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One form of astable or free-running blocking oscillators, is obtained by adding R1 C1 network in the Emitter circuit of a monostable blocking oscillator.

CIRCUIT DIAGRAM OF ASTABLE BLOCKING OSCILLATOR



astable blocking oscillator circuit




WORKING PRINCIPLE


Assume initially, that there is a voltage Vc on capacitor C1 larger than Vbb-Vg. Where Vg is the cut in base to Emitter voltage and g represent Gamma. Hence the transistor is off and the capacitor C1 discharges exponentially towards ground with a time constant R1 C1. When V1 becames equal to Vbb-Vg, the base current starts to flow and the transistor comes out of cutoff. The flow of current through collector further lowers the collector voltage and through inductive coupling of pulse Transformer increases the base voltage. This further increases the base current and hence the collector current.

This regenerative action quickly brings the transistor in saturation. The pulse is generated and during the pulse duration tp, the capacitor C1 is recharged and attains a voltage V1, which is larger than the value it had at the beginning of the pulse. The transistor comes out of saturation and the regenerative action brings it in the cutoff. The transistor now remains off for a time tp during which capacitor C1 discharges to the voltage at which transistor Q again enters its active region. At this point the cycle repeats itself.

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