discharging phases of the capacitor, which is determined by the resistors and capacitor in the circuit.

Frequency Calculation for 555 Timer IC

The frequency (f) of the astable multivibrator using a 555 timer can be calculated using the following formula:

[ f = \frac{1.44}{(R1 + 2R2) \times C1} ]

Where:

  • ( R1 ) and ( R2 ) are the resistances in ohms.
  • ( C1 ) is the capacitance in farads.

Table of Factors

Factor Description Impact on Frequency
( R1 ) Resistance connected to the charging path of the capacitor. Higher ( R1 ) decreases frequency.
( R2 ) Resistance connected to both the charging and discharging paths of the capacitor. Higher ( R2 ) decreases frequency.
( C1 ) Capacitance that charges and discharges to create the oscillation. Higher ( C1 ) decreases frequency.
Supply Voltage (Vcc) The voltage supplied to the circuit. Affects the charging rate of the capacitor. Typically does not affect frequency directly, but a higher Vcc can increase the charging rate, slightly increasing frequency.
Temperature The ambient temperature can affect the resistance and capacitance values due to their temperature coefficients. Varies depending on the components' temperature coefficients.

Example

Let's calculate the frequency of an astable multivibrator with the following component values:

  • ( R1 = 1k\Omega )
  • ( R2 = 10k\Omega )
  • ( C1 = 100nF )

Using the formula:

[ f = \frac{1.44}{(1000 + 2 \times 10000) \times 100 \times 10^{-9}} ] [ f = \frac{1.44}{(1000 + 20000) \times 100 \times 10^{-9}} ] [ f = \frac{1.44}{21000 \times 100 \times 10^{-9}} ] [ f = \frac{1.44}{2100000 \times 10^{-9}} ] [ f \approx 685.71 Hz ]

So, the frequency of the output square wave is approximately 685.71 Hz.