Ⅰ. What is a voltage comparator?

Simply put, the voltage comparator analyzes the magnitude of two analog voltages (there are also two digital voltages for comparison, but they will not be discussed here) and determines which has the larger voltage, as illustrated in Figure 1. Figure 1(a) depicts a comparator with two input terminals: a non-inverting ("+") and an inverting ("-") input terminal, as well as an output terminal Vout (output level signal). The power supply V+ and ground (this is a single power supply comparator), the non-inverting terminal input voltage  VA, and the inverting terminal input  VB  are also present. Figure 1 depicts the changes in  VA and  VB (b). VA>VB during time 0-t1; VB>VA during time t1-t2; VA>VB during time t2-t3. Figure 1(c) shows the output of Vout in this case: When VA>VB, Vout produces a high level of output (saturated output); when VB>VA, Vout produces a low level of output. You can tell which voltage is greater by looking at the output level.

Figure. 1

When VA is applied to the inverting terminal and VB to the non-inverting terminal, the voltage changes of VA and VB remain the same as in Figure 1(b), and the Vout output remains the same (d). The output level is inverted in comparison to Fig. 1(c). The input terminals of VA and VB are related to the output level change together.

Figure 2(a) shows a comparator with two power supplies (positive and negative power supplies). Figure 1(b) shows the output characteristic of a VA, VB input voltage. Figure 2 shows the output characteristic of a VA, VB input voltage (b). Vout outputs a saturated negative voltage when VB>VA.

Figure. 2

As indicated in Figure 3, the input voltage VA is compared to a fixed voltage VB (a). This VB is referred to as the reference voltage, threshold voltage, or reference voltage. This reference voltage is typically employed for zero-crossing detection if it is 0V (ground level), as shown in Figure 3(b).

Figure. 3

Ⅱ. How does comparator work?

The comparator is derived from the operational amplifier, and the comparator circuit can be thought of as an operational amplifier application circuit. Special comparator integrated circuits have been developed due to the widespread use of comparator circuits.

The input voltage VA is split by the voltage divider R2, R3 and then connected to the non-inverting terminal, VB is connected to the inverting terminal by the input resistor R1, and  RF  is the feedback resistor in Figure 4(a). The output voltage Vout, VA, VB, and the four resistors have the following connection when considering the input offset voltage: Vout=(1+RF/R1)〃R3/(R2+R3)VA-(RF/R1)VB. If R1=R2, R3=RF, Vout=RF/R1(VA-VB), where RF/R1 is the amplifier gain. Vout= when R1=R2=0 (equivalent to R1, R2 short circuit), R3=RF= (equivalent to R3,  RF open circuit), and R3=RF= (equivalent to R3, RF open circuit). Figure 4 shows the circuit diagram for when the gain becomes infinite (b). The comparator circuit, which is the differential amplifier, is in an open-loop state. In actuality, the op amp's gain is not infinite in the open loop condition, and the Vout output is the saturation voltage, which is smaller than the positive and negative supply voltages and cannot be infinite.

Figure. 4

The comparator circuit is a differential amplifier circuit in which the operational amplifier circuit is in an open-loop state, as shown in Figure 4.

Figure 5 depicts a non-inverting amplifier circuit. If RF= in Fig. 5 and R1=0, the comparator circuit is the same as in Fig. 3. (b). In Figure 5, Vin corresponds to VA in Figure 3. (b).

Figure. 5

Ⅲ. Difference between comparator and op amp

Comparator circuits can be made with op amps, however superior performance comparators offer higher open-loop gain, smaller input offset voltages, larger common-mode input voltage ranges, and faster slew rates than general-purpose op amps (making the comparators respond faster). Furthermore, the comparator's output stage normally has an open-collector layout, as shown in Figure 6, which requires an external pull-up resistor or direct driving of loads with varying power supply voltages, making it more adaptable in application. However, there are complementary output comparators that do not require pull-up resistors.

Figure. 6

It should be noted, however, that the comparator circuit has technical criteria as well, including accuracy, reaction speed, propagation delay time, sensitivity, and so on. The majority of the parameters are the same as the op amp. A general-purpose op amp can be utilized as a comparator circuit when the criteria are not stringent. In the A/D converter circuit, for example, a precise comparator circuit is required.

