Astable multivibrator using transistor

[..........So, if you have at least two transistors (NPN), four resistors (each two of same value), two capacitors and a power supply then you are ready to go for the project/experiment.]

Astable multivibrator is a type of multivibrator that doesn't have any stable state meaning it will create oscillation when used in a circuit. Astable multivibrator can be made using transistor. It is easy to build because only 8-10 small components are needed.

Multivibrators are special type of transistor switch. If you don’t know how to use transistor as a switch then please follow this link to get an idea about transistor switching. Transistor T₁ and T₂ (Figure 1) are connected in common emitter configuration feeding each transistors output to the input of the others over a capacitor. If transistor T₁ and T₂ would be identical then this circuit would have been of no use. But we know that no transistors are identical. So there must be at least one property which is identical for each transistor. This property is the β value of a transistor which determines that how fast a transistor will start conducting current when a circuit is being connected to a source.

Figure 1: Astable multivibrator using transistor

Consider the following case; let’s assume that transistor T₁ conducts more rapidly than transistor T₂. As a result transistor T₁ turns on and T₂ turns off. As transistor T₁ turns on, current started flowing through it reaching its saturation state. So we get no voltage difference at output1 (collector emitter voltage V_CE1). Since transistor T­₁ is fully on Capacitor C₁ starts to charge up through resistor R_b1 towards V_cc (Supply Voltage). When plate B₁ of capacitor C₁ is charged up to +0.7 Volt, transistor T₂ turns on. In the meantime, plate B₂ of capacitor C₂ was charged through resistor R_c2 up to its peak because transistor T₂ was turned off. As transistor T₂ turns on plate B₂ of capacitor C₂ releases its charge through base emitter junction of transistor T₂. Hence, plate A₂ of capacitor C₂ has its negative peak value which turns the transistor T₁ hard off. Thus raise the output1 voltage to its maximum. Same thing happen with the output2 but in opposite phase with respect to output1. So we get square wave output from both the output terminals.

In the type of multivibrator circuit shown in figure 1, only the base resistors and capacitors determine the frequency of oscillation. Periodic time,

T= T₁ + T₂

T₁ = 0.693 R_b1 C₁

T₂ = 0.693 R_b2 C₂

And frequency,

f = 1/T = 1/(T₁ + T₂) = 1/0.693(R_b1C₁ + R_b2C₂)

Now if R_b1 = R_b2 = R and C₁ = C₂ = C then,

f = 1/1.386RC

This is how we will determine the frequency. And if we want some specific frequency then doing some reverse calculation we can determine the resistor or capacitor value.

So, if you have at least two transistors (NPN), four resistors (each two of same value), two capacitors and a power supply then you are ready to go for the project/experiment.