4-23
With no input, D1 is forward biased and the +10 V battery is the output. C1 will charge to +10 V and
hold this charge until the first pulse is applied. The battery establishes the dc reference level at +10 volts.
The input wave shape at the top of view (B) is a square wave which alternates between +25 and
-
25 volts.
The output wave shape is shown at the bottom half of view (B).
Here, as with previous circuits, let's apply Kirchhoff's voltage law to determine circuit operation.
With no input signal, the output is just the +10 volts supplied by the battery.
At time T0 the
-
25 volt signal applied to the circuit is instantly felt across R1 and D1. The
-
25 volt
input signal forward biases D1, and C1 quickly charges to 35 volts. This leaves +10 volts across the
output terminals for much of the period from T0 to T1. The polarity of the charged capacitor is, from the
left to the right, minus to plus.
At T1 the 35 volts across the capacitor is series aiding with the +25 volt input signal. At this point
(T1) the output voltage becomes +60 volts; the voltage across R1 and D1 is +50 volts, and the battery is
+10 volts. The cathode of D1 is positive with respect to the anode and the diode does not conduct. From
T1 to T2, C1 discharges only slightly through the large resistance of R1. Assume that, because of the size
of R1 and C1, the capacitor discharges just 2 volts (from +35 volts to +33 volts) during this period. Thus,
the output voltage drops from +60 volts to +58 volts.
At T2 the
-
25 volt input signal and the +33 volts across C1 are series opposing. This makes the
voltage across the output terminals +8 volts. The cathode of the diode is 2 volts negative with respect to
its anode and D1 conducts. Again, since the forward-biased diode is essentially a short, C1 quickly
charges from +33 volts to +35 volts. During most of the time from T2 to T3, then, we find the output
voltage is +10 volts.
At T3 the +25 volts of the input signal is series aiding with the +35 volts across C1. Again the output
voltage is +60 volts. Observe that at T3 the conditions in the circuit are the same as they were at T1.
Therefore, the circuit operation from T3 to T4 is the same as it was from T1 to T2. Circuit operation
continues as a duplication of the operations which occurred from T1 to T3.
By comparing the input and output wave shapes, you should note the following: (1) The peak-to-
peak amplitude of the input wave shape has not been changed in the output (for all practical purposes) by
the action of the clamper circuit; (2) the shape of the input wave has not been changed; (3) the output
wave shape is now clamped above +10 volts. Remember that this clamping level (+10 volts) is
determined by the bias battery.
Positive-Diode Clamper With Negative Bias
View (A) of figure 4-20 is a positive clamper with negative bias. Observe that with no input signal,
the capacitor charges through R1 to the bias battery voltage; the output voltage equals
-
10 volts. The
circuit has negative bias because the positive side of the battery is grounded. The output waveform is
shown in view (B). Study the figure and waveforms carefully and note the following important points.
Once again the peak-to-peak amplitude and shape of the output wave are, for all practical purposes, the
same as the input wave. The lower extremity of the output wave is clamped to
-
10 volts, the value of the
battery. Let's look at the circuit operation. The capacitor is initially charged to
-
10 volts with no input
signal, and diode D1 does not conduct.
