Basic Transistor Astable Multivibrator Design

Astable multivibrator is a oscillator and timing circuit commonly called ‘Astable’, which means the circuit has two different states but it is not set to any of those states  but rather oscillates between them. It is employed to generate signals for applications where we need clock pulse train. It also find application as a frequency divider.

Multivibrator Circuit

The transistor multivibrator shown in the figure apply two transistors use as switches; output from one transistor is the inverse of the other which is not perfectly square but you could modify the basic circuit to obtain perfect square waves. The is a familiar circuit described in many texts similar to a bistable except it has no stable status. To design, construct or work on a multivibrator, you may need to consider the following for it will work fine:

  1. The voltage on either transistor collector is expected to be a square wave which oscillates between the power supply voltage and the transistor saturation voltage (check this transistor operation)which is typically 0.1 V.
  2. For the circuit to work, the ratio of R/S about I/3 of the transistor β (β (beta) of a transistor is the gain or amplification factor of a transistor. It is the factor by which current is amplified in the circuit) – you may need to check this from a datasheet! If R is too big the transistor will refuse to oscillate and voltage at both collectors will be greater than the transistor saturation voltages. Meanwhile, should be large enough. If R is too small, the time constant CR and CS would be about the same thing and so the collector voltage cannot change sufficiently rapidly to maintain oscillations, therefore both collector voltages will remain at the transistor saturation voltages.
  3. The highest frequency at which a multivibrator will oscillate is about 100 KHz, which is about 10 microsecond of oscillations (from the formula for T = 1/f). At higher frequencies C becomes so small that stray and transistor capacitance become significant and prevent oscillation.
  4. When operating at very low-frequency the capacitor will be very large and must use electrolytic types and of course with NPN transistor, the negative terminal of the capacitor should connect to the base of the transistors.
  5. If the power supply voltage increases slowly from zero then both transistors will conduct and the circuit will not oscillate, therefore the power supply must switched ON rapidly.

Reference:

BROWN, B. H., & SMALLWOOD, R. H. (1981). Medical Physics and Physiological Measurement. Oxford: Blackwell Scientific Publications