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AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2


Free Running (Astable) Multivibrators

This cumulative action cuts off V1 and leaves V2 conducting heavily. The avalanche ceases when V1 is cut off by the fall of voltage at point B relative to its cathode. At this point:

  • V1a has risen by, say, 150V towards its final h.t. value of + 300V.
  • V2g1K is driven above zero volts.
  • VK has risen to about 100V.
  • V2a has fallen to its working value (say + 150V).

Interval A to B. With V1 cut off, C2 charges through RL1 and RK, causing V1a to rise exponentially towards h.t. + and V2g1K to fall back towards zero volts. As V2g1K falls, VK falls with it (due to the fall in current through RK from V2). The voltage across CLR1 is VK and during this period C1 charges through R1 causing V1g1K to rise towards cut-off (see Fig 17c).

Instant B. When the voltage across R1 is sufficiently small, V1glK reaches cut-off...

V1 conducts causing V1a and V2g1K (via C2) to fall...

... The fall in V2g1K causes V2a to rise and VK to fall (less current through RK).

... The fall in VK (at point A) leaves V1g1K (point B) positive, since Cl cannot discharge instantaneously...

... V1 thus conducts more heavily causing V1a and V2g1K to fall further.

V2 now cuts off and V1 conducts heavily. VK cannot immediately fall because of the charge on C1; similarly, V1glK is held positive; V1a falls to its working value; V2g1K falls by the same amount well below cut-off; and, with V2 cut off, V2a rises immediately to h.t. +.

Interval B to C. C1 now discharges rapidly through RK and V1 grid-cathode path. V1glK is thus returned to zero volts and VK falls exponentially to a low value. C2 discharges through R2 towards the aiming voltage and V2g1K rises exponentially towards h.t. +.

Instant C. V2g1K reaches cut-off and the action as from instant A is repeated.

A negative-going output may be taken from the anode of V2(V2a) and a positive-going output from the common cathode (VK). V 2a output has a good waveform because there is no capacitor connected to V2 anode. The output from the cathode has a low source impedance due to the cathode follower action of V2 and this may be useful for matching. Since both edges of V2a Waveform are caused by an avalanche, very steep edges result - ideal for timing.

The period T1 in Fig 17 is determined by the time constant C1R1 and varying R1 varies the pulse duration. The period T2 depends upon C2R2 and varying R2 (or the aiming voltage) varies the p.r.f. Since C1R1 can be made much smaller than C2R2 a very asymmetrical output may be obtained from this circuit.

Multivibrator using Transistors

In many radio circuits the thermionic valve can be replaced by its semiconductor equivalent, the transistor. A version of a basic multivibrator using transistors - a collector-coupled circuit - is shown in Fig 18a.

As we have done throughout these notes we shall consider only the p-n-p transistor, although the n-p-n type could be used equally well if we reversed the polarity of the supply voltages. It will be remembered that the current through a p-n-p transistor increases as the base is made more negative relative to emitter. To cut off a p-n-p transistor we make the base positive with respect to emitter. Note that a valve cuts off when Vg falls below the cut-off point at a negative value equal to the grid base; a p-n-p transistor, on the other hand, cuts off as soon as its base goes positive with respect to its emitter, i.e. its 'grid base' is zero.

Because the polarities in a p-n-p transistor circuit are opposite to those in valve circuits, the waveforms associated with a p-n-p transistor are often drawn 'negative up' as in Fig 18b. This also means that the circuit action can be tied more closely with that of a circuit using valves. In some textbooks p-n-p transistor waveforms are drawn in the more conventional 'positive up' style as in Fig 18c.


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