handbook of modern sensors

172 5 Interface Electronic Circuits
(A)
(B)
Fig. 5.19. Square-wave oscillators: (A) with two logic inverters; (B) with a comparator or
OPAM.
When the voltage at its input crosses the threshold, the inverter generates the output
signal of the opposite direction; that is, if the input voltage is ramping up, at the
moment when it reaches one-half of the power supply, the output voltage will be a
negative-going transient. Timing properties of the oscillator are determined by the
resistor R and the capacitor C. Both capacitors should be of the same value. Stability
of the circuit primarily depends on the stabilities of the R and C.
A very popular square-wave oscillator (Fig. 5.19B) can be built with one OPAM
or a voltage comparator. 2 The amplifier is surrounded by two feedback loops: one is
negative (to an inverting input) and the other is positive (to a noninverting input). The
positive feedback (R 3 ) controls the threshold level, and the negative loop charges
and discharges the timing capacitor C 1 , through the resistor R 4 . The frequency of this
oscillator can be determined from
f = 1 [ (
ln 1 + R )] −1
1||R 2
, (5.22)
R 4 C 1 R 3
where R 1 ||R 2 is an equivalent resistance of parallel-connected R 1 and R 2 .
The two circuits (A and B) shown in Fig. 5.20 can generate sine-wave signals.
They use the n-p-n transistors as amplifiers and the LC networks to set the oscillating
frequency.The (B) circuit is especially useful for driving the linear variable differential
transformer (LVDT) position sensors, as the sensor’s transformer becomes a part of
the oscillating circuit.
A radio-frequency oscillator can be used as a part of a capacitive occupancy detector
to detect the presence of people in the vicinity of its antenna (Fig. 5.21). 3 The
antenna is a coil which together with the C 2 capacitors determines the oscillating
frequency. The antenna is coupled to the environment through its distributed capaci-
2 A voltage comparator differs from an operational amplifier by its faster speed response and
the output circuit which is easier interfaceable with TTL and CMOS logic.
3 See Section 7.3 of Chapter 7.