Multivibrator astable

The multivibrator astable provokes two stages of functioning that are replaced spontaneously. The tackles are not of electromagnetic origin, as we have seen in the oscillator, but these devices use the properties that present two trasistores where the unfreezing of one assures the blockade of other, so that turnan in these positions.

Here we have a basic multivibrator circuit astable that big watch seemed with the basic circuto of an amplifier of two stages, but that presents some special peculiarities.

First of all we observe that the exit of the transistor T2 is connected by the condenser C2 to the base of the transistor T1, for what we are opposite to a circuit of feedback which functioning is the following one: At the moment when the Switch (I) closes, the current proceeding from the device happens across the issuer to feed the base of the transistor T1, going on seguidamente to the base of T2, which to his you see he becomes an assistant and derives all his negative current across the condenser C2 towards the base of T1, which becomes more an assistant, the base re-feeds again of T2 and the step of the current increases, etc., in a permanent and rapid succession of amplifications that last until alcanzen the maximum values that the device allows. In this moment, one of the transistors, the T2, is opened, and the relief begins to be established between two transistors by virtue of the following consequence:

When the T1, for example, reaches his conduction maximum the collector's tension of this disminuy transistor, circumstance that is transmitted, logically, to the base of the transistor T2. But also, these tension changes become positive, what blocks the base of T2. This is the reason by which the said transistor is blocked, situation that is supported only a brief period of time.

The tension in the condenser C1, come the moment, is diminishing and by the base resistance R3 a negative step is prepared for feeding of the base T2 across the negative of the network, situation that materializes when the tension of C1 is below the negative tension of this point. This way when T2 it receives negative tension in the base a rapid amplification of the current takes place until the moment of the conduction comes to the maximum of T2, and then a depression originates in the tension that he returns us to the same situation of the previous case, since the transistor T2 is blocked.

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Bistable multivibrator
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