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AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2

CHAPTER 4

Limiting Circuits

The waveform may be limited to any positive or negative reference level by holding the appropriate electrode of the diode at the required value of bias voltage. Whether the waveform is positively limited or negatively limited to this reference value depends upon how the diode is connected.

Fig 7 shows examples of biased diode parallel limiters. Series limiters may be shown in a similar manner.

In Fig 7a, with a positive reference bias applied to the cathode, the diode is cut off until the input voltage rises above the bias level. When the diode is cut off, practically the whole of the input voltage is developed across it and appears as Vout. When the diode is conducting the voltage developed across it is practically zero and Vout is then limited to the positive bias level E. We therefore have positive limiting above the bias level. The waveforms of the other circuits in Fig 7 may be deduced in a similar manner.

Although bias batteries have been shown in each of the previous examples, in practice the reference bias voltage is usually obtained from a more convenient source, e.g. from a potential divider connected across a supply network. In addition, although all the inputs so far have been shown as sine waves any waveform may be used.

A typical circuit, which gives negative limiting to a positive bias level, is shown in Fig 8. This circuit is using a series negative limiter with the resistor connected to a reference bias voltage of +30V. Because of this, the diode will conduct only when the input voltage rises above +30V. If a square wave input of 50V amplitude is applied to a short CR differentiating circuit the input waveform to the limiter is a series of positive- and negative-going pips each of 50V amplitude as shown in Fig 8. The limiter diode conducts only on the peaks of the positive-going pips when the input voltage is above the reference bias level. Thus the output from the limiter is a series of very narrow pulses of 20V amplitude, positive-going from + 30V. These pulses may be used as timing or triggering pulses.

Combined Positive and Negative Limiting

Combined positive and negative limiting may be obtained from one circuit using two diodes arranged as shown in Fig 9. Each diode conducts in turn so that both half-cycles of the input are limited.


 

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