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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

Note however that the waveforms of Fig 18b and c are exactly equivalent. They are merely different ways of looking at the same thing. Throughout these notes we shall draw p-n-p transistor circuit waveforms 'negative up'. If for any reason 'positive up' versions are required, the waveforms are simply turned upside down.

The circuit action of a collector-coupled multivibrator is basically the same as that of the free-running astable multivibrator using valves. We shall assume that TR2 is conducting and TR1 cut off but with its base voltage just about to reach cut-off (zero volts). Then initially VC2 is at its low working voltage, VB2 is limited by base current to zero volts, VC1 is at the negative supply voltage (- 12V) and VB1 is approaching cut-off (using Fig 18b waveforms).

Instant A. VB1 reaches cut-off...

... TR1 conducts and VC1 falls from - 12V towards its low working value...

... This fall is transferred through C2 to TR2 base causing VB2 to fall from zero...

...The fall in VB2 causes VC2 to rise towards - 12V...

... The rise in VC2 is transferred through C1 to TR1 base causing VB1 to rise further above zero and the action is cumulative.

An avalanche results which cuts off TR2 and leaves TR1 conducting. VC1 falls rapidly to its working value, VB1 rises above zero volts, VC2 rises by the same amount and VB2 falls to a positive value well beyond cut-off.


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