Page 306 - Mechatronics with Experiments

JWST499-Cetinkunt
JWST499-c05
292 MECHATRONICS Printer: Yet to Come October 28, 2014 11:15 254mm×178mm
V V
C1 C1
R
1
V v
ref R in v
- v 0 1 - 0
+ +
R
2 v in V ref
R
2

v
in

v
0
v
sat

- v
sat

v
0
v
sat

- v
sat

FIGURE 5.29: Open loop op-amp used as a comparator: V ref reference voltage and V in input
voltage (i.e., voltage from a sensor) are switched at the op-amp terminals between the two
cases. The net result is the output polarity change.

Op-Amps with Positive Feedback Modifications to the op-amp feedback loop
on the non-inverting terminal can provide hysteresis in the comparator to implement
ON/OFF relay control with hysteresis, such as is used in home temperature control. A
well-known circuit with this function is the Schmitt trigger (inverting and noninverting
types, Figure 5.31). The op-amp uses positive feedback which results in a hysteresis rela-
tionship between input and output. Positive feedback is the key to the hysteresis function
realization. Such circuits are not only used in ON/OFF control systems with desired hys-
teresis, but also reject undesirable noise in the signal provided the noise magnitude is
smaller than the hysteresis magnitude (Figure 5.31). A more general form of the Schmitt
trigger op-amp circuit allows adjustment of both the hysteresis magnitude and center input
voltage value about which the hysteresis is designed. Schmitt trigger circuits are also used