Let's look at a series-positive limiter with positive bias as shown in figure 4-4, views (A) and (B). The diode will conduct until the input signal exceeds +5 at T1 on the positive alternation of the input signal. When the positive alternation exceeds +5 volts, the diode becomes reverse biased and limits the positive alternation of the output signal to +5 volts. This is because there is no current flow through resistor R1 and battery voltage is felt at point (B). The diode will remain reverse biased until the positive alternation of the input signal decreases to just under +5 volts at T2. At this time, the diode again becomes forward biased and conducts. The diode will remain forward biased from T2 to T3. During this period the negative alternation of the input is passed through the diode without being limited. From T3 to T4 the diode is again reverse biased and the output is again limited.
Figure 4-4A. - Series-positive limiter with positive bias.
Figure 4-4B. - Series-positive limiter with positive bias.
Now let's look at what takes place when reverse bias is aided, as shown in figure 4-5, view (A). The diode is negatively biased with -5 volts from the battery. In view (B), compare the output to the input signal applied. From T0 to T1 the diode is reverse biased and limiting takes place. The output is at -5 volts (battery voltage) during this period. As the negative alternation increases toward -10 volts (T1), the cathode of the diode becomes more negative than the anode and is forward biased. From T1 to T2 the input signal is passed to the output. The diode remains forward biased until the negative alternation has decreased to -5 volts at T2. At T2 the cathode of the diode becomes more positive than the anode, and the diode is again reverse biased and remains so until T3.
Figure 4-5A. - Series-positive limiter with negative bias.
Figure 4-5B. - Series-positive limiter with negative bias.
In view (A) of figure 4-6, the SERIES-NEGATIVE LIMITER limits the negative portion of the waveform, as shown in view (B). Let's consider the input signal and determine how the output is produced. During T0 to T1 (view (B)), the anode is more positive than the cathode and the diode conducts. Current flows up through the resistor and the diode, and a positive voltage is developed at the output. The voltage across the resistor is essentially the same as the voltage applied to the circuit.
Figure 4-6A. - Series-negative limiter.
Figure 4-6B. - Series-negative limiter.
During T1 to T2 the anode is negative with respect to the cathode and the diode does not conduct. This portion of the output is limited because no current flows through the resistor.
As you can see, the only difference between series-positive and series-negative limiters is that the diode is reversed in the negative limiters.
SERIES-NEGATIVE LIMITER WITH BIAS. - View (A) of figure 4-7 shows a series-negative limiter with negative bias. The diode is forward biased and conducts with no input signal. In view (B) it will continue to conduct as the input signal swings first positive and then negative (but only to -5 volts) from T0 through T1. At T1 the input becomes negative with respect to the -5 volt battery bias. The diode becomes reverse biased and is cutoff until T2 when the anode again becomes positive with respect to the battery voltage (-5 volts) on the cathode. No voltage is developed in the output by R1 (no current flow) and the output is held at -5 volts from T1 to T2. With negative bias applied to a series-negative limiter, only a portion of the negative signal is limited.
Figure 4-7A. - Series-negative limiter with negative bias.
Figure 4-7B. - Series-negative limiter with negative bias.
Now let's look at a series-negative limiter with positive bias, as shown in figure 4-8, view (A). Here we will remove all of the negative alternation and part of the positive alternation of the input signal. We have given a full explanation of the series-positive limiter, series-positive limiter with bias, series-negative limiter, and series-negative limiter with negative bias; therefore, you should have little difficulty understanding what is happening in the circuit in the figure.
Figure 4-8A. - Series-negative limiter with positive bins.
Figure 4-8B. - Series-negative limiter with positive bins.
The series-negative limiter with positive bias is different in only one aspect from the series-positive limiter with bias (figure 4-5) discussed earlier. The difference is that the diode is reversed and the output is of the opposite polarity.
Q.1 Which portion of a sine-wave input is retained in the output of a series-positive