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AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2


Free Running (Astable) Multivibrators

The series of events may be summed up with reference to the waveforms of Fig 9:

Instant A. V1g rises above cut-off and V1a falls...

...V2g falls (through C2) and V2a rises

... V1g rises further (via C1) and an avalanche results. V1a falls to its working value and V2g falls by the same amount. V1g rises to just above zero, where it is held by grid current limiting, and V2a rises by the same amount.

Interval A to B. In the relaxation period C2 discharges through R2 and V2g rises exponentially towards zero; C1 charges through RL2 and V2a, rises exponentially to h.t. +. V1g is held at zero volts by grid current limiting and V1a is steady at its working value.

Instant B. V2g rises to cut-off and the action is the same as that described for instant A with the roles of the valves reversed. Thereafter the action is repeated.

From each anode we have anti-phase 'square wave' voltages. The shapes of these wave-forms may be improved by applying the outputs to limiting and clamping circuits. The 'pips' in the Vg waveforms, which are reflected also in the Va waveforms, are caused by grid current limiting which 'pulls' the grids back to zero volts. They may be reduced by inserting grid stoppers in each stage.

Control of Multivibrator Frequency

The time period T1 in Fig 9, during which V2 is cut off, corresponds to the time taken for C2 to discharge through R2 to the cut-off value and is determined by the time constant C2R2 seconds. Similarly, T2 is determined by C1R1.

If both stages are using identical components then C1R1 = C2R2 and T1 = T2. The waveform of voltage at either anode is then symmetrical. If T1 = T2 = 1000 uS, the time taken for one complete cycle is 2000 uS and the frequency of the output is 1/2000 uS or 500 Hz. Thus the frequency at which a free-running astable multivibrator oscillates is determined by the time constants C1R1 and C2R2 and depends upon the relationship:

    1/(C1R1 + C2R2)

To change the frequency, any of the four components may be altered. However, if we wish to change the frequency without affecting the symmetry we must change both time constants by the same amount. This may be done by the frequency control Rf shown in Fig 10.

Control of Output Symmetry

If one valve in a multivibrator remains cut off for a longer period than the other, T1 no longer equals T2 and the output becomes asymmetrical. An asymmetrical output can be obtained fairly easily by making the time constant C1R1 different to that of C 2R2. However, in practice we often need to be able to vary the symmetry without affecting the frequency.


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