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circdiodo65-1J.jpg
Figure 65-01

    Working algebraically the equation, we get:

    I = 19.3 / 6k = 3.22   mA

    Therefore, the output voltage V0 will be:

    V0 = -10 + 1k x 3.2 mA = - 6.78   volts

    And if V0 = - 6.78 V, then Va = - 6.08 V. Therefore, this premise does not satisfy the solution of the problem either, because with Va = - 6.08 V it means that D1 is conducting.

circdiodo65-1K.jpg
Figure 65-02

    Therefore, we must consider the alternative in which D1 and D2 are conducting. If D1 is conducting, then Va = - 0.7 V. In Figure 65-02 we show the circuit with the indication of currents. Knowing Va we can calculate V0, just add the voltage drop over D2. Then:

    V0 = - 1.4   volts

    To complete the solution of the problem we will calculate the currents in the circuit.

    I1 = (10 - (-0.7)) / 5k = 2.14   mA
    I0 = (10 - 1.4) / 1k = 8.6   mA

    From the circuit, we are known that I0 = I1 + ID. So:

    ID = I0 - I1 = 6.46   mA

    Therefore, after establishing several premises it was found that the only viable one was the last one. The other two previous ones conflicted.

    This example served to illustrate one of the most commonly used techniques for solving this type of problem. On the Problems tab there are several proposed circuits with their respective solutions.


    3.   Limiting Circuits

    In many applications there is a need to limit the waveform to a certain voltage or current level, passing only the signal that occurs above or below a predetermined value. This feature is used in applications that include the limitation of excessive amplitudes, formation of certain types of waveforms and also in the control of the power delivered to a charge.

    The diodes can be combined with resistors to perform the function of a limiter. Circuit transfer characteristics are obtained using the voltage drop circuit model diode constant (VD = 0.7 V). However, a smooth transition between the linear and saturation regions of the transfer characteristics is assumed.

    We can have a simple limiter when only one of the waveform polarities is limited. Or, a double limiter, when both the positive and negative part of the waveform are limited.


        3.1   Simple Limiter

    First, let's study the simple type limiter and analyze its behavior when subjected to a sine wave.

circdiodo65-2J.jpg
Figure 65-03
graphdiodo65-1J.jpg
Figure 65-04

    In the Figure 65-03, we present the scheme of a simple limiter composed of a diode and a resistor. When the voltage Vi (by assumption a sine wave) has a positive peak and reaches a value of 0.7 V, the D diode goes into conduction not allowing the voltage Vo exceed this value. For the negative cycle of Vi, the diode is cut-off and in Vo input voltage appears without any change.

    In the Figure 65-04, we present the graph of the circuit transfer characteristic. Note that for negative values of Vi we have a linear response to the input voltage. For positive values, the response is not linear, setting the output voltage to 0.7 V. Of course we can obtain multiple voltages of 0.7 V at the output, provided that diodes are added in series.

graphdiodo65-3J.jpg
Figure 65-05

    See Figure 65-05 for the representation of a sine waveform at the input and the limited wave at the positive peak appearing at the output. Note that a voltage with 1 volt peak was used at the input. If the input peak voltage is less than 0.7 V, so there will be no limiting as the input voltage does not exceed the diode conduction voltage. Therefore, we can use this circuit whenever we need to protect the input of some circuit against overvoltage.

    In practice, there are situations where there is a need to limit the output to a voltage other than of 0.7V. In this case, it is possible to add a voltage source in series with the diode to move the actuation point of the diode.

circdiodo65-3J.jpg
Figure 65-06

    In the Figure 65-06 see an example circuit for this case. We show the modified circuit where a voltage source Vd, this value, which is how much we want to move the limiting point, was added in series with the diode. So the point of limiting will be given by Vd + 0.7. Note that if the peak value of the input voltage Vi not reach the value Vd + 0.7, the sine wave does not change any output.

    Of course if we want a limitation only on the negative peak, then we must invert the position of the diode and the source Vd in the circuit above. With this, we will be causing a limitation of the negative part of the sine wave, while not changing the positive peak.


        3.2   Double Limiter
circdiodo65-13J.png
Figure 65-07

    The double limiter acts on both positive and negative parts of the sinusoid. The most common circuit for obtaining this limitation is that shown in Figure 65-07. They are two diodes connected in parallel and in the arrangement called counter phase.

    The diode D1 acts on the positive part of the sinusoid, while D2 acts on the negative part.

graphdiodo65-4J.png
Figure 65-08

    In the Figure 65-08 we present the graph of the transfer characteristic of the double limiter circuit. Note that the output voltage is limited between the voltages + VD and - VD. Thus, even if the input voltage is significantly increased, the output voltage will remain between the two values above.

    Of course, in the double limiter the same concept applies as adding a voltage source in series with the diode to shift the wave limit point at the output. Making the voltage sources with different values, we will have limiting points different, that is, at the output we will get an asymmetric waveform.

circdiodo65-14J.png
Figure 65-09

    In Figure 65-09 we can see a double limiting circuit with voltage sources connected in series with the diodes. This causes a shift of the limit point at the circuit output. Note that the two sets are in parallel. If VD1 > VD2 or VD1 < VD2, let's get an asymmetric waveform on the output.

