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Circuits Peak Clipper by Diode :

 

4. - INDIVIDUAL CIRCUITS

Let's end this lesson with a few simple circuits, able to provide an approximately square voltage from a sinusoidal voltage or to modify the reference potential of a rectangular signal, without changing its characteristics.

These circuits are called scouters in the first case and restorers in the second case.

4. 1. - STRIPPING CIRCUITS

These circuits scouters (also called Limiters) are intended to change the amplitude of a voltage or more precisely to remove a part.

They allow for example to obtain a square voltage from a sinusoidal wave.

a) DIODE STRETCHING

Clippers using diodes can be classified into three categories :

negative clippers

positive clippers

the mixed clippers.

1°) NEGATIVE SCRATTERS

The diagram of a negative clipper is given in Figure 1-a.

Ecreteur_negatif.GIF

This assembly is a simple alternator rectifier. During the positive half-cycle of the input signal, the diode D is conductive.

The current flowing through the resistor R produces a voltage drop therein, having the form of the positive half-cycle of the input signal.

During the negative half cycle, diode D polarized in the opposite direction of its conduction, has a very high internal resistance.

The negative current flowing through the resistor R is therefore very small and consequently the voltage drop across its terminals is negligible.

Figure 1-b shows the shapes of the voltages at the input and output of the assembly. The signal obtained at the output is not comparable to that delivered by a multivibrator, but in some cases it can be likened to a rectangular signal.

The same result can be obtained by mounting the diode D in parallel on the resistor R (Figure 2).

During the positive half-cycle of the input signal Ve, the diode D is off. All the current goes through the resistance R.

Ecreteur_negatif_1.GIF

During the negative half cycle, the diode D is conductive, its internal resistance being very low, almost all of the current passes through the diode.

If the resistance R is very large in front of the internal resistance of the diode D, we can consider that the current through R during the negative half-wave of Ve is practically non-existent.

These two montages totally eliminate the negative alternation of the input signal.

Sometimes it is useful to delete only part of the negative half cycle. For this, use is made of the circuit shown in Figure 3, which is a negatively polarized negative diode negative clipper.

Ecreteur_negatif_polarise_negativement.GIF

The diode D has its anode brought to a negative potential - E, by a battery or a source of DC voltage.

As a result, the diode can conduct only when its cathode is at a higher negative potential than that of its anode.

During the positive half cycle, the diode is blocked and all the voltage is present across the resistor R.

From t1 to t2 (Figure 3-b), the diode has its cathode less negative than the anode and it does not conduct.

The AC voltage is present at the output.

From t2 to t3, the diode is busy. The alternating current flows to ground through D and the voltage source - E.

At the output, we obtain the continuous potential - E.

From t3 to t4, the diode is again blocked and the alternating voltage appears across the resistor R.

HAUT DE PAGE 2) POSITIVE CLIPPERS

These clippers are based on the same principle as that of the negative clippers and we will not go back over their operation.

To convert a negative clipper into a positive clipper, simply reverse the diode's connection direction.

Figure 4 shows a positive series diode clipper, as well as the voltage forms at the input and output of the assembly.

Ecreteur_positif.GIF

The diode D passes the negative alternations, but opposes a very high resistance to the positive alternations.

The assembly of Figure 5 carries out the same operation, but a diode is used in parallel on the resistor R.

Ecreteur_positif_1.GIF

Finally, the assembly of Figure 6 makes it possible to clip only a portion of the positive half cycle.

Ecreteur_positif_polarise_positivement.GIF

For this, the cathode of the diode D is positively biased by a direct voltage source + E.

HAUT DE PAGE 3) MIXTE CLIPPER

This assembly, shown in Figure 7 is the most interesting. It consists of a positively polarized positive clipper and a negatively polarized clipper, each of the diodes conducting alternately, one during a fraction of the negative half cycle, the other during a fraction of the positive half cycle.

Ecreteur_mixte.GIF

From t1 to t3, the two diodes are blocked and the input voltage is retransmitted integrally at the output.

From t2 to t3, the diode D2 is conductive, the potential + E appears across the resistor R.

From t3 to t5, the two diodes are again blocked and the voltage Vs takes the form of the input sinusoid.

From t5 to t6, the diode D1 is conductive and the potential - E appears at the output.

From t6 to t7, the two diodes are blocked and the output voltage goes to the value - E at the value 0, following a portion of sinusoid. At the output, a substantially rectangular voltage is obtained.

To obtain steep (almost vertical) edges, it is necessary that the input sinusoidal voltage, has a magnitude of very high value with respect to the + E and - E polarization voltages.

We end our explanations on the negative, positive, mixed limiters, and we will take another lesson on rectifiers, filtering, doublers, triplers and quadruplers.

 

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