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AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2

CHAPTER 7

Monostable and Bistable Multivibrators

 

This represents one stable state in which V1 is cut off and V2 is conducting. This state can be held indefinitely because there are no capacitors in the circuit to discharge and lift Vg1 to the cut-off point, ie there is no true 'relaxation period' in this circuit. To switch over to the other stable state we can apply either a positive pulse to V1 grid to cut on V1 or a negative pulse to V2 grid to cut off V2. Of the two methods the application of a negative pulse to the conducting valve is more common because a larger triggering voltage is usually required to cut on a non-conducting valve.

If we apply a negative trigger pulse to the grid of the conducting valve V2, its anode current falls and Va2 rises. This causes Vg1 to rise and V1 conducts. An avalanche thus occurs to switch the circuit over to its other stable state in which V1 is conducting and V2 is cut off. Again it will remain in this state until triggered out of it. This is done by applying another negative trigger pulse, to the grid of V1 this time. The waveforms for this circuit are shown in Fig 7. Note that the p.r.f. and the pulse duration are both determined by the trigger pulses.

 

 

 

 

 

 

 

An Eccles-Jordan circuit which avoids the need for a separate bias supply is shown in Fig 8. In this circuit, either V1 or V2 is conducting during the stable states and the resulting current through RK provides the appropriate bias voltage. The capacitor CK ensures that the d.c. bias is not affected when switching from one stable state to the other.

The circuit and waveforms of an Eccles-Jordan circuit using transistors are shown in Fig 9. The action of this circuit is virtually the same as that just described for the valve version and may be deduced from the remarks of the previous paragraphs.

Note that the pips on the output waveforms are the result of grid current or base current limiting at the input. If the valve or transistor is worked under bottoming conditions the pips at the grid or the base will not be reflected in the output waveforms.


 

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