EXAMPLE 2.20 Determine the output waveform for the network of Fig. 2.74.

Figure 2.74 Series clipper for

Example 2.20.

Solution
Past experience suggests that the diode will be in the “on” state for the positive re-
gion of vi —especially when we note the aiding effect of V  5 V. The network will
then appear as shown in Fig. 2.75 and vo  vi  5 V. Substituting id  0 at vd  0 for
the transition levels, we obtain the network of Fig. 2.76 and vi  5 V.

Figure 2.75 vo with diode in

the “on” state.

– + vd = 0 V

+ 5 V id = 0 A
+
vi R vo = vR = iR R = id R = (0) R = 0 V
Figure 2.76 Determining the
– – transition level for the clipper of
Fig. 2.74.

For vi more negative than 5 V the diode will enter its open-circuit state, while
for voltages more positive than 5 V the diode is in the short-circuit state. The input
and output voltages appear in Fig. 2.77.

vo
vi

20 vi + 5 V = 20 V + 5 V = 25 V

5V vo = 0 V + 5 V = 5 V
–5V T T t 0 T T t
2 2
Transition vo = –5 V + 5 V = 0 V
voltage

Figure 2.77 Sketching vo for Example 2.20.

The analysis of clipper networks with square-wave inputs is actually easier to an-
alyze than with sinusoidal inputs because only two levels have to be considered. In
other words, the network can be analyzed as if it had two dc level inputs with the re-
sulting output vo plotted in the proper time frame.

78 Chapter 2 Diode Applications

Repeat Example 2.20 for the square-wave input of Fig. 2.78. EXAMPLE 2.21

Figure 2.78 Applied signal for

Example 2.21.

Solution
For vi  20 V (0 → T/ 2) the network of Fig. 2.79 will result. The diode is in the short-
circuit state and vo  20 V  5 V  25 V. For vi  10 V the network of Fig. 2.80
will result, placing the diode in the “off” state and vo  iRR  (0)R  0 V. The re-
sulting output voltage appears in Fig. 2.81.

– + + – + +
+ 5V – 5V
20 V R vo 10 V R vo = 0 V
– +
– –

Figure 2.79 vo at vi  20 V. Figure 2.80 vo at vi   10 V. Figure 2.81 Sketching vo for

Example 2.21.

Note in Example 2.21 that the clipper not only clipped off 5 V from the total
swing but raised the dc level of the signal by 5 V.

Parallel
The network of Fig. 2.82 is the simplest of parallel diode configurations with the out-
put for the same inputs of Fig. 2.67. The analysis of parallel configurations is very
similar to that applied to series configurations, as demonstrated in the next example.

+ R +
vi vo

– –

vi vo vi vo

V V

0 t 0 t 0 t 0 t

–V –V –V –V

Figure 2.82 Response to a parallel clipper.

2.9 Clippers 79
 EXAMPLE 2.2 Determine vo for the network of Fig. 2.83.

Figure 2.83 Example 2.22.

Solution
The polarity of the dc supply and the direction of the diode strongly suggest that the
diode will be in the “on” state for the negative region of the input signal. For this re-
gion the network will appear as shown in Fig. 2.84, where the defined terminals for
vo require that vo  V  4 V.

– R +

vi vo = V = 4 V

V 4V
+ – Figure 2.84 vo for the negative
region of vi.

The transition state can be determined from Fig. 2.85, where the condition id 
0 A at vd  0 V has been imposed. The result is vi (transition)  V  4 V.
Since the dc supply is obviously “pressuring” the diode to stay in the short-
circuit state, the input voltage must be greater than 4 V for the diode to be in the “off”
state. Any input voltage less than 4 V will result in a short-circuited diode.
For the open-circuit state the network will appear as shown in Fig. 2.86, where
vo  vi. Completing the sketch of vo results in the waveform of Fig. 2.87.

Figure 2.85 Determining the

transition level for Example 2.22.

Figure 2.87 Sketching vo for

Example 2.22.

Figure 2.86 Determining vo for

the open state of the diode.

To examine the effects of VT on the output voltage, the next example will spec-
ify a silicon diode rather than an ideal diode equivalent.

80 Chapter 2 Diode Applications

Repeat Example 2.22 using a silicon diode with VT  0.7 V. EXAMPLE 2.23
Solution
The transition voltage can first be determined by applying the condition id  0 A at
vd  VD  0.7 V and obtaining the network of Fig. 2.88. Applying Kirchhoff’s volt-
age law around the output loop in the clockwise direction, we find that
vi  VT  V  0
and vi  V  VT  4 V  0.7 V  3.3 V

Figure 2.88 Determining the

transition level for the network of
Fig. 2.83.

For input voltages greater than 3.3 V, the diode will be an open circuit and
vo  vi. For input voltages of less than 3.3 V, the diode will be in the “on” state and
the network of Fig. 2.89 results, where
vo  4 V  0.7 V  3.3 V

Figure 2.89 Determining vo for

the diode of Fig. 2.83 in the “on”
state.

The resulting output waveform appears in Fig. 2.90. Note that the only effect of VT
was to drop the transition level to 3.3 from 4 V.

Figure 2.90 Sketching vo for

Example 2.23.

There is no question that including the effects of VT will complicate the analysis
somewhat, but once the analysis is understood with the ideal diode, the procedure,
including the effects of VT, will not be that difficult.

Summary
A variety of series and parallel clippers with the resulting output for the sinusoidal
input are provided in Fig. 2.91. In particular, note the response of the last configura-
tion, with its ability to clip off a positive and a negative section as determined by the
magnitude of the dc supplies.

2.9 Clippers 81
 Figure 2.91 Clipping circuits.

82 Chapter 2 Diode Applications