Hysteresis
comparators
Simple comparators have two
drawbacks:
For a very slowly varying input
output switching can be
rather slow.
For a noisy input signal the
output may make several
undesirable transitions
(commutations) as the input
passes through the threshold
voltage value (trigger point)
no more undesirable transition
How can we implement
such a VTC ?
1/8
Solution:
Two threshold values VThH and VThL
Two distinct output values: VOH and VOL
the commutation takes place at VThH only if vO=VOH
the commutation takes place at VThL only if vO=VOL
⇒ The threshold values should depend on the output
value → The output voltage should be fed back to
the input to contribute to the threshold values:
positive feedback (PF) (to strengthen the effect)
Feeding back one fraction of the output voltage to
the non-inverting input by means of a resistive
divider
2/8
Inverting hysteresis
comparator
VTC ?
R1
v =
vO
R1 + R2
+
v _ = vI
R1
vD =
vO − v I
R1 + R2
R1
vD = 0
vO = VTH
R1 + R2
R1
VThH =
VOH
R1 + R2
R1
VThL =
VOL
R1 + R2
3/8
∆VTh = VThH − VThL
R1
(VOH − VOL )
=
R1 + R2
moving direction on the hysteresis
at a certain moment only one threshold is “active”
hysteresis comparators are bistable circuits
the input signal triggers the switching of the output, the
switching process being sustained by the PF
suppose vO=VOL , vI>VThL , vI ↓, when vI passes through VThL
vI ↓, vD ↑, vO ↑, v+↑, vD↑, vO ↑
PF
once the vO starts to change its value the transition is
sustained by the circuit itself due to its PF
⇒ fast (accelerated) switching
Bistable multivibrator circuit or Schmitt triggers
4/8
Illustration
CSTV ?
5/8
Inverting comparator with
asymmetric thresholds
Circuit ?
R1
R2
v =
vO +
VREF
R1 + R2
R1 + R2
R1
R2
+
−
vD = v − v =
vO +
VREF − vI
R1 + R2
R1 + R2
+
VThL
VThH
R1
R2
=
VOL +
VREF
R1 + R2
R1 + R2
R1
R2
=
VOH +
VREF
R1 + R2
R1 + R2
6/8
Non-inverting hysteresis comparators
R1
R2
vD = v − v =
vO +
vI − 0
R1
R1 + R2
R1 + R2
VThL = −
VOH
R1
R2
R2
+
−
R1 + R2
VTh
vO +
R1 + R2
R1
=−
vO
R2
VP = 0
R1
VThH = −
VOL
R2
7/8
Illustration
R1
R2
vD = v − v =
vO +
v I − VREF
R1 + R2
R1 + R2
+
−
8/8