Monostable Blocking oscillator consists of a transistor with an Emitter Resistor Re and a 3 winding pulse Transformer. One winding is in the collector circuit, the second winding with n times as many turns is in the base circuit, the third winding, with n1 times as many turns as the collector winding, feeds a resistor RL, Which may be the load or may be required for damping. The base and collector turns must be connected for regenerative feedback, but the relative winding polarity of the third leg of the transformer is arbitrary. It may be chosen to provide either a positive or negative output pulse across the load. The Emitter resistor Re controls the pulse duration.


monostable blocking oscillator circuit diagram

Working of Monostable Blocking Oscillator

In the quiescent state, the transistor is OFF. Vbb is of the order of a few tenths of a volt. Even though it can be Zero, it is chosen as a fraction of volt in order to avoid false triggering due to noise spikes. When a triggering signal is momentarily applied at the collector to lower its potential, by transformer action and with the winding polarity the base potential rises.

When the base to emitter voltage exceeds the cut in voltage, the transistor comes out of cutoff. This makes the collector current to flow, further causing decrease in collector potential, Which in turn causes increase in base potential. If the loop gain is more than unity, regeneration takes place and the transistor is quickly driven to saturation. During the saturation period, when the collector voltage is constant, the collector current continues to increase at a constant rate due to transformer inductance. The emitter current is virtually constant and is determined by the transformer feedback and the emitter circuit resistor Re. Thus the base current continues to fall at a constant rate. When the base current is no longer sufficient, the transistor comes out of saturation and regeneration cuts off the transistor. The pulse duration depends upon Re.

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