Non - inverting amplifier from a voltage comparator - LM139

This circuit uses an LM139 (LM339) voltage comparator to form a Non-inverting amplifier. The LM139 comparator can be used as an operational amplifier in DC and very low frequency AC applications (<100Hz). An interesting combination is to use one of the comparators as an op amp to provide a DC reference voltage for the other three comparators in the same package. Another useful application of an LM139 has the interesting feature that the input common mode voltage range includes ground even though the amplifier is biased from a single supply and ground. These op-amps are also low power drain devices and will not drive large load currents unless current boosted with an external NPN transistor. The largest application limitation comes from a relatively slow slew rate which restricts the power bandwidth and the output voltage response time.