Download Notes on Schmidt Trigger Physics 120, David Kleinfeld, Spring 2015

Notes on Schmidt Trigger
Physics 120, David Kleinfeld, Spring 2015
This circuit uses feedback to minimize the effect of noise on a threshold transition. It involves
hysteresis (shown below), in terms of a higher level for the up than down transition, with the
output from the comparator. First, the case without feedback:
Add feedback resistor R4.
1 K V L gives
!! !!!
!!
!
+ !! +
!
!! !!!"#
!!
=0
Case of Vout = 0
The NPN output transistor acts as a short, so K V L gives
V! − V! V! V!
+
+
=0
R!
R! R!
so
V! =
R!R!
V
R ! R! + R ! R! + R ! R ! !
Usually R4 is large (the fedback voltage is small) so that, expanding to order (1/R4),
V! ≈
R!
R ! R!
1−
R! + R !
R ! R! + R !
V!
and we see that the feedback lowers the lower threshold relative to the case without feedback.
Case of Vout ≠ 0
The NPN output transistor acts as an open circuit, so K V L gives
V! − V! V! V! − 𝑉!"#
+
+
=0
R!
R!
R!
and
This leads to a morass of algebra
𝑉! =
V!"# − V! V!"# − V!
+
=0
R!
R!
R! + R ! 𝑅! 𝑅! 𝑅!
𝑅! 𝑅! + 𝑅! 𝑅! + 𝑅! 𝑅! R! + R ! − 𝑅! 𝑅! 𝑅!
𝑉!
𝑉!
+
R! R! + R !
that simplifies when R4 is large and becomes:
𝑉! ≈
R!
R! V!
1+
V
R! + R !
R ! V! !
and we see that the feedback raises the upper threshold relative to the case without feedback.
•
When the value of the input heads from high to low, the "0" to "VP" transition
causes an increase in threshold that adds hysteresis and prevents jitter.
•
When the value of the input heads from low to high, the "VP" to "0" transition
causes a decrease in threshold that adds hysteresis and prevents jitter.
2 Example
Simple case is R! = R ! ; R ! = 10R! ; V! = V! ∴ the threshold voltages are:
V! = 𝑉!
1
1−
≈ 0.45 𝑉! ; 𝑉!"# 𝑡 = 0! = 0
2
10
𝑉!
1
1+
≈ 0.55 𝑉! ; 𝑉!"# 𝑡 = 0! ≈ 1.05 𝑉!
2
10
3