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AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2


Monostable and Bistable Multivibrators


The astable multivibrator considered in the previous chapter is a circuit which has no stable state, ie it is a free-running relaxation oscillator. Although it may be synchronized by applying sync pulses, it is capable of free-running with no input. The monostable and bistable multi-vibrators on the other hand both require a triggering input pulse to make them operate. We shall see how this is done in this chapter. We shall consider the monostable circuit first and then go on to the bistable circuit.

Monostable Multivibrator (Flip-flop)

Monostable multivibrators are circuits with one stable state. They remain in the stable state until triggered, when they then 'flip' over to the other state. They remain in the unstable state for a time decided by the circuit constants and then, of their own accord, 'flop' back to the original stable state. Flip-flops find many applications in radar. They may be used to generate rectangular pulses 'locked' to precise time intervals, to reshape pulse trains which have deterior-ated in shape, to stretch narrow pulses into wider ones or to generate a time delay. Fig 1 illus-trates how a time delay may be produced. The flip-flop produces a rectangular wave whose leading edge is coincident in time with the trigger pulse and whose trailing edge may be varied with time. The output from the flip-flop may be differentiated and then negatively limited to give a series of pulses which have a controlled variable time delay in relation to the original trigger pulses. There are a number of variations of the monostable multivibrator and in the following paragraphs we shall consider some of these basic circuits.

Anode-coupled Flip-flop

The circuit of a basic anode-coupled flip-flop and its associated waveforms are illustrated in Fig 2. The circuit is similar to that of the free-running anode-coupled multivibrator discussed in p 110. The main differences are:

a. The grid of one valve (V1 in this circuit) is taken to a negative bias voltage instead of to earth or a positive aiming voltage. This bias is sufficient to hold V1 below cut-off in the absence of a trigger pulse.

b. The cross-coupling impedance between V2 anode and V1 grid is normally a resistance (R3 in this circuit). A capacitance may however be used in some circuits. 


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