4-3
The output during each negative alternation of the input is approximately the same as the input (
-
10
volts) because most of the voltage is developed across the resistor.
Ideally, the output wave shape should be exactly the same as the input wave shape with only the
limited portion removed. When the diode is reverse biased, the circuit has a small amount of reverse
current flow, as shown just above the 0-volt reference line in figure 4-2. During the limiting portion of the
input signal, the diode resistance should be high compared to the resistor. During the time the diode is
conducting, the resistance of the diode should be small as compared to that of the resistor. In other words,
the diode should have a very high front-to-back ratio (forward resistance compared to reverse resistance).
This relationship can be better understood if you study the effects that a front-to-back resistance ratio has
on circuit output.
Figure 4-2.—Actual output of a series-positive limiter.
The following formula can be used to determine the output amplitude of the signal:
Let's use the formula to compare the front-to-back ratio of the diode in the forward- and reverse-
biased conditions.
Given:
