Multivibrators are nothing but the non-sinusoidal oscillators which are usually two-stage amplifiers with positive feedback. These circuits generally comprise of passive components like resistors and capacitors together with active elements which can be BJTs (Bipolar Junction Transistors) or FETs (Field Effect Transistors) or Vacuum Tubes or Op-Amps or 555 timer ICs. However while designing them, care must be taken so as to ensure that the two-stages of the circuit continuously alter their states between cutoff and saturation regions.
Multivibrators can be of three types viz., astable multivibrators, monostable multivibrators and bistable multivibrators. Astable multivibrators are the multivibrators which have no stable state i.e. the multivibrators in which the output continuously oscillates between two permissible states. As a result, they produce square-wave at their output and are regarded to be free-running in-nature. Further, these multivibrators do not require any kind of external triggering, except the DC supply, due to which they fall under the category of relaxation oscillators.
Figure 1 shows such a circuit designed using two BJTs Q1 and Q2, two capacitors C1 and C2 and four resistors RC1, RC2, R1 and R2. The frequency of the output signal can be varied by varying the values of the capacitors and the resistors present in the circuit. In this kind of circuit, the application of the supply voltage VCC causes one of the transistors to turn ON earlier than the other due to the inevitable difference in their electrical properties. Now, let us assume that the Q2 switches ON first, causing the flow of collector current through RC2.
This causes the right-plate of the capacitor C2 to be grounded due to which even its left-plate will be grounded (assuming C2 to be uncharged). This inturn turns OFF the transistor Q1 which causes the left-plate of the capacitor C1 to float. However its right-plate will acquire a charge of 0.7 V as it is connected to the base of the forward biased Q2. At this state, the output O1 will be high while that at O2 will be low. From then on, C1 continues to charge via RC1 until its left-terminal acquires a voltage of VCC. In addition, at the same time, even C2 charges via the resistor R2 increasing the voltage at its left-terminal.
As this continues, a situation arises wherein the voltage at the left-plate of C2 becomes equal to 0.7 V, which forward biases the BE junction of Q1, causing the device to turn ON. When this happens, both the collector terminal of Q1 as well as the left-terminal of C1 will be shorted to ground. This causes the right-plate of C1 to acquire a voltage of –VCC, turning OFF Q2. This results in the output O2 to go high, while the output O1 goes low. Next, C1 starts to charge through R1, continuing the cycle when its right-plate becomes 0.7 V. This cyclic phenomenon gives rise to the oscillatory waveform where complementary square signals are generated at the collector terminals of either transistors.