4-30
Figure 4-24B.—Common-base configuration clamper.
Look at view (B) as we continue. The input signal is a square-wave pulse type signal that varies in
amplitude. Without an input signal, Q1 conducts and provides current through R1. This develops the
output (collector to ground) potential which is assumed to be approximately
-
5 volts (VCC
-
ER1) for this
discussion.
From T0 to T1 the output follows the input because of the increasing emitter-base forward bias.
However, at T1 the collector voltage reaches
-
2 volts and D2 is forward biased. D2 conducts and limits
the output to
-
2 volts (the value of B3). D2 conducts until T2 when the input decreases below
-
2 volts. At
this time, D2 cuts off and the output again follows the input because of the decreasing forward bias on
Q1. At T3 the input reaches
-
8 volts and forward biases D1. D1 conducts and any further increase
(beyond
-
8 volts) of the input has no effect on the output. When the input returns to a value more positive
than
-
8 volts, D1 cuts off and the output again follows the input. This circuit action is the same for all
inputs. The output remains the same as the input except that both positive and negative extremes are
clamped at
-
2 and
-
8 volts, respectively.
SHAPING CIRCUITS
Timing circuits and circuits which require a particular shape or "spike" of voltage, may use
SHAPING circuits. Shaping circuits can be used to cause wave shapes, such as square waves, sawtooth
waves, and trapezoidal waves, to change their shape. Shaping circuits may be either series RC or series
RL circuits. The time constant is controlled in respect to the duration of the applied waveform. Notice that
the wave shapes mentioned did not include the sine wave. These RC or RL shaping circuits do not change
the shape of a pure sine wave.
The series RC and RL circuits electrically perform the mathematical operations of INTEGRATION
and DIFFERENTIATION. Therefore, the circuits used to perform these operations are called
INTEGRATORS and DIFFERENTIATORS. These names are applied to these circuits even though they
do not always completely perform the operations of mathematical integration and differentiation.
