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AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2


Limiting Circuits

When the valve is operating under Class A conditions the output is symmetrical. The effect of varying the bias either side of the Class A biasing point is illustrated in Fig 14. The output is now asym-metrical. Note however that the slope of a sine wave decreases towards the peaks. Hence the use of a large bias to produce narrow pulses results in deterioration in the steepness of the edges of the square wave output.

Cathode Follower Limiting

The cathode follower circuit is considered in some detail in Part 1 of these notes. It is an amplifier in which the output is taken across the cathode load resistor, the phase of the output being the same as that of the input. Since the cathode resistor is not decoupled, 100 per cent voltage negative feedback is applied between output and input. This has two main effects:

    a. The voltage gain of the amplifier is always less than unity, i.e. output smaller than input.
    b. The input impedance is very high and the output impedance very low. It therefore provides a means of matching.


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