As the name implies. a voltage comparator is a device used to compare .wo voltage levels. The output of the :;OTr” :>tor reveals which of its two inputs is larger, so it is basically a switching device, producing a high output when one input is the larger, and switching to a low output if the other input larger. n operational amplifier is used as a voltage comparator by operating it open-loop (!’IO feedback) and by connecting the two voltages to be compared to the inverting and nornn erting inputs. Because it has a very large open-loop gain, the amplifier’s output is driven all the way to one of its output voltage limits when there is a very <-nall difference between the input levels. For example, if the + input voltage is slightly greater than the – input voltage, the amplifier quickly switches to its maximum positive output, and when the – input voltage is slightly greater than the + input voltage, the amplifier switches to its maximum negative output. * This behavior is illustrated.
Note that Figure 15-1(b) is a transfer characteristic showing output voltage versus differential input voltage, v+ – V-. It can be seen that the output switches when v+ – u: passes through O. To further clarify behavior of the comparator, Figure 15-2 shows he output waveform when the noninverting input is a 10- V-peak sine wave and a +6- V-dc source is connected to the inverting input. The comparator output is assumed to switch between :t15 V. Notice that the output switches to +15 Veach time the sine wave rises through +6 V, because u’ – u: = (6 V) – (6 V) = 0 V at those points in time, The output remains high so long so v+ – o: > 0, i.e., v+ > 6 V, and when v+ falls below 6 V. the comparator output switches to -15 V, As an exercise, plot the output when the sine wave is connected to the inverting input and the +6- V-de source is connected to the non inverting input. In some applications, either the inverting or noninverting input is grounded, so the comparator is effectively a zero-crossing detector. It switches output states, Many authors use the term saturation voltage to describe the minimum or maximum output voltage of the amplifier. We are avoiding this use only to prevent confusion with the saturation voltage of a transistor ••which, as we have seen. is near O.
when the ungrounded input passes through O. For example, if the inverting input is grounded, the output switches to its maximum positive voltage when v+ is slightly positive and to its maximum negauvc voltage when u” is slightly negative. The reverse action occurs if the noninverting input is grounded. The transfer characteristics for these two cases’ In the context of a voltage comparator, the if/PIli offset voltage is defined to be the minimum differential input voltage that will cause the output to switch from 1111′ comparator 0″11′”/ switches to + V””•., when v’· – v – > 0 V, which corresponds 10 the time points where u I rises through +6 V. The output remains high as long as u’ – u – > 0, or u’ > 6 V .
One state to the other. The smaller the input offset voltage, the more accurate the voltage comparator in terms of its ability to detect the equality of two input levels. Clearly, the greater the open-loop gain, the smaller the input offset voltage. For example, a gain of 20,000 will cause the output to switch from -10 V to +10 V when u” – v- is (20 V)/20,OOO = 1 my.
Two other important characteristics of a voltage comparator are its response time and rise time, illustrated in Figure 15-4. The response time IS the delay between the time a step input is applied and the time the output begins to change state. It is measured from the edge of the step input to the time point where the output reaches a fixed percentage of its final value, such as 10% of + Vnllu’ (For clarity,the output switching from 0 V toward +Vmax ; in many applications, one of the output levels actually is 0 V.) Response time is strongly dependent on the amount of overdrive in the input: the voltage in excess of that required to cause switching to occur. The greater the overurive, the shorter the response time. Rise time is defined in the usual way: the time requires for the output to change from 10% of its final value to 90% of its final value. Recall that rise time is inversely proportional to amplifier bandwidth: I, .35/BW. Note this important point: A large voltage gain improves the input offset voltage (reduces it), but lengthens the rise time, because large gains mean smaller bandwidths, the gain-bandwidth product being constant.
Although general-purpose operational amplifiers can be, and are. used as voltage omparators in the way we have described, there are also m re elaborate, specially designed operational amplifiers manufactured and marketed specifically for voltage-comparator applications. One feature of some comparators is their ability to switch between output levels that are not necessarily related to the amplifier supply voltages. These are useful in digital systems where level is necessary to interface logic circuitry of different types. For example, one part of a digital system may be designed to operate with logic levels of “one” (high) = +5 V and “zero” (low) = 0 V, while another part of the system uses “one” = o V and “zero” = -10 V. A level-shifting voltage comparator can be used to make these components compatible.