Astable Multivibrator Using Op-Amp

Experiment 5

Aim:

To Construct and test a simple square wave generator (astable multivibrator) using op-amp 741

Apparatus Required:

SL. NO.	Name of the Apparatus	Range	Type	Qty
1.	Regulated Power Supply	+/- 12V	DC	1 No.
2.	Resistors	10 K Ohm		2 No.
3.	Resistors	100 K Ohm		1 No.
4.	Capacitors	0.01 uF		1 No.
5.	Operational Amplifier		IC 741	1 No.
6.	CRO			1 No.
7.	Breadboard			1 No.
8.	Connecting wires			Required Amount

Theory:

• The astable multivibrator has two quasi-stable states i.e., no stable states. The circuit conditions oscillate between two quasi-stable states. No external signal is required to produce the changes in state. The components value used determines the time for which circuit remains in each state.

• Usually, as the astable multivibrator oscillates between the states, it is used to produce square wave. It looks like a comparator with hytersis, except that the input voltage is replaced by a capacitor. The circuit has time dependent elements such as resistance and capacitor to set frequency of oscillation.

• When power is turned ON, Vo automatically swings either to +Vsat or to – Vsat since these are the only stable states allowed by Schmitt trigger. Assume it swings to +Vsat with Vo=+Vsat we have Vp=Vut and capacitor charges towards +Vsat through R f. As long as VcVut, negative terminal goes positive w.r.t. positive terminal of op-amp. This switches the output voltage to –Vsat and we have VP=Vlt , which is negative with respect to ground.

Circuit Diagram

Tabular Column

Waveform	Amplitude (No. of div* Volts per div	Time period (No. of div* Time per div)
Output Voltage (Vo)
Capacitor Voltage (Vc)

Model Graph

Procedure

1. Construct the astable multivibrator circuit as per the circuit diagram.

2. Sketch the capacitor and output waveforms and record the waveform

   amplitude and frequency.

3. Double the capacitor of Ci by paralleling with another 0.1Mf capacitor.

4. Note the effect on the capacitor and output waveform.

Link to experiment

Go here and select Experiment 5

Demonstration Video