    The graph of the circuit transfer characteristic is similar to that of Figure 65-08 . Of course, for the positive part of the sinusoid, we must replace the value VD by VD1 + VD. And for the negative sinusoid cycle, we must replace - VD by - (VD2 + VD ). To eliminate voltage sources in series with the diodes, we can use zener diodes. We will study these settings in the next item.


    4.   Zener Diode Limiting Circuits

    One way to avoid using diode voltage sources in order to achieve a certain level of limitation is to use the zener diode. Since we have zener diodes with various operating voltages, we can choose the one that best suits our needs.


        4.1   Simple Limiter with Zener Diode

circdiodo65-5K.png
Figure 65-10

    In Figure 65-10 we show the circuit using zener as a voltage limiter. Note that in this case, the zener will not allow at the output the positive peak of the input signal to exceed the working voltage of the zener. Thus, at the output we will have a limited signal at the positive peak with a maximum voltage equal to Vz.

circdiodo65-6J.png
Figure 65-11

    However, this circuit presents a problem when the input voltage passes to the negative part of the sine. This is due to the fact that in the negative part the zener diode will behave like a common diode, that is, the negative peak at the output will be limited to 0.7 V . See Figure 65-11 what the output voltage would look like using a zener with an operating voltage of the order of 6.2 volts and the input voltage being given by Vi = 10 sin ω t. Note that the positive part of the sinusoid is limited by 6.2V, while the negative part is limited by - 0.7V.

    Then, the maximum variation (peak to peak) of the signal at the circuit output will be equal to the difference of the two voltages, that is, Vo = 6.2 - (- 0.7) = 6.9 volts.

    But for everything there is solution. So if we add a common diode in series with the zener, wired as shown in Figure 65-12, in principle, we solve the problem.

circdiodo65-9J.png
Figure 65-12

    We say " in principle" , because if we pay close attention to Figure 65-12 we will realize that the output voltage of the circuit will be given by Vo = Vz + VD. That is, in the output voltage we have to add the voltage drop in the diode, in addition to the zener voltage. Of course this only applies to the positive part of the sinusoidal input voltage.

    As soon as the input voltage is negative, the diode will enter the cut-off zone, resulting in an open circuit, as if there were no diode and zener. Therefore, the negative part of the sinusoid appears entirely at the output without any changes. Therefore, with the addition of a diode, we can again limit only the positive part of the input voltage.

    Of course, if we want a limiter with a cut in the negative part of the sine, just reverse the direction of the two components.


        4.2   Zener Diode Double Limiter

    To achieve a double limiter, we can combine zener with zener or zener with diode. Let's look at some possible configurations.


        4.2.1   Two Zener in Series

circdiodo65-10J.png
Figure 65-13

    In the Figure 65-13 we show how we can perform a double limiter. Note that the connection is in the serial configuration, with the components in the so-called counter-phase array. In the figure, we are representing the situation where the input voltage is in the positive part of the sine.

    This way, the zener of the upper part of the figure will behave like zener, while the lower zener will behave like a common diode. This is explicit with the indication of Vz and VD next to each zener. So in this configuration, with the positive part of the sinusoid, we get the output voltage given by the equation Vo = Vz + VD.

circdiodo65-10K.png
Figure 65-14

    Now, in Figure 65-14 we are representing the situation where the input voltage is in the negative part of the sinusoid. Note the inversion of the indications of Vz and VD next to each zener, in relation to Figure 65-13. In this situation it is the lower zener that is acting as a zener, while the upper zener acts as a common diode.

    So in this configuration, with the negative part of the sinusoid, we get the output voltage given by Vo = - Vz - VD or Vo = - (Vz + VD ).


    Attention
    "It should be noted that the two zener need not have the same operating voltage. Obviously, in this case the limitation will not be symmetrical. In the Problems tab we will see in more detail."


        4.2.2   Two Sets of Zener + Diode in Parallel

    Initially, we could think of placing two zener diodes in parallel to obtain a double limiter. But if we look carefully at the situation we will conclude that putting two zener diodes in parallel is the same as putting two common diodes in parallel. And as we know, in this case, there will be + 0.7 V limitation on the positive part of the sinusoid and - 0.7 V on the negative part of the sinusoid. Then we should discard this setting. However, based on the circuit shown in Figure 65-12 showing the addition of a diode in series with zener, we have the solution to this problem.

circdiodo65-12J.png
Figure 65-15
    In Figure 65-15 we show a circuit pertinent to our goals. Note that the diode + zener set highlighted in orange will act when the input voltage is at the positive part of the sinusoid. As already mentioned, the output voltage will be given by Vo = Vz + VD.

    Of course, for the positive part of the sinusoid, the circuit highlighted in green does not work because the diode is polarized inversely. However, when the input voltage is in the negative part of the sinusoid, the circuit highlighted in green is acting. And of course the circuit highlighted in orange is operating in cut-off. So for the negative part of the sinusoid the output voltage will be given by Vo = - (Vz + VD ).

    The graph of the circuit transfer characteristic is similar to that of Figure 65-08. In this way we get a limiting circuit that acts on both sine polarities. If you are interested in working with different limiting voltages on the positive and negative part of the sine, simply use zener with different operating voltages.