Comparator (analog technology)
A comparator is an electronic circuit that compares two voltages . The output shows in binary / digital form which of the two input voltages is higher. It is practically a 1-bit analog-digital converter .
function
A signal is available at the output of the comparator which indicates which of the two input voltages is higher. If the voltage at the positive, non-inverting input is higher than the voltage at the negative, inverting input, the output voltage approaches the positive supply voltage. In the opposite case, the output voltage goes against the negative supply voltage.
If the input voltage with a noise voltage "contaminated", or has only a small slope , a is positive feedback ( regenerative feedback ) is used, which causes the comparison voltage as a function that changes from the previous state. This prevents the output voltage from switching too frequently and irregularly in the event of very small fluctuations in the input voltage. This special circuit of a comparator with hysteresis is also called a Schmitt trigger .
A window comparator is made up of two comparators with a common output, which indicates whether the input signal lies between two defined minimum and maximum values (which form the "window").
Internal circuit
A comparator is always a differential amplifier (yellow) followed by an amplifier (orange) and an output stage (blue). In contrast to the operational amplifier , it intentionally contains no frequency compensation in order to achieve a high edge steepness at the output . Therefore it cannot be operated with negative feedback , it would act as an oscillator with a frequency that is difficult to define.
Conventional operational amplifiers, which are designed for linear operation with negative feedback, should not be used as comparators, since their recovery times from saturation are usually too long or undefined. In addition, almost all operational amplifiers have an internal capacitor for frequency response compensation, which reduces the gain at high frequencies. This is exactly what is undesirable with comparators. Integrated circuits like LM339 or LM393 have several comparators on one chip and often have an open collector output. The output transistor is switched through when the inverting input has a higher potential than the non-inverting input.
| Entrances | output |
|---|---|
| -> + | closed, 0 |
| +> - | open, 1 |
Applications
A comparator is the simplest circuit to digitize an analog signal - for example the signal from a sensor -, i.e. to make a yes / no decision. This is used to check whether there is a sound, whether a preselected temperature is exceeded or the necessary operating voltage of a device is not reached. Microcontrollers often have comparator inputs to make external circuits superfluous. Examples from image processing are explained in more detail using threshold value methods .
Analog-digital converters often contain entire chains of comparators in order to be able to detect even small changes in analog signals.
Comparators are frequently used components or circuit elements, e.g. B.
- Two-point and three-point controllers , e.g. B. Temperature switches / thermostats
- Trigger circuits , e.g. B. in frequency counters and oscilloscopes
- in analog-to-digital converters
- in analog timers, e.g. B. Staircase light
- in switched-mode power supplies for regulation and current limitation
- as a pulse width modulator
- as protective circuits against over / undervoltage
Web links
- LT1016 data sheet (PDF file; 1.19 MB)
- Comparator explanation in the "Electronics Compendium"
- From window comparator to precision Schmitt trigger
literature
- Ulrich Tietze, Christoph Schenk: Semiconductor circuit technology , Berlin: Springer 2002. ISBN 3-540-42849-6
Individual evidence
- ↑ Data sheet LMx39-N, LM2901-N, LM3302-N Low-Power Low-Offset Voltage Quad Comparators , TI , document: SNOSBJ3E, November 1999, revision December 2014