Op Amp Comparators
Op Amp circuits can be designed that will make “decisions” based on the values of the voltages at their 2
inputs. Recall that because of the very large open loop gain of the Op Amp, the output voltage will be
either at +Vsat or –Vsat for any value of input voltage greater that 100 µV or so.
Basic Principle of Op Amp Comparators
Whichever input has the largest voltage, dictates the state of the output. Remembering these
conditions will make your understanding of Op Amp comparators simple.
If V2 > V1 then Vout will be at +Vsat
If V1 > V2 then Vout will be at -Vsat
Whenever the 2 input voltages are equal (V2 = V1) the output voltage Vout will be zero.
Comparator Configurations
Zero Voltage Referenced Comparators
In this configuration the input voltage is compared to zero volts (the other input is grounded). As with
Op Amp amplifiers there are Non Inverting and Inverting Configurations. Triangular wave inputs are
best suited to demonstrating how a comparator works. In these examples VIN is 10 V P-P.
Non Inverting Zero Voltage Referenced Comparator
In this configuration the output changes state from –Vsat to +Vsat when Vin at the non-inverting input rises
a few mV above zero volts. The comparator switches state at the same reference voltage as shown in
the waveforms below and they are in phase.
Zero switching voltage
+VSAT
-VSAT
1
Inverting Zero Voltage Referenced Comparator
In this configuration the output changes state from +Vsat to -Vsat when Vin at the Inverting input rises a
few mV above zero volts. The comparator switches state at the same switching voltage and the
waveforms are out of phase.
Zero switching voltage
0 volts
Non Zero Voltage Referenced Comparators
In this configuration the input voltages is compared to a non zero reference voltage.
Non Inverting Non Zero Voltage Referenced Comparator
In this configuration the output changes state from –Vsat to +Vsat when Vin at the non-inverting input rises
a few mV above the reference voltage (VREF) of 2 volts. The comparator switches state at the same
switching voltage as shown in the waveforms below and they are in phase.
2 volt switching
0 volts
2
In another configuration the output changes state from –Vsat to +Vsat when Vin at the non-inverting input
rises a few mV below the reference voltage (VREF) of -8 volts. The comparator switches state at the same
switching voltage as shown in the waveforms below and they are in phase.
-8 volt switching
0 volts
Inverting Non Zero Voltage Referenced Comparator
In this configuration the output changes state from +Vsat to -Vsat when Vin at the Inverting input rises a
few mV above the reference voltage of +5 V. The comparator switches state at the same switching
voltage and the waveforms are out of phase.
VREF
3
In another configuration the output changes state from +Vsat to -Vsat when Vin at the inverting input rises
a few mV below the reference voltage (VREF) of -5 volts. The comparator switches state at the same
switching voltage as shown in the waveforms below and they are out of phase.
VREF
Comparators with Hysteresis
All of the comparators we have examined so far have the same switching or reference voltage where
they change state either from +Vsat to -Vsat or from -Vsat to +Vsat.
In example 5 above the comparator changed state at + 5V. Imagine that the input voltage to this
comparator was from a sensor monitoring the temperature of a room. When the temperature
increased and VIN rose a few mV above the reference point, the furnace would turn OFF and the room
would start to cool. A short time later the temperature would decrease and VIN would drop a few mV
below the reference point and the furnace would turn ON. This would result in an unacceptable
situation where the furnace would be cycling on and off many times in a short time.
The solution to this situation is to design a comparator circuit with different ‘turn-on’ and ‘turn-off’
voltages. This type of a circuit is called a Comparator with Hysteresis. The ‘turn-on’ and ‘turn-off’
voltages are referred to as the upper and lower threshold voltages – VUT and VLT.
Hysteresis Voltage VH
The hysteresis voltage VH is the difference between the upper and lower threshold voltages
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VH = VUT - VLT
A comparator with hysteresis also has noise immunity to noise voltages less than the hysteresis voltage
VH.
Center Voltage
The center voltage Vcenter is the midpoint between VUT and VLT.
Vcenter = (VUT + VLT)/2
Zero Centered Comparator with Hysteresis
A comparator circuit with hysteresis uses positive feedback as shown in the circuit to create a situation
with hysteresis. Be sure that you see the difference between a circuit with positive feedback (a
comparator) and a circuit with negative feedback (an amplifier).
Assume that VSAT = ± 11 V.
The feedback voltage is
VFB = R1/(R1 + R2) x VOUT and VUT = VLT = ± VFB
The upper threshold voltage VUT = 1 kΩ/(1 kΩ + 10 kΩ) x 11 V = 1 V
The lower threshold voltage VLT = 1 kΩ/(1 kΩ + 10 kΩ) x -11 V = -1 V
The hysteresis voltage VH is
VH = VUT - VLT = 1 V – (-1V) = 2 V
This circuit has a noise immunity of 2 V
The center voltage Vcenter is
Vcenter = (VUT + VLT)/2 = (1 V + (-1))/2 = 0 V
The waveforms for this comparator are shown below. Note that this comparator is using an Inverting
configuration. A Non Inverting comparator circuit is also possible.
5
VUT
Vcenter
VLT
Non Zero Centered Comparator with Hysteresis
In the previous example the upper and lower threshold voltages were centered around zero volts. In
this example they are not.
Calculate the Upper and Lower Threshold Voltages (turn-on and turn-off voltages) for this comparator.
The Upper and Lower threshold voltages are at the non-inverting input. Assume Vsat = ± 12 V. You
need to use the Superposition Theorem to calculate these voltages.
Upper Threshold Voltage
Contribution due to Vref - ground Vout
VUT,1 = 80 k/(20 K + 80 k) X 4 V = 3.2 V
Contribution due to Vout - ground Vref
VUT,2 = 20 k/(20 K + 80 k) X 12 V = 2.4 V
Total VUT = 3.2 V + 2.4 V = 5.6 V
Lower Threshold Voltage
Contribution due to Vref - ground Vout
VLT,1 = 80 k/(20 K + 80 k) X 4 V = 3.2 V
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Contribution due to Vout - ground Vref
VLT,2 = 20 k/(20 K + 80 k) X - 12 V = - 2.4 V
Total VLT = 3.2 V + (- 2.4 V) = 0.8 V
Hysteresis Voltage
VH = VUT - VLT = 5.6 V – 0.8 V = 4.8 V
Center Voltage
VCenter = (VUT + VLT)/2 = (5.6 V + 0.8 V)/2 = 3.2 V
VUT
VLT
The waveforms for this comparator are shown above. Note that this comparator is using an Inverting
configuration. A Non Inverting comparator circuit is also possible as well as negative reference voltages.
Application Note
Written by David Lloyd
Computer Engineering Program
Humber College
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