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AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2


Limiting Circuits

This means that if we vary Vgk positively above Vgk = - 2V, there is no change in Ia and so Va remains at 50V. This condition where a change of Vgk does not cause a change in either Ia or Va is known as anode bottoming. It results from the overlap of the curves below the knee of the characteristic at large values of anode load. The effect on the Ia-Vgk curve is shown in Fig 12b. In this example, anode bottoming occurs at Vgk = - 2V. However by increasing the value of anode load resistor RL, bottoming at even greater negative values of Vgk can be obtained.

A pentode amplifier may be used as a squarer to produce square waves from a sine wave input by limiting both the positive and negative half-cycles of the input. The positive half-cycle is limited by bottoming and the negative half-cycle by cut-off (Fig 13). Although the grid stopper in a pentode squarer does not usually have any effect on the output waveform it is still retained to limit the positive swings of the Vgk waveform. By limiting the grid current the input impedance of the amplifier is kept at a high value.

The output from a pentode squarer is a better square wave than that from a triode squarer. The top and bottom of the waveform are perfectly flat. In addition, a pentode with a short grid base can be selected and this, coupled with the high gain of a pentode, gives a steep-sided output waveform. It can be made more steep if a very large amplitude input sine wave is used so that the 'effective' portion of the input has almost vertical edges. For this reason inputs of 100V peak are common.

Symmetry of Output from Squarers

The symmetry of the square wave output from triode and pentode squarers depends upon the operating point selected for a given valve. This may be adjusted by varying the bias.


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