Astable Multivibrator

Astable multivibrator is an assembly which has two quasi-stable states in which it goes through during the transitional mode. The simple circuit of astable multivibrator is shown on Picture 1. It contains two amplifiers connected in common emitter connection which works as inverters. Also, a strong positive feedback is implemented from the collector of the one transistor to the base of the other, and vice versa. As it can be expected for the system with positive feedback, the assembly has regenerative properties if the circularly amplification is higher than 1, which can be achieved if the values for the base and collector resistors are selected to satisfy: R2/R1 < hfe - parameter of the selected transistor. The frequency of oscillation will be determined with the base resistors R2, R3 and with capacitors C1, C2. Because the transistors alternately goes from mode of saturation into blocking mode, the wave form of the voltage on their collectors it's basically rectangular, so the outputs V1 and V2 are complimentary, or phase-shifted for 180 degrees.  

Picture 1: Astable Multivibrator


At the astable multivibrators where the base current of the transistors is far more higher than the current needed for their saturation, it may occur the state where the assembly is blocked if both transistors are in saturation at same time. In order to prevent this kind of blocking of the assembly, the saturation factor of the transistors should be decreased by increasing the values of resistance of the base resistors R2 and R3, or by decreasing the values of collector resistors R1 and R4. As can be seen from Picture 1, the values of R1 = R4 = 1K, and R2 = R3 = 28K ohms.

Time-domain analysis

The results of the transient analysis for this circuit in time domain is shown on Picture 2. On same picture are plotted both wave forms of the outputs V1 (green line plot) and V2 (blue line plot). The high level of the outputs is about 12 V, while the low level is near to zero or about 54 mV, which is actually the saturation voltage Vce(sat) of the transistors BC547C for this circuit configuration, the models used in this simulation. The duration of high/low levels is about 0.5 ms, and this value can be altered with different values for the capacitors C1 and C2, and the base resistors R2 and R3.

Picture 2: Transient analysis - output voltage V1 and V2 wave forms (time-domain)