handbook of modern sensors

5.3 Excitation Circuits 171
atures. Naturally, the reference voltage from the combination is higher than from a
single zener diode. A commercial monolithic version of this circuit is available from
Motorola (part 1N821).
The so-called band-gap references are often useful substitutes for zener diodes.
They have typically 10 times lower output impedance than low-voltage zeners and
can be obtained in a variety of nominal output voltages, ranging from 1.2 to 10 V.
Currently, a large variety of high-quality voltage references with selectable outputs
is available from many manufacturers.
5.3.3 Oscillators
Oscillators are generators of variable electrical signals. Any oscillator is essentially
comprised of a circuit with a gain stage, some nonlinearity and a certain amount of
positive feedback. By definition, an oscillator is an unstable circuit (as opposed to an
amplifier which better be stable!) whose timing characteristics should be either steady
or changeable according to a predetermined functional dependence. The latter is called
a modulation. Generally, there are three types of electronic oscillators classified according
to the time-keeping components: the RC, the LC, and the crystal oscillators.
In the RC oscillators, the operating frequency is defined by capacitors (C) and resistors
(R); in the LC oscillators, it is defined by the capacitive (C) and inductive (L) components.
In the crystal oscillators, the operating frequency is defined by a mechanical
resonant in specific cuts of piezoelectric crystals, usually quartz and ceramics.
There is a great variety of oscillation circuits, the coverage of which is beyond
the scope of this book. Below, we briefly describe some practical circuits which can
be used for either direct interface with sensors or may generate excitation signals in
an economical fashion.
Many various multivibrators can be built with logic circuits, (e.g., with NOR,
NAND gates, or binary inverters). Also, many multivibrators can be designed with
comparators or operational amplifiers having a high open-loop gain. In all of these
oscillators, a capacitor is being charged, and the voltage across it is compared with
another voltage, which is either constant or changing. The moment when both voltages
are equal is detected by a comparator. A comparator is a two-input circuit which
generates an output signal when its input signals are equal. A comparator is a nonlinear
element due to its very high gain that essentially results in the saturation of
its output when the input signals differ by a relatively small amount. The indication
of a comparison is fed back to the RC network to alter the capacitor charging in the
opposite direction, which is discharging. Recharging in a new direction goes on until
the next moment of comparison. This basic principle essentially requires the following
minimum components: a capacitor, a charging circuit, and a threshold device (a
comparator). Several relaxation oscillators are available from many manufacturers,
(e.g., a very popular timer, type 555, which can operate in either monostable or actable
modes). For illustration, below we describe just two discrete-component square-wave
oscillators; however, there is a great variety of such circuits, which the reader can
find in many books on operational amplifiers and digital systems, (e.g., Ref. [3]).
Asimple square-wave oscillator can be built with two logic inverters (e.g., CMOS)
(Fig. 5.19A). A logic inverter has a threshold near a half of the power supply-voltage.