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AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2


Monostable and Bistable Multivibrators

Instant A. When a positive trigger is applied to V1 a point is reached (the 'trigger-on' level) where the cathode bias due to RK is overcome and V1 conducts. V1a therefore falls, V2g falls and, because the fall in V2g has been amplified by V1, the decrease in current through RK due to V2 is greater than the increase due to V1, so that VK also falls. The fall in VK increases V1 conduction, initiating an avalanche which rapidly switches the circuit over to its other stable state in which V1 is on and V2 off.

Interval A to B. The circuit remains in this stable state in which V2a is at h.t. + and V1a is at a low working value.

Instant B. The trailing edge of the trigger pulse reduces V1g, causing V1a to rise. V2g therefore rises and when the trailing edge of the trigger pulse has fallen to the 'trigger-off' level, V2g rises above cut-off and V2 conducts. Since the rise in V2g has been amplified by V1, V2 anode current increases by more than V1 anode current decreases, so that VK also rises. The rise in VK reduces V1 conduction still further and a second avalanche occurs which rapidly switches the circuit back to its original stable state in which V1 is off and V2 on.

Interval B to C. The circuit remains in this stable state until another trigger pulse applied to V1 raises V1g sufficiently to overcome the cathode bias VK (at instant C).

With the arrangement shown in Fig 16 the circuit is being triggered rapidly from one stable state to the other by small positive-going trigger voltages applied to V1 grid. (Larger negative-going triggers to V2 grid produce similar results). At each avalanche an amplified trigger voltage appears at both anodes (in anti-phase with each other) and by making RK variable the input voltage necessary to start an avalanche can be made very small, e.g. 0.1v. Like the Eccles-Jordan circuit, a speed-up capacitor may be shunted across R2. Since, in this application, the leading and trailing edges of the output pulse are locked to the corresponding edges of the trigger pulse, we are effectively converting a weak input pulse into a larger amplitude output pulse of the same p.r.f. and pulse duration.

A transistor version of the Schmitt trigger is illustrated in Fig 17. The action is somewhat similar to that just described for the valve version, but for p-n-p transistors, polarities are reversed.

The resistor values are so chosen that when no voltage is applied to the input, TR1 is off and TR2 on. If a voltage more negative than Vu (the 'trigger-on' level) is applied to the input, TR1 cuts on and TR2 cuts off. So long as the input is then held more negative than VL (the 'trigger-off' level) TR1 remains on and TR2 off.

When the input falls below VL towards zero, TR1 cuts off again and TR2 conducts. Thus the state in which the circuit is operating depends upon whether the input is above Vu or below VL.

In the application shown in Fig 17, the Schmitt trigger is being used as a squarer. By adjusting the mean input level (zero in Fig 17) we can produce a symmetrical square wave output from a sine wave input. The Schmitt trigger also has many other useful applications.


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Constructed by Dick Barrett

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ęCopyright 2000 - 2002 Dick Barrett

The right of Dick Barrett to be identified as author of this work has been asserted by him in accordance with the Copyright, Designs and Patents Act 1988.