Electronics

1. Schmitt Trigger Circuits

2. Op Amps
Previously in ET1 we investigated the Operational Amplifier. The
Op Amp consisted of
• V in
• V out
• A feedback loop
O V O V
V in
V out
Feedback Loop
A Non Inverting Op Amp
It is possible to tell if an Op Amp is Inverting or Non-Inverting by the
location of V in. If it went to the positive terminal of the Op Amp then
the circuit was Non Inverting and vice versa.

3. Op amps as Comparators
When using the Op Amp in this format we need to consider the
value of the Supply Rails it is connected to. The values of the
inputs at the + and - terminals then determine if V out would be
positive or negative.
If A is higher than B then V out = – 9V
If B is higher than A then V out = + 9V
-9 V
V out
-9 V
+ 9 V
+ 9 V
A
B

4. Op Amps as Schmitt Triggers
Op Amps can also be made to behave in a very similar way to
Schmitt Invertors. These appeared in ET2 where they solved the
problem of a NOT gate hunting at its switching threshold.
voltage
time
If a voltage signal varies at the switching threshold of an Invertor it
can cause it to burn out.

5. Op Amps as Schmitt Triggers
When using an Op Amp as a Schmitt Trigger we now have to
consider both the voltage values at the input terminals and the
voltage values of the dual rail supply. A feedback loop is also used.
Inverting Schmitt Trigger
0 V
V out
0 V
+ V
+ V
V in
V non - inv
V in
V out
Switching thresholds

6. Op Amps as Schmitt Triggers
We cannot assume that we know which voltage rail the Op Amp will
take for it’s starting voltage. We only know that when Vin exceeds
Vnon inv then the circuit will INVERT and the output will equal the
other voltage rail instead.
Thus, we can recognise an inverting Schmitt Trigger by the fact that
Vin is connected to the negative terminal.

7. Switching Thresholds
The switching threshold of a Schmitt Trigger does not have to be
the same as the voltage rails available. In the diagram below we
can see that resistors have been added to create a Potential
Divider. By selecting the correct values of these resistor we can
change the Switching Threshold of the Op Amp
(the value of V non – inv).
V non – inv = Vout x R2
V non – inv = Vout x 50
V non – inv = 10
Inverting Schmitt Trigger
0 V
V out
0 V
+ 14 V
+ 14 V
V in
V non - inv
Two resistors
added to
divide the
voltage
10 k
50 k
R1 + R2
10 + 50
+ - 12 V

8. Non Inverting Schmitt Triggers
In order to create a Non Inverting Schmitt Trigger you must connect
a reference voltage to the input (Vref). This reference voltage is
constant and thus the circuit will switch when the positive terminal
(Vin) equals the negative terminal. The resistors can be treated as
a voltage divider again to make calculations easier.
Non - Inverting Schmitt Trigger
0 V
V out
0 V
+ V
+ V
V in
V ref
Vp