4-5
Figure 4-3A.—Positive and negative bias. POSITIVE BIAS.
Figure 4-3B.—Positive and negative bias. NEGATIVE BIAS.
When the dc aids forward bias, as in view (A), the diode conducts even with no signal applied. An
input signal sufficiently positive to overcome the dc bias potential is required to reverse bias and cut off
the diode.
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.
