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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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