Determining if one voltage is greater than or less than another voltage is the job for a comparator. With two analog voltages as inputs and a digital output, this device is sometimes called a one-bit A/D converter. The comparator looks like an operational amplifier (op amp) in an open-loop configuration.

However, there are several significant differences between the two devices. The transfer function for a comparator is shown in Figure 1.

Figure 1: Comparator transfer function
(Click on image to enlarge)

From the transfer function shown in the figure, you can see that as long as In+ is greater than In-, the output is high. When In+ is less than In-, the output is low. In common applications, one of the inputs is held at a fixed reference voltage and the signal being tested moves above and below the reference. However, some applications have both signals moving.

Just as with op amps, the comparator input stage is characterized by offset voltage, bias current and common mode voltage range. The value of these parameters is of concern only as it impacts the switching point.

Since the comparator is a transition device between analog and digital, there is a time specification on the action of the device. This is the propagation time: the time delay from when the input crosses the switch point to the output switching. The output actually switching is a function of the level of overdrive. Propagation delay may be given for 10 mV and 100 mA overdrive. Additionally, the device may not be symmetric in its response, so there may be one set of specs for the low-to-high transition, and another for the high-to-low transition.

One characteristic that divides the comparators into two groups is the output stage design shown in Figure 2. One group has a full push-pull stage, while the second group has an open collector or open drain stage.

Figure 2: The two output stages of comparators.
(Click on image to enlarge)

There are several application differences between the two output types. The push-pull output swing is limited to the supply rails, while the upper limit for the open-collector stage is determined by the voltage that the output transistor can tolerate. The output of the push-pull stage is actively pulled in each direction, while the open collector only has an active pull down.

Because of this, the low-to-high transition time is set by the pull-up resistor. It is possible to connect several open-collector devices in parallel, which will result in what is termed a collector-or configuration. The actual logic function is NOR. If any device is activated, the common output line is pulled low.

In special situations you could use an op amp as a comparator, but never try to use a comparator as an op amp. Op amps are designed for linear operation, with high open-loop gain and for stable operation at low gains. None of these concerns were addressed in the comparator design of a. Therefore, a comparator should not be considered in place of an op amp.

Some op amps may be considered for comparator service. First, determine if the op amp has internal circuitry to limit the voltage between the input pins, such as anti-parallel diodes. This op amp design feature would eliminate the device from consideration. Be sure to check the data sheet to be sure the absolute maximum differential input voltage is not violated. Typical propagation times for op amps used as comparators will be very slow. Since an op amp used as an open-loop comparator is always in saturation, the saturation recovery time becomes critical.

About the author



William P. (Bill) Klein is a Senior Applications Engineer with the High Performance Analog group at Texas Instruments. Bill joined TI through its acquisition of Burr-Brown in August 2000. His experience as an analog circuit designer covers over 40 years in fields ranging from mineral exploration to medical nuclear imaging. One current role Bill has is hosting the Analog e-LAB Web Cast, presenting real world solutions to real world problems in analog circuit design. In addition to a BSEE from Arizona State University and registration as a Professional Engineer in the State of Arizona, he has authored numerous magazine articles, application notes and conference papers.

Previous installments of this series:

• "SIGNAL CHAIN BASICS (Part 15): Analog/digital converter—dynamic parameters", www.planetanalog.com/features/showArticle.jhtml;?articleID=208401183, click here
• "SIGNAL CHAIN BASICS (Part 14): Analog/digital converter—static parameters", www.planetanalog.com/features/showArticle.jhtml;?articleID=207800114, click here
• "SIGNAL CHAIN BASICS (Part 13): Putting the Bode plot to use", www.planetanalog.com/features/showArticle.jhtml;?articleID=207403561, click here
• "SIGNAL CHAIN BASICS (Part 12): The Bode plot, an essential ac-parameter display tool", www.planetanalog.com/features/showArticle.jhtml;?articleID=207403561, click here
• "SIGNAL CHAIN BASICS (Part 11): Introducing voltage- and power-conditioning circuits", www.planetanalog.com/features/showArticle.jhtml;?articleID=207001505, click here
• "SIGNAL CHAIN BASICS (Part 10): Exploring the Delta-Sigma Converter", www.planetanalog.com/features/showArticle.jhtml;?articleID=206903892, click here
• "SIGNAL CHAIN BASICS (Part 9): SAR Converter Operation Explored", www.planetanalog.com/features/showArticle.jhtml;?articleID=206901015, click here
• "SIGNAL CHAIN BASICS (Part 8): Flash- and Pipeline-Converter Operation Explored", www.planetanalog.com/features/showArticle.jhtml;?articleID=206504089, click here
• "SIGNAL CHAIN BASICS (Part 7): Op Amp Performance Specification--Bias Current", www.planetanalog.com/features/showArticle.jhtml;?articleID=206101908, click here
• "SIGNAL CHAIN BASICS (Part 6): Op Amp Input Voltage Offset", www.planetanalog.com/features/showArticle.jhtml;?articleID=205901111, click here
• "SIGNAL CHAIN BASICS (Part 5): Introduction to the Instrumentation Amplifier", www.planetanalog.com/features/showArticle.jhtml;?articleID=205208593, click here
• "SIGNAL CHAIN BASICS (Part 4): Introduction to analog/digital converter (ADC) types", www.planetanalog.com/features/showArticle.jhtml;?articleID=204803631, click here
• "SIGNAL CHAIN BASICS (Part 3): Analog and the digital world", www.planetanalog.com/features/showArticle.jhtml;?articleID=204400376, click here
• "SIGNAL CHAIN BASICS (Part 2): Op Amp--Basic operations", www.planetanalog.com/features/showArticle.jhtml;?articleID=203101699, click here
• "SIGNAL CHAIN BASICS: Operational Amplifier--The Basic Building Block", www.planetanalog.com/features/showArticle.jhtml;?articleID=202801320, click here