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 Common Diode Applications: Clippers,... Chapter Summary

Clippers

A clipper is a circuit that is used to eliminate a portion of an input signal. There are two basic types of clippers: series clippers and shunt clippers. As shown in Figure 4-1, the series clipper contains a diode that is in series with the load. The shunt clipper contains a diode that is in parallel with the load.

FIGURE 4-1 Basic clippers.

The series clipper is a familiar circuit. The half-wave rectifier is nothing more than a series clipper. When the diode in the series clipper is conducting, the load waveform resembles the input waveform. When the diode is not conducting, the output is approximately 0 V (Figure 4.2). The direction of the diode determines the polarity of the output waveform. If the diode symbol (in the schematic diagram) points toward the source, the circuit is a positive series clipper, meaning that it clips the positive alternation of the input. If the diode symbol points toward the load, the circuit is a negative series clipper, meaning that it clips the negative alternation of the input (Figure 4.11).

Ideally, a series clipper has an output of when the diode is conducting (ignoring the voltage across the diode). When the diode is not conducting, the input voltage is dropped across the diode, and .

Unlike a series clipper, a shunt clipper provides an output when the diode is not conducting. For example, refer to Figure 4-1. When the diode is off (not conducting), the component acts as an open. When this is the case, and form a voltage divider, and the output from the circuit is found using

When the diode in the circuit is on (conducting), it shorts out the load. In this case, the circuit ideally has an output of . Again, this relationship ignores the voltage across the diode. In practice, the output from the circuit is generally assumed to equal 0.7 V, depending upon whether the circuit is a positive shunt clipper or a negative shunt clipper. The direction of the diode determines whether the circuit is a positive or negative shunt clipper. The series current-limiting resistor () is included to prevent the conducting diode from shorting out the source.

A biased clipper is a shunt clipper that uses a dc voltage source to bias the diode. A biased clipper is shown in Figure 4-2. (Several more are shown in Figures 4.9 and 4.10). The biasing voltage () determines the voltage at which the diode begins conducting. The diode in the biased clipper turns on when the load voltage reaches a value of . In practice, the dc biasing voltage is usually set using a potentiometer and a dc supply voltage, as shown in Figure 4.10.

FIGURE 4-2 A biased clipper.

Clippers are used in a variety of systems, most commonly to perform one of two functions:

1. Altering the shape of a waveform
2. Protecting circuits from transients

The first application is apparent in the operation of half-wave rectifiers. As you know, these circuits are series clippers that change an alternating voltage into a pulsating dc waveform. A transient is an abrupt current or voltage spike of extremely short duration. Left unprotected, many circuits can be damaged by transients. Clippers can be used to protect sensitive circuits from the effects of transients, as illustrated in Figure 4.12.

Clampers (DC Restorers)

A clamper is a circuit that is designed to shift a waveform above or below a dc reference voltage without altering the shape of the waveform. This results in a change in the dc average of the waveform. Both of these statements are illustrated in Figure 4-3. (The clamper has changed the dc average of the input waveform from 0 V to +5 V without altering its shape.)

FIGURE 4-3 A clamper with its input and (ideal) output waveforms.

There are two basic types of clampers:

• A positive clamper shifts its input waveform in a positive direction, so that it lies above a dc reference voltage. For example, the positive clamper in Figure 4-3 shifts the input waveform so that it lies above 0 V (the dc reference voltage).
• A negative clamper shifts its input waveform in a negative direction, so that it lies below a dc reference voltage.

Both types of clampers, along with their input and output waveforms, are shown in Figure 4.17. The direction of the diode determines whether the circuit is a positive or negative clamper.

Clamper operation is based on the concept of switching time constants. The capacitor charges through the diode and discharges through the load. As a result, the circuit has two time constants:

• For the charge cycle, and (where is the resistance of the diode)
• For the discharge cycle, and (where is the resistance of the load)

Since is normally much greater than , the capacitor charges much more quickly than it discharges. As a result, the input waveform is shifted as illustrated in Figure 4.16.

A biased clamper allows a waveform to be shifted above (or below) a dc reference other than 0 V. Several examples of biased clampers are shown in Figure 4-4.

FIGURE 4-4 Several biased clampers.

The circuit in Figure 4-4(a) uses a dc supply voltage (V) and a potentiometer to set the potential at the cathode of . By varying the setting of , the dc reference voltage for the circuit can be varied between approximately 0 V and the value of the dc supply voltage.

