SEC. 22-1 COMPARATORS WITH
ZERO REFERENCE
A comparator with a reference voltage of
zero is called a zero-crossing detector.
Diode clamps are often used to protect the
comparator against excessively large input
voltages. Comparators usually interface
their outputs with digital circuits.
SEC. 22-2 COMPARATORS WITH
NONZERO REFERENCES
In some applications a threshold voltage
different from zero may be preferred.
Comparators with a nonzero reference
voltage are sometimes called limit
detectors. Although op amps may be used
as comparators, IC comparators are
optimized for this application by removing
the internal compensating capacitor. This
increases the switching speed.
SEC. 22-3 COMPARATORS WITH
HYSTERESIS
Noise is any kind of unwanted signal that
is not derived from or harmonically
related to the input signal. Because noise
can cause false triggering of a
comparator, positive feedback is used to
create hysteresis. This prevents noise from
producing false triggering. The positive
feedback also speeds up the switching
between output states.
SEC. 22-4 WINDOW
COMPARATOR
A window comparator, also called a
double-ended limit detector, detects
when the input voltage is between two
limits. To create the window, a window
comparator uses two comparators with
two different trip points.
SEC. 22-5 THE INTEGRATOR
An integrator is useful for converting
rectangular pulses into linear ramps.
Because of the large input Miller
capacitance, only the earliest part of an
exponential charge is used. Since this
early part is almost linear, the output
ramps are almost perfect. Integrators are
used to create the time bases of
oscilloscopes.
SEC. 22-6 WAVEFORM
CONVERSION
We can use a Schmitt trigger to
convert a sine wave to a rectangular
wave. An integrator can convert a
square wave to a triangular wave.
With an adjustable resistor, we can
control the duty cycle with a limit
detector.
SEC. 22-7 WAVEFORM
GENERATION
With positive feedback, we can build
oscillators, circuits that generate or create
an output signal with no external input
signal. A relaxation oscillator uses the
charging of a capacitor to generate an
output signal. By cascading a relaxation
oscillator and an integrator, we can
produce a triangular output
waveform.
SEC. 22-8 ANOTHER TRIANGULAR
GENERATOR
The output of a noninverting Schmitt
trigger can be used to drive an integrator.
If the output of the integrator is used as
the input to the Schmitt trigger, we have
an oscillator that produces both square
waves and triangular waves.
SEC. 22-9 ACTIVE DIODE
CIRCUITS
With op amps, we can build active halfwave
rectifiers, peak detectors, clippers,
and clampers. In all these circuits, the
closed-loop knee voltage equals the knee
voltage divided by the open-loop voltage
gain. Because of this, we can process lowlevel
signals.
SEC. 22-10 THE DIFFERENTIATOR
When a square wave drives an RC
differentiator, the output is a series of
narrow positive and negative voltage
spikes. With an op amp, we can improve
the differentiation and get a low output
impedance.
SEC. 22-11 CLASS-D AMPLIFIER
The Class-D amplifier uses output
transistors operated as switches. Instead of
operating in a linear region, these transistors
are alternately driven into saturation and
cutoff by the output signal of a comparator
circuit. The Class-D amplifier is capable of
very high circuit efficiencies and is gaining
popularity in portable equipment needing
audio amplification.
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