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AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2


Free Running (Astable) Multivibrators


When the voltage at V1 grid rises above cut-off, V1 anode voltage falls by an amount depend-ing upon the gain of the stage...

... This fall is transferred through C2 to V2 grid and, since C2 cannot change its charge instantly, V2 grid voltage falls from zero to a negative value causing V2 anode voltage to rise...

... This rise is transferred through C1 to V1 grid causing V1 grid to rise further above cut-off and V1 anode voltage to fall even more...

... This further fall in V1 anode voltage is again transferred through C2 to V2 grid and the action is cumulative.

The result is that the voltages at V1 grid and V2 anode rise very rapidly and those at V2 grid and V1 anode fall very rapidly, giving the avalanche effect. Because of the rapidity of the action and the cumulative amplification that takes place, the time for such an avalanche is extremely small - a fraction of a microsecond - and steep-sided waveforms are produced. At the end of the avalanche the roles of the two stages are reversed, i.e. V1 is now conducting and V2 is cut off (see Fig 7b).

During the relaxation period which follows the avalanche two things happen simultaneously:

a. With V2 cut off, V2 anode voltage tries to rise to h.t. + but it can only do so exponentially as C1 charges through RL2 (see Fig 8a). This gives the curve in V2 anode waveform.

b. V1 is conducting and its anode voltage is now steady at its working value. C2 was initially charged to the h.t. voltage and now starts to discharge through R2 (Fig 8b). V2 grid voltage therefore rises exponentially from its negative value towards earth (zero volts) with a time constant of C2R2 seconds. When V2 grid voltage rises above cut-off, V2 is then in exactly the same state as was V1 at the beginning of the action. Another avalanche then takes place and the action is repeated continuously.


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