Clipper circuits-nonlinear waveshaping circuit
Nonlinear waveshaping circuits are circuits which incorporate at least one nonlinear device, a circuit component in which the volt-ampere characteristic curve is not a straight line.
The most common nonlinear components are the semiconductor junction diode and the semiconductor three-element devices (example—the transistor). These components exhibit exponential volt-ampere characteristic curves.
4.1 DEFINITION OF A CLIPPER CIRCUIT
The clipper circuit is a common form of the nonlinear waveshaping circuit.
Either it restricts the amplitude of a voltage and/or current waveform to some finite value or it cuts off the positive or negative peak of a voltage and/or current waveform at some finite value.
Refer to Fig. 4-1.
Figure 4-1 Clipper Circuit
Technically, the circuit which restricts (or limits) the amplitude of a voltage and/or current waveform is called a limiter, while the one which cuts off (or clips) the positive or negative peak of a voltage and/or current waveform is called a clipper circuit.
In the latter circuit, it is only the clipped peaks which become the output voltage.
Practically, however, both types of waveshaping circuits are interchangeably called limited or clipper circuits. In this text, each will be referred to as a "clipper circuit."
The input-voltage waveform to a clipper circuit may have any voltage waveshape. That selected for illustration in Fig. 4-1 is a sine wave.
Clipper circuits are classified according to the active element they employ. The two most common types are those employing the semiconductor diode and those employing the transistor.
The diode clipper circuits are further classified according to the placement of the diode in the circuit. This placement determines whether the circuit is a series-diode, or a shunt-diode, clipper circuit.
The operation of a clipper circuit, in which a practical semiconductor junction diode is used is different from the hypothetical operation of the same circuit in which an ideal diode is assumed to be used.
4.2 IDEAL SEMICONDUCTOR DIODE
The semiconductor junction diode is the simplest circuit element which may be used as a switch. The ideal diode may be thought of as a simple ON-OFF switch. Refer to Fig. 4-2(a). The volt-ampere characteristic curve of an ideal junction diode is shown in Fig. 4-2(b).
Figure 4-2 Switch Volt-Ampere Characteristic
The switching action of the junction diode is determined by the polarity of the voltage applied across the diode. When the latter is forward biased in excess of the barrier potential, it acts as a closed switch; when reverse biased, it acts as an open switch.
4.3 SERIES-DIODE (IDEAL) CLIPPER CIRCUIT
Refer to Fig. 4-3.
Figure 4-3 Series-Diode Clipper Circuit
The schematic is identical to that of the half-wave voltage rectifier circuit. When this circuit is used in the study of pulse circuits, it is referred to as a series-diode clipper circuit.
It operates on the following principle:
When there is a current flow through the resistor, there is an output voltage across resistor R.
When the diode is forward biased, and hence acting as a closed switch, there is a current flow through the resistor.
Therefore, the positive half cycle of the input voltage forward biases the diode; hence, the applied voltage effectively appears across the resistor R. During the negative half cycle, the diode is reverse biased and acts as an open switch; hence, no current flows through resistor R and, in turn, there is no voltage drop across it.
The resultant output-voltage waveform is illustrated in Fig. 4-3.
4.4 SHUNT-DIODE (IDEAL) CLIPPER CIRCUIT
Refer to the shunt-diode clipper circuit shown in Fig. 4-4.
Figure 4-4 Shunt-Diode Clipper Circuit
In this circuit, there may be an output voltage only when the diode (acting as a switch) is not conducting.
When the diode does conduct, it acts as a short circuit to the output; hence, the output voltage then is zero. The resultant output-voltage waveform is illustrated in Fig. 4-4.
4.5 PRACTICAL SEMICONDUCTOR JUNCTION DIODE
Refer to Fig. 4-5.
Figure 4-5 Practical Junction Diode
Compare this volt-ampere characteristic curve for a practical junction diode with the ideal junction diode volt-ampere characteristic curve shown in Fig. 4-2.
The practical diode does not conduct heavily for very small values of forward bias (for example, 0.1 V). This is due to the barrier potential formed when the diode was manufactured.
If the forward bias is in excess of the barrier potential, the current will be high but not infinite. The diode, therefore, has some resistance when conducting.
This resistance is referred to
as the dc forward resistance of the diode. It is symbolized Rf and expressed in
ohms.
When the practical diode is reverse biased, a very small amount of
current flows.
The amplitude of the current is in the order of nanoamperes or, at most, microamperes for the normal silicon diode used in switching circuits.
Therefore, the practical junction diode has a very high resistance when reverse biased. This resistance is referred to as the dc reverse resistance of the diode. It is symbolized Rr and expressed in ohms.
The volt-ampere characteristic curve of the practical junction diode is expressed by Eq. 17. When the appropriate signs are used, this equation is valid for forward, as well as reverse, current flow.
