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AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2


Free Running (Astable) Multivibrators

In some circuits the sync pulse is applied to the anode instead of directly to the grid. This has the same effect, because the pulse is transferred from the anode to the appropriate grid through the coupling capacitor, which cannot change its charge instantaneously.

Electron-coupled Multivibrator

The output waveforms from a triode multivibrator have curved rising edges, due to the charging of the coupling capacitors, and have 'tails' on the falling edges, also due to charging of the coupling capacitors. The waveforms can be made more square by using pentodes arranged as an electron-coupled oscillator (Fig 16a). In this circuit the screen grids are acting as the 'anodes' of the oscillator and the waveforms at the grids and screens are the same as those that appear at the grids and anodes of the triode multivibrator (see Fig 16b).

As the grid voltage of a valve falls below cut-off, the screen and anode voltages of that valve both rise towards h.t. +. However, there are no coupling capacitors connected to the anodes so that the anode voltages can rise almost immediately, whereas the screen voltages can only rise exponentially as their coupling capacitors charge. The outputs from the anodes are therefore much more square.

In addition, if the anode loads RL1 and RL2 are large enough to cause bottoming before Vg rises to zero volts, the positive pips on the grid waveforms are not reproduced as tails in the output waveforms.

The other advantage of the electron-coupled multivibrator is that the output is taken from an electrode which is not concerned in the multivibrator action and the load does not interfere with the operation of the circuit. Thus, variation in the load conditions has little effect on the multivibrator frequency, and stability is greatly improved.

Cathode-coupled Multivibrator

The circuit and waveforms of a cathode-coupled multivibrator are illustrated in Fig 17. In this circuit V2 is connected to V1, not by the usual RC coupling from anode to grid, but by feedback through a common cathode resistor RK, as in the long-tailed pair discussed in Part 1 of these notes. V1 is a low-current valve, and V2 passes a high current.


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