Operational Amplifier Basics. 1st year electronics

1. Operational Amplifier Basics
Revision Question 4
An inverting comparator is shown in Figure 4.
Calculate the switching threshold voltages and
the hysteresis for VREF=+2.5V.

2. Operational Amplifier Basics
Revision Question 4
Allow a current to flow from VREF through R1. Since
no current can flow into the Op Amp Non-inverting
Terminal. The current must continue through the
feedback resistor R2 to the output. Let the
potential at the Non-inverting terminal be V+.
Ideal conditions so the current flowing through R1
equals the current flowing through R2.
VREF - V V - VO - VREF - VO
R1 R2 R1 R 2

3. Operational Amplifier Basics
Revision Question 4
and as a potential divider
VREFR2 VOR1
V=
R1 R 2 R1 R 2
When the output voltage is high, the threshold
voltage will be given by:
+ R2 R1
V H=VREF VH
R1 R2 R1 R2
Where VH = +5V

4. Operational Amplifier Basics
Revision Question 4
Similarly, when the Output Voltage is low, the
threshold voltage is given by:
+ R2 R1
V L=VREF VL
R1 R2 R1 R2
Where VL = -5V
The hysteresis is given by the difference between VH & VL
R1
V H V L (VH VL)
R1 R2

5. Operational Amplifier Basics
Revision Question 4
Substituting values
+ 15000 10000
V H=2.5 ( 5) 3.5V
10000 15000 10000 15000
And
+ 15000 10000
V L=2.5 ( 5) 0.5V
10000 15000 10000 15000
10000
V H V L (5 ( 5)) 4V
10000 15000