Unsteady multivibrator is a simple oscillating circuit. It can generate its own rectangular pulse continuously and periodically without external excitation signal. Pulse is composed of fundamental wave and multiple harmonics, so it is called multivibrator circuit. The multivibrator can be composed of triode, 555 or general gate circuit. A multivibrator composed of two triodes is usually called a triode unstable multivibrator.

principle of operation

Positive feedback: at the moment when BG 1 is saturated, VC 1 suddenly changes from +EC to near zero, forcing the base potential VB2 of BG2 to drop to near -EC instantly, so BG2 is reliably turned off.

2. The first transient:

C 1 discharge:

C2 charging:

3. Flip: When VB2 rises to +0.5V with the discharge of C 1, BG2 starts to conduct, the positive feedback of BG 1 stops, and BG2 is saturated.

Positive feedback:

4. The second transient:

C2 discharge:

C 1 charging:

5. Repeat the cycle to form self-excited oscillation.

6. Oscillation period: t = t1+t2 = 0.7 (rb2 * c1+rb1* c2) =1.4 rb * c.

7. oscillation frequency: f =1/t = 0.7/Rb * C.

8. Improvement of waveform: The correction diode circuit can be used as the monostable circuit.