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AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2


Limiting Circuits

By making the reference bias voltages applied to the diodes equal, an approximate square wave output is obtained from a sine wave input. A double diode limiter used for this purpose is sometimes called a diode squarer. The output amplitude is of course smaller than that of the input.

Limiting in a Triode Amplifier

There are two ways in which a triode amplifier may be used as a limiting circuit:

    a. The input waveform may be made large enough to drive the valve beyond cut-off at the peak of the negative half-cycles. The anode current Ia is then zero and the anode voltage Va remains at h.t. + whilst the valve is cut off (Fig l0a). Grid cut-off limiting gives negative limiting below cut-off at the grid of the valve.

    b. A resistor of high value may be inserted in the grid-cathode path of the valve. The grid and cathode act as a diode, with the triode grid functioning as the diode 'anode'. Thus the grid-cathode circuit with its large series resistor (known as a grid stopper) forms an effective diode parallel limiter (Fig l0b). If the grid is driven positive by the input signal, grid current flows and the grid-cathode path of the valve has a very low resistance, of the same order as that of the conducting diode in a diode limiter. Thus most of the input is developed across the grid stopper and the grid-cathode voltage Vgk is practically zero. Conversely when the grid goes negative the effective grid-cathode resistance is very much higher than that of the grid stopper and most of the input voltage appears as Vgk. This action is known as grid current limiting and gives positive limiting above zero volts at the grid of the valve.

Both processes may be used together to give combined negative (grid cut-off) and positive (grid current) limiting. Such an arrangement may be used to convert a sine wave input to an approximate square wave output. When used in this manner a triode limiter is referred to as a squarer. A triode squarer produces a better square wave output than that from a diode squarer because of the amplifying properties of the triode.


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