4-21
At T1 the +25 volts already across the capacitor and the +25 volts from the input signal are in series
and aid each other (SERIES AIDING). Thus, +50 volts appears across R1 and D1. At this time, the
cathode of D1 is positive with respect to the anode, and the diode does not conduct. From T1 to T2, C1
discharges to approximately +23 volts (because of the large values of R and C) and the output voltage
drops from +50 volts to +48 volts.
At T2 the input signal changes from +25 volts to
-
25 volts. The input is now SERIES OPPOSING
with the +23 volts across C1. This leaves an output voltage of
-
2 volts (
-
25 plus +23 volts). The cathode
of D1 is negative with respect to the anode and D1 conducts. From T2 to T3, C1 quickly charges through
D1 from +23 volts to +25 volts; the output voltage changes from
-
2 volts to 0 volts.
At T3 the input signal and capacitor voltage are again series aiding. Thus, the output voltage felt
across R1 and D1 is again +50 volts. During T3 and T4, C1 discharges 2 volts through R1. Notice that
circuit operation from T3 to T4 is the same as it was from T1 to T2. The circuit operation for each square-
wave cycle repeats the operation which occurred from T2 to T4.
Compare the input wave shape of figure 4-18, view (B), with the output wave shape. Note the
following important points: (1) The peak-to-peak amplitude of the input wave shape has not been changed
by the clamper circuit; (2) the shape of the output wave shape has not been significantly changed from
that of the input by the action of the clamper circuit; and (3) the output wave shape is now all above 0
volts whereas the input wave shape is both above and below 0 volts. Thus, the lower part of the input
wave shape has been clamped to a dc potential of 0 volts in the output. This circuit is referred to as a
positive clamper since all of the output wave shape is above 0 volts and the bottom is clamped at 0 volts.
The positive clamper circuit is self-adjusting. This means that the bottom of the output waveform
remains clamped at 0 volts during changes in input signal amplitude. Also, the output wave shape retains
the form and peak-to-peak amplitude (50 volts in this case) of the input wave shape. When the input
amplitude becomes greater, the charge of the capacitor becomes greater and the output amplitude
becomes larger. When the input amplitude decreases, the capacitor does not charge as high as before and
clamping occurs at a lower output voltage. The capacitor charge, therefore, changes with signal strength.
The size of R1 and C1 has a direct effect upon the operation of the clamper. Because of the small
resistance of the diode, the capacitor charge time is short. If either R1 or C1 is made smaller, the capacitor
discharges faster (TC = R · C).
The ability of a smaller value capacitor to quickly discharge to a lower voltage is an advantage when
the amplitude of the input wave shape is suddenly reduced. However, for normal clamper operation, quick
discharge time is a disadvantage. This is because one objective of clamping is to keep the output wave
shape the same as the input wave shape. If the small capacitor allows a relatively large amount of the
voltage to discharge with each cycle, then distortion occurs in the output wave shape. A larger portion of
the wave shape then appears on the wrong side of the reference line.
Increasing the value of the resistor increases the discharge time (again, TC = R · C). This increased
value causes the capacitor to discharge more slowly and produces an output wave shape which is a better
reproduction of the input wave shape. A disadvantage of increasing the resistance value is that the larger
resistance increases the discharge time of the capacitor and slows the self-adjustment rate of the circuit,
particularly in case a sudden decrease in input amplitude should occur. The larger resistance has no effect
on self-adjustment with a sudden rise in input amplitude. This is because the capacitor charges through
the small resistance of the conducting diode.