Because the internal structure of the comparator and the op amp is almost the same, the majority of its parameters (electrical characteristic parameters) are nearly identical to the op amp's parameter items (such as input offset voltage, input offset current, and input bias current, etc.).

Ⅳ. Comparator typical application circuit

Here are two simple comparator circuits as examples to illustrate their applications.

1. Cooling fan automatic control circuit

Some high-power devices or modules will generate additional heat during operation in order to raise the temperature. To maintain normal operation, heat sinks and fans are typically utilized to cool them. As shown in Figure 7, a very simple temperature control circuit is introduced. To detect the temperature of the power device, a negative temperature coefficient (NTC) thermistor  RT  is pasted on the heat sink (the temperature on the heat sink is slightly lower than the temperature of the device). There is a voltage VA when 5V is applied to the  RT and R1 resistors. The resistance of the thermistor RT decreases as the temperature of the heat sink rises, causing VA to rise. Figure 8 depicts the temperature characteristics of RT. It is a single-valued function, despite the fact that its resistance and temperature change curves are not linear (that is, when the temperature is constant, its resistance value is also a certain single-valued). The cooling fan should be turned on if the temperature is set to 80°C. The set threshold temperature TTH is 80°C, and the characteristic curve shows the resistance value of RT corresponding to 80°C. The VA value at 80°C may be determined if the resistance value of R1 remains unchanged (it is installed on the circuit board, and the value of R1 can be considered unchanged when the ambient temperature changes slightly).

Figure. 7

A voltage divider is formed by R2 and RP. Adjusting RP can affect the voltage of VB when the 5V power supply voltage is stable (excellent voltage stability) (the voltage value of the potentiometer center head). The threshold voltage set by the comparator, called VTH, is the VB value.

When designing, it is desired that once the heat sink temperature surpasses 80 °C, the cooling fan will be activated to achieve heat dissipation, and the value of VTH will be equal to the K value at 80 °C. The comparator outputs a low signal when VA>VTH, the relay K pulls in, and the cooling fan (DC motor) is powered to cool the high-power device. Figure 8 depicts the properties of VA, VTH voltage variation, and comparator output voltage Vout. It should be noted that when VA exceeds VTH, the fan activates, but the heat sink retains a significant amount of heat, and it takes some time to drop the temperature to below 80°C.

Figure. 8

It is highly convenient to adjust the threshold temperature TTH, as shown in FIG. 7, as long as the VTH value is modified as well. When the VTH value rises, the TTH rises as well; vice versa, the adjustment is quite simple. R1, R2, and RP can be easily derived once RT and the temperature parameters of RT have been determined (set the currents flowing through RT, R1, R2, and RP to be 0.1 to 0.5 mA respectively).

2. Window Comparator

The window comparator is usually made up of two comparators (dual comparator), each of which has two threshold voltages: VTHH (high threshold voltage) and VTHL (low threshold voltage), and the voltage VA are compared to the two threshold voltages. Vout outputs a high level if VTHLVAVTHH is true; if VAVTHH is true, Vout outputs a low level, as seen in Figure 10. A refrigerator alarm circuit is shown in Figure 9. The refrigerator's regular working temperature is set to 0 to 5°C (0°C to 5°C is a "window"). The comparator outputs a high level (showing that the temperature is normal) within this temperature range; if the refrigerator temperature is below 0V or above 5°C, the comparator outputs a low level, and the low level signal voltage is supplied to the microcontroller (C) as an alert signal.

Figure. 9

NTC thermistor RT is used as the temperature sensor. At 0°C, RT has a resistance of 333.1k while at 5°C, it has a resistance of 258.3k. It is roughly 1.5 uA, based on the working voltage of 1.5V and the current flowing through R1 and RT. R1's value should be determined. After determining the value of R1, the VA value at 0°C may be computed as 0.5V (where R1=665k in Figure 9) and the VA value at 5°C can be calculated as 0.42V, resulting in VTHL=0.42V and VTHH=0.5V. Figure 10 shows the current I=(1.5V-0.5V)/665k=0.0015mA flowing through the resistors R2, R3, and R4 when R2=665k is used. R4=280k may be acquired by pressing 0.5V=(R3+R4)0.0015mA and then R3=53.3k can be obtained by pressing R4I/=0.42V.

Figure. 10

The LT1017, a low-voltage, low-power, complementary-output dual-comparator with no external pull-up resistors, is used in this application.