Op Amp LM 311 is capable of driving DTL, RTL, TTL, or MOS logic circuits etc. The output can switch voltages to 50 volts at currents to 50 mA. IC LM311 operates between 5 and 30 volts single supply so it can be used to drive Relays, Lamps, Solenoids etc. Compared to other general purpose Op Amps, the pin connection of LM311 is different.
LM 311 Inputs
Like all other single Op Amps, LM 311 also has two inputs – Inverting input and Non inverting input. In typical Op Amps, the inverting input (negative) is pin 2 and Non-inverting (positive) input is pin3. But is LM311, the condition is just reversed. Pin 2 is non inverting input and pin3 is inverting input.
Output stage of LM 311 is different from a typical Op Amp like 741. This modification enhances the flexibility of LM 311. It has two outputs. Pin 7 is the positive output and forms the collector of the NPN output transistor. Pin 1 is the negative output that forms the emitter of the output transistor. Usually pin 1 is grounded to allow collector-emitter flow of current. Since the positive output is the collector of the output transistor, it is current sinking. Normally in the off state, the collector of output transistor is pulled to Vcc .The output transistor saturates when its base gets more than 0.7 volts. When this happens, the output is pulled to emitter voltage. That is the load turns on when the output goes low. In typical Op Amps, the output is current sourcing and gives current when the output becomes high.