Op-amp Comparators part I of 3

Comparators The comparator is an op-amp circuit that compares two input voltages and produces an output indicating the relationship between them. The inputs can be two signals (such as two sine waves) or a signal and a fixed dc reference voltage. Comparators are most commonly used in digital applications. Digital circuits respond to rectangular or square waves, rather than sine waves. These waveforms are made up of alternating (high and low) dc levels and the transitions between them (as shown in Figure 1).

Comparators The purpose served by a comparator is as illustrated. Assume that the digital system is designed to perform a specific function when a sine wave input reaches a value of 10 V. Unfortunately, digital systems respond to waveforms like the one in the top Figure, not sine waves. Using a comparator between the signal source and the digital system, as shown in bottom Figure, solves the problem.

Comparators With the inverting input of the comparator tied to a reference voltage (Vref ), the output from the comparator remains low as long as the value of the input waveform is less than Vref . As soon as the value of the input exceeds Vref , the comparator output goes high. When the value of the input falls below Vref , the comparator output goes low again.

Comparators The dc output voltage from the comparator indicates the polarity (or magnitude) relationship between the two inputs. Comparators are not normally used to convert sine waves into square waves. A Schmitt trigger usually performs this function. When a comparator circuit is used to compare an input signal to a fixed dc voltage, the circuit is commonly referred to as a level detector.

Comparators The most noticeable circuit recognition feature of the comparator circuit is the lack of any feedback path. (See the comparator in the next slide.) Without a feedback path, the voltage gain of the circuit is equal to the open loop gain (AOL ) of the op-amp. With such high gain, even the slightest difference voltage at the inputs results in the output going to one or the other of the voltage extremes (depending on the input polarity).

Comparators Just as the name implies when an op-amp is used as a comparator it is comparing one signal to another. In this particular example of a zero-level detector the op-amp is in open loop configuration. The incoming signal drives the op-amp into saturation producing a square-wave output. Remember that the op-amp is driven into saturation easily when in open-loop configuration. Fig 13-1a&b zero-level config. and signals

Comparators With nonzero-level detection the voltage divider or zener diode sets the reference voltage at which the op-amp turns goes to the maximum voltage level. Fig. 13-2c&d zener ref. w/waveforms

Comparators Most electronic systems have one or more supply voltages. The reference voltage for a comparator is usually derived from one of them. Figure illustrates how a voltage-divider circuit is used to establish a reference voltage using the supply voltage of the circuit.

Comparators Remember that the comparator is configured in open-loop making the gain really high. this is open-loop configuration. This makes the comparator very susceptable to unwanted signals (noise) that could cause the output to arbitrarily switch states. Fig 13-6a&b op-amp w/noise signals

Comparators The effects of the noise can be reduced with feedback for hysteresis. This sets the trigger upper and lower trigger points, set by the voltage divider, such that it is unaffected by the minor variations caused by noise. This type of op-amp configuration is sometimes called a Schmitt trigger.

Comparators If the level of the pulse must be less than the output of a saturated op-amp a zener-diode can be used to limit the output to a particular voltage. This is called output bounding. Either positive, negative, or both halves of the output signal can be bounded by use of one or two zener diodes respectively. Fig 13-12 double-bounded coparator