Diode Eq. [17]
I = current flow through diode (A)
Ir = reverse saturation current at the reverse-bias voltage rating of the diode (A)
ε = a constant, 2.718 (base of the Napierian logarithm)
q = the charge of an electron = 1.602 X 10-19 coulomb (C)
K = Boltzmann's constant, 1.38 X 10-23 joules per degree centigrade
T = absolute temperature in degree Kelvin (K°)
V = applied voltage across the diode (V)
kT/q = 26 mV at 25°C (ideal)
In Eq. 17, the diode current varies with the applied voltage and the temperature. The other parameters in the equation are constants.
The diode current equals the reverse saturation current within the limits of the reverse voltage rating of the diode, when reverse bias is applied.
The reverse diode current varies with temperature, however.
Refer to Fig. 4-3.
Recall that this basic clipper circuit is schematically simple but in practice it is fairly complex. This complexity is the result of the resistance of the diode, large when reverse biased and small when forward biased.
Although it is the province of the engineer to design the circuit, the technician is expected to understand its operation and limitations and to determine a working value for the resistor R.
4.6 PRACTICAL DIODE CLIPPER CIRCUIT
In the practical diode clipper circuit shown in Fig. 4-3, the value of the resistor R must satisfy two completely divergent circuit conditions.
These are:
1. The amplitude of the output voltage must equal the amplitude of the input voltage when the diode is forward biased.
2. The amplitude of the output voltage must be zero when the diode is reverse biased.
Because the resistance of the diode, when the latter is forward biased, is small and the resistor R is in series with the diode, the circuit is that of a simple series voltage divider.
For the output voltage to equal the input voltage, the value of R must be substantially greater than the resistance of the diode. Hence, when the diode is forward biased, the value of R should be large.
Because the resistance of the diode, when the latter is reverse biased, is large and the resistor R is in series with the diode, the circuit is that of a simple series voltage divider.
For the output voltage to equal zero, the value of R must be substantially less than the resistance of the diode. Hence, when the diode is reverse biased, the value of R should be small.
Since neither of these conditions may be satisfied in the practical circuit, a compromise value for R must be used.
To determine this value, solve for the geometric mean of the forward resistance of the diode (symbolized Rf) and the reverse resistance of the diode (symbolized Rr). Equation 18 expresses this relationship.
R = (RfRr)˝ (˝ = Radical)
R = resistance of resistor to be used in diode clipper circuit (Ω)
Rt = diode forward resistance (Ω)
Rf = diode reverse resistance (Ω)
To apply this equation to a specific circuit problem, determine the numerical values for the forward resistance (Rf) and the reverse resistance (Rr) of a specific diode.
Refer to Fig. 4-6 which represents a straight-line approximation of the volt-ampere characteristic curve of a semiconductor junction diode.
Figure 4-6 Junction Diode Volt-Ampere Characteristic (Idealized)
The forward resistance of the diode may be approximated by the determination of the reciprocal of the slope of the forward biased portion of the characteristic curve of the diode.
The solution for Rf, stated mathematically, is Rf = Δf/ΔIf.
Assume point 2 in Fig. 4-6 to be at zero current and at zero voltage.
The values of point 1 may be ascertained from the manufacturer's specification sheet.
For 1N914 diode. These are IF = 10 mA and VF = 1 V dc.
Hence:
The reverse resistance of the diode may be approximated by determining the reciprocal of the slope of the reverse biased portion of the characteristic curve of the diode.
The solution for Rr, stated mathematically, is Rr « ΔVr/ΔIr. Assume point 3 in Fig. 4-6 to be at zero current and at zero voltage. The values for point 4 may be ascertained from the specification sheet.
These are Ir = 5 µA and VR = 75 V.
Hence:
Having thus established the numerical values for Rf and RT) for a specific diode, the value of the resistance R may be determined by the use of Eq. 18.
R = 39 kΩ - closest standard value
The determination of R by the use of this method is approximate, but practical for the technician.
The engineer more accurately determines the value of R by the use of the diode equation (Eq. 17). To be cognizant of the complexities of the circuit, the technician should be familiar with the diode equation.
4.7 BIASED SHUNT-DIODE CLIPPER
Refer to Fig. 4-7(a), a schematic of a battery-biased shunt-diode clipper.
Figure 4-7 Biased Shunt Clipper
In this circuit, when the diode D is forward biased (acting as a closed switch), the output voltage equals the battery voltage (+E V).
When diode D is reverse biased (acting as an open switch), the output voltage equals the input voltage. If no input voltage is applied and the signal source has zero resistance, the battery voltage E reverse biases the diode.
Hence, the output voltage equals the input voltage, that is, zero volts.
During the negative half cycle of the input, the signal voltage is in series aiding with respect to the battery voltage E. The battery voltage E is of a polarity to reverse bias diode D.
