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Comparators
An operational amplifier without feedback, as in Fig. 1.61, represents the basic circuit of a comparator. Its output voltage is given by
Owing to the high gain, the circuit responds to very small voltage differences U1 - U2. It is thus suitable for the comparison of two voltages, and operates with high accuracy.
Fig. 1.61 - Operational amplifier as Fig. 1.62 - Transfer characteristic. comparator.
At zero crossing of the input voltage difference, the output voltage does not immediately reach the saturation level because the transition is limited by the slew rate. For frequency-compensated standard operational amplifiers it is about 1 V/ms. A rise from – 12 V to + 12 V therefore takes 24 ms. An additional delay is incurred due to the recovery time needed after the amplifier has been saturated. Fig. 1.63 - Comparator with logic output. Fig. 1.64 - Transfer characteristic.
y = 1 for U1 >U2
As the amplifier possesses no negative feedback, it also does not require any frequency compensation. Its omission can improve the slew rate and recovery time by a factor of about 20.
Considerably shorter response times can be attained when using special comparator amplifiers which are designed for use without feedback and have especially small recovery times. However, the gain and hence the accuracy of the threshold are somewhat lower than those of operational amplifiers. Usually, the amplifier output is directly connected to a level-shift circuit which permits compatible operation with logic circuits. Its application and characteristics are shown in Figs. 1.63 and 1.64 A number of commonly used comparators are listed in Fig. 1.65. Fig. 1.65 - Examples of comparators.
Date: 2015-01-12; view: 911
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