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AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2


Other Square Wave Generators


Although the majority of pulse generators are based on one of the family of multivibrators discussed in the preceding chapters there are occasions when other circuits capable of producing a rectangular wave output are preferred. Many of these circuits use only one stage and are used mainly for special applications. We shall consider some of these circuits in this chapter.

Complementary Regenerative Pair Circuit

This circuit uses transistors of opposite polarity-one p-n-p and the other n-p-n - and has no valve equivalent. One version of the circuit is illustrated in Fig 1. It is, in effect, a monostable or flip-flop circuit. In this arrangement TR1 is a p-n-p transistor and TR2 a n-p-n type. TR1 therefore requires a negative voltage to its base to cut it on, and TR2 base must be positive with respect to its emitter before it conducts. In the quiescent condition both transistors are off. Before the application of a trigger pulse, with no current to its base, TR1 is cut off and its collector voltage is at -Vc. There is therefore no drive voltage to the base of TR2 which is also cut off. Under these conditions C2 is discharged and the output is at earth. This is the stable state.

If we apply a negative trigger pulse to TR1 base, TR1 cuts on and its collector voltage falls towards earth because of the voltage drop across RL1 (Note that we are using the 'negative up' convention in this circuit). TR2 base is therefore driven down and becomes positive with respect to its emitter so that TR2 also cuts on. TR2 collector voltage then rises towards -Vc because of the voltage drop across RL2, and the output also rises. This rise is applied through C2 (which cannot charge instantaneously) to the base of TR1 where it drives TR1 hard on. An avalanche therefore occurs which 'flips' the circuit into the state where both transistors are conducting (and saturated). This is the unstable state in which TR1 collector voltage is steady at just above zero volts and TR2 collector voltage is steady at just below -Vc. The duration of the unstable state depends upon the time taken for TR2 to come out of saturation.



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