Hence, any increase in amplitude of the reverse-bias voltage will sustain the open switch condition of the diode. Therefore, the output voltage will equal the input voltage during the negative half cycle.
During the positive half cycle of the input, the signal voltage is in series-opposing with respect to the battery voltage E.
The bias applied across the diode equals the difference between the signal voltage and the battery voltage and has the polarity of the larger voltage.
Therefore, the output equals the input when the input is equal to or less than the battery voltage E.
When the input is greater than the battery voltage, the diode is forward biased (acting as a closed switch); hence, the output voltage equals the battery voltage E. The resultant output voltage is shown in Fig. 4-7 (c).
A zener diode may be used to replace the battery E in the shunt-diode clipper circuit in Fig. 4-7(a). The resultant zener diode circuit is shown in Fig. 4-7(b). Which has a constant voltage drop across it.
The output voltage is constant, and the amplitude equals the zener voltage. Figure 4-7 (c) illustrates this resultant output-voltage waveform. It has been assumed in this example that the battery voltage equals the zener voltage.
In general, to ensure sharp clipping action in a clipper circuit, the non-linearity of the diode must be overcome. To accomplish this, the amplitude of the input voltage must greatly exceed the barrier potential of the diode.
The technician should be aware of the disadvantages of the diode clipper circuits.
When the diode in a series clipper circuit is reverse biased or OFF, the junction capacitance of the diode causes it to function as a coupling capacitor which transmits high-frequency voltage signals when the diode is OFF.
The shunt-diode clipper circuit also has a disadvantage.
When the diode is OFF, in a shunt-diode clipper circuit, the junction capacitance of the diode causes it to function as a low-impedance path to the high-frequency components. Hence, the corners of the output-voltage waveform are rounded.
Some functions of the clipper circuit are the removal of unwanted voltage spikes ; the squaring of deteriorated square waves ; and the comparison of two voltages, in which capacity the circuit is called a voltage comparer or a comparer circuit.
LABORATORY EXPERIMENT
CLIPPER CIRCUITS NONLINEAR WAVESHAPING CIRCUITS
OBJECT:
1. To analyze series and shunt clipper circuits and to illustrate how the practical circuitry verifies the theory
2. To analyze the biased-diode clipper circuit
3. To analyze the operation of the zener diode as a reference voltage
MATERIALS:
1 Sine wave generator (20 Hz to 200 kHz)
1 Oscilloscope, dc time base; frequency response dc to 450 kHz; vertical sensitivity, 100 mV/cm
2 Silicon junction diodes with manufacturer's specification sheet (example1N914 or equivalent)
2 Silicon zener diodes with manufacturer's specification sheet (example 1N4728: 3.3 V, 1W or equivalent)
2 Transistor power supplies (0 to 30 V) or equivalent batteries
1 Resistor substitution box (10 Ω to 10 MΩ, 1 W)
PROCEDURE:
Figure 4-1X
1. Determine the value of the resistor in the clipper circuits of Fig. 4-1X.
(a) Determine the value of Rr and Rf from the manufacturer's specification sheet.
(b) Substitute the numerical values obtained in Step a for Rr and Rf in Eq. 18, in order to determine the value of R.
2. Connect circuit 1 in Fig. 4-1X. For use in this circuit, select a standard color-coded resistor, the value of which is closest to that determined in Step 1.
3. Apply a sine wave input of 4 V rms at a frequency of 1000 Hz.
4. Draw a schematic of the circuit on graph paper.
5. Measure the input-voltage and output-voltage waveform displayed on the dc oscilloscope. On graph paper, draw the input-voltage waveform and, below it, to the same convenient time base, draw the output-voltage waveform as it would appear on a dc oscilloscope. Label all pertinent voltages.
6. In detail, explain how the theory justifies the function of the circuit. What factors determined the shape of the resultant output-voltage waveform?
7. For circuits 2 through 13, repeat Steps 3 through 6.
QUESTIONS AND EXERCISES
1. Draw the schematic of a series clipper circuit which will produce the same resultant output voltage as that produced by circuit 5 in Fig. 4-lX.
2. Repeat Problem 1 for circuit 6.
3. Repeat Problem 1 for circuit 7.
4. Repeat Problem 1 for circuit 8.
5. How does the output voltage of circuit 8 compare to the output voltage of circuit 9? Explain.
6. Explain why the value of the forward resistance of a junction diode is not constant.
7. The solution for the proper value of the resistance to be used in a clipper circuit presents a problem. What is it, and how is it solved?
8. What is the principal disadvantage of a shunt-diode clipper circuit?
9. In what respect is a zener diode similar to a battery?
10. Under what circuit condition will a zener diode perform as a battery?
11. Determine the proper value of resistor R to be used in a clipper circuit employing a 1N916B silicon diode.
