Op-Amp Application: Voltage Comparators

Two reasons why LM101 is well suited to comparator applications are it has a large differential input voltage range and the output is easily clamped to make it compatible with various driver and logic circuits. LM101 doesn’t have the speed of the LM7104 (10us versus 40 ns, under equivalent conditions) but spees is not a problem in many linear applications and the lower input currents along with higher voltage ability of the LM101 is a great benefit. The following circuit diagram show two comparator circuits using the LM101.

The upper schematic is a clamping scheme which makes the output signal directly compatible with DTL or TTL integrated circuits. The output at 0V or 4V in the low or high states respectively clamped by LM103 breakdown diode. Has a sharp breakdown and low equivalent capacitance are the reasons why this particular diode was chosen. The amplifier operates open loop so normally no frequency amends is needed when working as a comparator. Nonetheless, to prevent low level oscillations when the comparator is in the active region, the stray capacitance between pins 5 and 6 of the amplifier should be minimized. A 3 pF capacitor on the normal compensation terminal will eliminate it if this becomes a problem.

The connection of the LM101 shown in the lower figure  is employed as a comparator and lamp driver. Q1 switches the lamp, with R2 limiting the current surge resulting from switching on a cold lamp. The base drive to Q1 is determined by R1 while D1 keeps the amplifier from putting too much excessive reverse bias on the emitter-base junction of Q1 when it switches off.