The zener clamper in Figure 4-4(b) uses a zener diode to set the dc reference voltage for the circuit. The dc reference voltage for this circuit is approximately equal to . Note that zener clampers are limited to two varieties:

• Negative clampers with positive dc reference voltages
• Positive clampers with negative dc reference voltages

Voltage Multipliers

FIGURE 4-5 Voltage doublers.

A voltage multiplier provides a dc output voltage that is a multiple of the circuit’s peak input voltage. For example, a voltage doubler with a peak input of 10 V provides a dc output that is approximately 20 V. Two voltage doublers are shown in Figure 4-5.

Each of the circuits in Figure 4-5 provides a dc load voltage that is approximately twice the value of the peak source voltage. The half-wave doubler gets its name from the fact that the output capacitor () is charged during the positive half-cycle of the input signal, as shown in Figure 4.21. In contrast, the output capacitor in the full-wave doubler () is charged during both alternations of the input cycle, as shown in Figure 4.23. Note that the output from a full-wave doubler has less ripple than the output from a comparable half-wave doubler.

The voltage tripler is very similar to the half-wave voltage doubler. If you compare the tripler shown in Figure 4-6 to the circuit in Figure 4-4(a), you will see that the circuit made up of , , , and is actually a half-wave voltage doubler. This circuit charges to a value of . During the negative alternation of the input cycle, is charged to approximately . The voltage across the series combination of and is approximately . Since and the load are in parallel with the series combination of and , and are also approximately equal to .

FIGURE 4-6 A voltage tripler.
The voltage quadrupler contains two half-wave voltage doublers, as shown in Figure 4-7. The circuit made up of , , , and charges to a value of . The circuit made up of , , , and charges to a value of . The combined charge of is applied to (the filter capacitor) and the load.

FIGURE 4-7 A voltage quadrupler.

Voltage multipliers reduce source current by roughly the same factor that they increase source voltage. For example, a voltage tripler produces a dc output voltage that is approximately three times the peak source voltage. At the same time, its maximum output current is roughly one-third the value of the source current. As such, voltage multipliers are commonly used in high-voltage, low-current applications. They can also be used to produce dual-polarity output voltages in power supply applications (Figure 4.26).

LED Applications

LEDs are most commonly used as power indicators, level indicators, and as the active elements in multisegment displays.

The power indicator on any electronic component is most likely an LED. When the component is turned on, power is supplied to the LED. The LED lights, indicating that the component is on. A level indicator is used to indicate when a signal voltage reaches a designated level. Several examples of level indicators are shown in Figure 4.27.

LEDs are most commonly used in multisegment displays. These displays are used to display alphanumeric symbols, such as letters, numbers, and punctuation marks. A typical seven-segment display is shown in Figure 4.28. Several other common displays are shown in Figure 4.29.

Each type of display is available in either a common-anode or common-cathode configuration. A common-anode display has a single anode (+V) input that is applied to all the LEDs in the display. Individual segments are lighted by providing a ground path to the appropriate cathodes. In contrast, a common-cathode display has a single cathode (ground) pin that is connected to all LEDs in the display. Individual segments are lighted by providing a +V input to the appropriate anodes. Note that many multisegment displays require a current-limiting resistor in series with each LED in order to restrict device current.

Another type of multisegment display, called a liquid-crystal display (LCD), contains segments that reflect (or do not reflect) ambient light. LCDs typically require less power than LED displays and thus are better suited for use in low-power electronic systems, such as portable phones.

Diode Circuit Troubleshooting

A variety of fault symptom tables are listed in this chapter for clippers, clampers, multipliers, and displays:

• Shunt clipper faults, Table 4.1
• Clamper faults, Table 4.2
• Additional biased-clamper faults
• Additional zener-clamper faults
• Voltage multiplier faults Table 4.5

Multisegment displays are often controlled by ICs called decoder-drivers. These ICs provide the active +V (or ground) inputs required for the individual segments. The most common multisegment display fault is the failure of one or more segments to light. When this occurs, check the input to the common pin. Assuming that the potential there is correct, check the inputs from the decoder-driver. If the inputs to the display are correct, the display must be replaced. If not, the decoder-driver (and current-limiting resistor) must be tested.

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