A basic comparator is similar to a differential amplifier operating in the open-loop mode. Because of high gain, the output is normally saturated in either the high state or the low state depending upon the relative amplitudes of the two input voltages. With these conditions, the comparator provides a logic-state output which is indicative of the amplitude relationship between two analog input signals.
In typical applications, a comparator provides an indication of the relative state of the two input signals. Figure 2 -34 illustrates a basic comparator and its transfer function.
In the circuit in Figure 2-34, if a reference voltage is applied to the inverting input and an unknown potential to the noninverting input, the output will reflect the relationship between the two inputs. When VI is more negative that VREF, the device output will be in saturation at a logic low level. When V1 becomes more positive than VREF, the output will change states and become saturated at a logic high level.
Because comparators are normally used to drive logic circuits, the output must change states as rapidly as possible. High open-loop gain, wide bandwidth and slew rate are key factors in comparator speed. Operation in the open-loop mode (no feedback), with minimum or no frequency compensation, results in maximum gain-bandwidth product for best performance. Most comparators operate in this manner.
The ideal comparator has the same characteristics as the ideal operational amplifier. Those characteristics are as follows:
- Differential Gain = -> ∞
- Common-Mode Gain = 0
- Input Impedance = -> ∞
- Output Impedance = 0
- Bandwidth = -> ∞
- Offset Voltage and Current = 0
Initially operational amplifiers were used in the open-loop mode to perform comparator functions. However, devices designed specifically for this operation resulted in improvements in recovery time, switching speed, and output levels. Since the comparator amplifier stage is usually followed by a TTL logic stage, output logic-state levels normally match those required by TTL loads.
Circuits designed as a comparator use none of the phase/frequency compensation usually required for operational amplifier stabilization with feedback. In fact, these compensation components are detrimental because they slow the response time of the comparator. Although any operational amplifier may be used as a comparator, a compensated device (such as the TL071) will result in longer response times and an output that is not directly TTL compatible.