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AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2


Ringing and Blocking Oscillators

The circuit of Fig 14 contains a pulse transformer with three windings, the output being taken from the tertiary. This is often used when we require large-amplitude positive-going output pulses. The overswing diode in this case is connected to limit the negative overshoot caused by ringing.

Transistor Blocking Oscillator

The circuit of a transistor blocking oscillator consists essentially of a transistor with trans-former-coupled positive feedback from the collector to either the base or the emitter, and with a RC network in either the emitter or the base circuit. The four possible circuit arrangements are shown in Fig 15. The action of these four circuits is essentially the same and is very similar to the action of the valve blocking oscillator described earlier.

Fig 16 shows a practical triggered blocking oscillator based on the arrangement given in Fig 15d. A grounded-base circuit is used, with both the feedback winding L2 and the timing circuit C2R2 in the emitter circuit. Bias is applied to the emitter via R1, the values of R1 and R2 being such that the emitter is sufficiently negative with respect to its base to keep TR1 cut off. To cut the transistor on, a positive-going trigger pulse is applied to TR1 emitter via the coupling capacitor C1 and the resistor R3 (inserted for matching). A diode and a damping resistor R4 are connected across the transformer primary L1 to prevent excessive ringing at the collector.


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