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AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2


Other Square Wave Generators

With very little space current (and most of what there is going to the anode) the screen current is very small so that Vg2 rises instantly to just below h.t. +. As VK falls towards earth, the bias on the suppressor grid is reduced and more of the available space current reaches the anode. As 'Ia increases in this way, Va falls. It can fall only by a limited amount however, as explained above - in this circuit by an amount equal to the grid base of the valve plus the fall of Vk. This sudden fall of Va is the so-called 'Miller step' which we shall discuss in more detail later in these notes. In this circuit the Miller step is larger than normal because of the cathode follower action of Vk

Interval A to B. Vg1 now commences to rise as C1 discharges through R1. As Vg1 rises, the space current rises and so does Vk. The increased space current tends to cause Va to fall and this fall of Va is transferred through C1 to the grid to counteract the rise of Vg1. This effect is known as 'Miller feedback'. The feedback cannot stop the rise of Vg1 completely however because, owing to the fact that Vk is also rising, the negative bias on the suppressor grid is increasing and less of the available space current is reaching the anode. The effect of the Miller feedback is to 'slow down' the rise of Vg1, and Vg1 rises linearly. Since Vg1 is rising linearly, Va falls in a slow and linear manner. This variation of Va is termed the 'Miller rundown'. The rise of Vg1 increases the screen current and Vg2 falls slightly during the Miller rundown.

Instant B. Va reaches its bottoming voltage and with Va held at this value there is no further Miller feedback through C1 to the grid so that Vg1 now rises quickly. Owing to the cathode follower action, Vk also rises quickly, applying sufficient negative bias to the suppressor grid to reduce the anode current. Va therefore rises, this rise being fed back via C1 to the grid to cause Vg1 to rise further. The resulting increased space current increases Vk, Ia falls still more and the cumulative action quickly cuts off Ia. The total space current is thus diverted to the screen so that Vg2 falls to a low value as Va rises.

Interval B to C. The capacitor C1 now recharges quickly to its original value via RL and the grid-cathode path of the valve, and Va rises exponentially to h.t. + as C1 charges. The circuit is now in its initial stable state and will remain in this condition until the arrival of the next trigger pulse.

This circuit may also be triggered by applying a negative trigger pulse to the control grid and connecting the suppressor grid directly to earth. This reduces Vg1 so that the space current falls and so does Vk. This makes the suppressor grid less negative with respect to its cathode and anode current starts to flow. The action is then the same as from instant A.

Since the avalanche which ends the square wave and returns the circuit to its stable state occurs when the anode voltage bottoms (instant B), the pulse duration of the square wave output from the screen grid depends upon the Miller rundown time. This can be varied by modifying the basic circuit to include 'anode-catching diodes' as shown in Fig 4:

a.      The starting point of the rundown may be adjusted as in Fig 4a. The lower the setting of VR1 the lower is the initial anode voltage and the less is the rundown before the bottoming voltage is reached. Va cannot return to a higher value than that set by VR1 because as soon as it tends to do so the diode conducts to hold Va at the level set by VR1.

b.      An artificial bottoming voltage may be provided by stopping the anode voltage fall at any desired point by using a limiting diode as shown in Fig 4b. With the diode connected as shown, Va can only fall until the voltage across the diode is such that it will conduct. This level is set by VR2.


There are several forms of this circuit. A typical example with waveforms is shown in Fig 5. It produces an output similar to that of a phantastron but is more stable and reliable in operation. A likeness to the circuit of a monostable multivibrator may be noticed.


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Constructed by Dick Barrett

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ęCopyright 2000 - 2002 Dick Barrett

The right of Dick Barrett to be identified as author of this work has been asserted by him in accordance with the Copyright, Designs and Patents Act 1988.