Transistorized Astable Multivibrator is a cross-coupled transistor network capable of producing sharp continuous square waves. It is a free-running oscillator or simply a regenerative switching circuit using positive feedback.
Astable Multivibrator switches continuously between its two unstable states without the need for any external triggering. The time period of the astable multivibrator can be controlled by changing the values of feedback components such as coupling capacitors and resistors.
We have explained the NE 555 IC-based astable multivibrator circuit. Here we are dealing with a modified version: Transistorised Astable Multivibrator.
Circuit Diagram of Transistorised Astable Multivibrator
Components Required for Astable Multivibrator Using Transistors
- Resistors (1KΩx2, 1MΩx2)
- Capacitors (1µFx2)
- Transistors (BC548x2)
- DC power supply
Working Principle of an Astable Multivibrator Using Transistors
- Assume anyone of the transistors Q1 or Q2 turns ON due to parameter variation or due to some switching transients. Let it be Q1.
- Then the collector voltage of Q1=Vce(sat)=0.2V, it is cross coupled to base terminal of Q2 through C1, then Q2 remain in OFF state.
- During Q1 ON, the current path through R1 charges the capacitor C1, the capacitor C1 voltage is coupled to base of transistor Q2.
- While charging of C1, when the capacitor voltage exceeds 0.7V, Q2 turns ON.
- As soon as Q2 enters ON state, its collector voltage falls to Vce(sat)=0.2V. It is coupled to the base terminal of Q1 and so Q1 turns OFF.
- At the same time capacitor C2 starts charging through R2. When the volatge of C2 exceeds 0.7 V, Q1 turns ON due to cross coupling.
- This process continues.
- We can also take output from collector terminals of the transistors. Such variation of the circuit is show below.
- To know the working of the circuit below you must know how transistor acts as a switch.
The following is the output taken from the collector terminal of Q1.
Generalized Design of Transistorised Astable Multivibrator
If the reference transistor is Q1, the time period of the positive cycle of transistorized astable multivibrator is given by,
Ton=0.69 R1xC1 sec
We can also take output from Q2. Then,
Ton=0.69 R2xC2 sec
My Design of Transistorised Astable Multivibrator
I designed the circuit for 0.6 sec ON and 0.6 sec OFF.
Time Period (T)
T=Ton + Toff
= 0.69 (R1C1+R2C2)
Duty cycle (D)
Implementation of Astable Multivibrator Using Transistors in Breadboard
BC548 is the most common BJT used in an electronics project. If you are an electronics enthusiast, you must understand it well and know sufficient uses of it. And building an astable multivibrator using transistors falls into the primary realm of it.