Astable Multivibrator using Transistors

Multivibrator is an astable multivibrator or multivibrator that can be used freely and without any fixed condition. The output is constantly moving between two unstable states without the help of external stimulation. The length of each state is determined by the time constant of the resistance-capacitor (RC).

Multivibrator stable with cross-circuit

In the above diagram, we can find two wiring transformers as switches. Read the transistor section as a switch. When a transistor is active, the collector and the emitter work as a short circuit. But when it is out, they act as an open circuit. In the above circuit, when a transistor is in the OFF position, the collector has a Vcc voltage and when it is input, the collector is based. When one transistor is active, others are switched off. The transistor time is determined by the constant RC time.

When the circuit is running, one of the transistors is more conductive than the other due to a circuit diver or difference in transistor parameters. The more conductive transistor is gradually driven into saturation and the transistor is at least conductive to stop.


  • When the circuit is activated, one transistor goes to saturation (AR) and another goes on. Consider C1 AR and Q2 OUT.
  • Meanwhile, the C2 capacitor is being charged to R resistor in Vcc.
  • Q2 is disabled due to the passive voltage of C1 capacitor charge that was charged during the previous cycle. Therefore, the R1C1 constant determines the time from Q2.
  • After a period of time determined by the constant R1C1, capacitor C1 is completely discharged and starts with Q1 in the transfer process.
  • When a C1 capacitor voltage is charged at a voltage sufficient to provide a 0.7 V base-emitter voltage on a transistor Q2, it turns and starts releasing the C2 capacitor.
  • The negative voltage of the C2 capacitor leaves a Q1 transistor and the C1 capacitor starts by cutting Vcc through the resistor R and the base transistor R2. That is why the Q2 transistor is still in the state.
  • When the C2 capacitor is fully discharged, it begins to cut the opposite direction through R2 as in the previous state.
  • When the voltage across a C2 capacitor is sufficient to activate the transistor Q1, it is active and the C1 capacitor begins to relax.
  • This process is ongoing and generates rectangular waves in the transistor of each transistor.
  • Note: the billing time is shorter than the check time.


R – resistance of the resistor

The resistor R must be designed so that the current Ic has a safety limit.

R = V / Ic, where V is the voltage across the resistor R.

Usually V = (Vcc – Vce) = (Vce – 0.3), but when the load is charged like a connected LED

V = (Vcc – Vce – Vled) where Vled is the voltage drop between the lights.

Usually the IC collector current is higher than the current transmitter current, as in the case of LED. In these cases, the Ic must be selected in order not to exceed the current maximum transmitter current.


R = (Vcc – Vce – Vload) / Ic

R1 and R2 – resistance to the ground

Q1 and Q2 must be selected to provide the current collector required during the saturation state.

  • Mainstream, Ibmin = Ic / β, where β as hFE is the transistor
  • Safe soil bottom, Ib = 10 × Ibmin = 3 × Ic / β
  • R1, R2 = (Vcc-Vbe) / Ib

Period T1 and T2

  • T2 = Transistor time Off Q1 = ON transistor time Q2 = 0.693R2C2
  • T1 = Transistor transient period Q2 = transistor period AR Q1 = 0.693R1C1

From these comparisons, we can determine the value of C1 and C2.

service cycle

This is the Tc time ratio in which the output is high compared to the total time of the cycle T.

Therefore, a useful cycle = Toff (Toff + Tone) when the output is taken from the collector of the transistor T.