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AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2

CHAPTER 8

Other Square Wave Generators

As C2 charges through R2, TR1 base falls and TR1 collector (and TR2 base) rise towards -Vc. When TR2 base has risen sufficiently, TR2 comes out of saturation and the circuit 'flops' back to its original stable state where both transistors remain off until the next trigger arrives.

This circuit has the advantage that in the stable state both transistors are off and the circuit therefore requires no power until it is triggered. It can also handle large load currents and can sustain long duration pulses.

Transitron

The transitron, the circuit and waveforms of which are illustrated in Fig 2, is sometimes referred to as a one-valve flip-flop. It is a monostable circuit producing a square wave output whose p.r.f. is determined by that of the trigger pulses and whose duration may be varied.

The circuit depends for its action on the fact that the total space current in a pentode is determined by the control grid voltage Vg1, but the division of this current between anode and screen is determined by the suppressor grid voltage Vg3. Thus if Vg1 remains constant:

a.      When Vg3 rises the anode current Ia rises and the screen current Is falls.

b.      When Vg3. falls, Ia falls and Is rises.

Neglecting any control grid current which may flow, the sum of Ia and Is is at all times equal to the total space current.

Before the application of a trigger pulse, Vg1 is at zero volts and the valve is conducting normally with its space current shared between anode and screen. The anode voltage Va is therefore at its low working value and the screen voltage Vg2 is below h.t. + by an amount determined by R3 and the screen current 1s. C2 is charged to the screen voltage so that the suppressor grid voltage Vg3 is zero.


 

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