Comparator

A comparator, an electronic circuit that compares two voltages. The output displays in binary / digital form, which of the two input voltages higher. Thus it is practically a one -bit analog-to- digital converter.

Function

At the output of the comparator is a signal, which indicates which of the two input voltages higher. When the voltage at the positive, the noninverting input is higher than the voltage at the negative -inverting input, the output voltage approaches the positive supply voltage. In reverse conditions, the output voltage goes to the negative supply voltage.

If the input voltage " contaminated " with a noise voltage or does it only slight slope, a positive feedback ( positive feedback) is used, which causes changes the reference voltage depending on the previous state. This avoids the case of very slight variations of the input voltage, the output voltage too often and irregularly switches. This special circuit of a comparator with hysteresis is also a Schmitt trigger.

A window is formed of two comparators with a common output that indicates whether the input signal between two defined minimum and maximum values ​​( forming the " window " ) is located.

Internal circuit

A comparator is always a differential amplifier ( yellow) with subsequent amplifier (orange) and an output stage ( blue). It contains - in contrast to the operational amplifier - intentionally no frequency compensation in order to achieve a high slew rate at the output. Therefore it can not be operated with negative feedback, he would act as an oscillator with difficult definable frequency.

Conventional operational amplifiers which are designed for the linear feedback operation, should not be used as a comparator, since the recovery time of the saturation is usually too long and undefined. Moreover, almost all the operational amplifiers have internally a capacitor for frequency compensation, which reduces the gain at high frequencies. That is undesirable in comparators. Integrated circuits such as LM339 or LM393 have multiple comparators on a single chip and often have an open collector output. Here, the output transistor is switched on, if the inverting input has a higher potential than the non-inverting input.

Applications

A comparator is the simplest circuit to an analog signal - for example, the signal of the sensor - to digitize, so come to a yes / no decision. In order to check whether a sound is there or exceeded a preselected temperature or the necessary operating voltage of a device is exceeded. Microcontroller comparator inputs often have to make external circuits superfluous. Examples of image processing are explained in detail in thresholding.

Analog- to-digital converter is often contain all the chain of comparators in order to detect small changes in the analog signals can.

Comparators are blocks or circuit elements frequently used, eg

• Two -point and three -point controller, such as thermostats
• Trigger circuits, for example, frequency counters, and oscilloscopes
• In analog to digital converters
• In analog timers, such as staircase lighting
• In switching power supplies for control and current limiting
• A pulse width modulator
• As protection circuits against Over-/undervoltage