LM311 is an excellent Operational amplifier capable of driving DTL, RTL, TTL, or MOS logic. The output can switch voltages to 50 volts at currents to 50 mA. IC LM311 operates between 5 and 30 volts single supply. It can be used to drive Relays, Lamps, Solenoids etc. But when compared to other Op Amps, the pin connections of LM311 are different. This design note will help you to design circuits based on LM311.
Like all other single Op Amps, LM 311 also has two inputs. These are 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. Pin2 is non inverting input and pin3 is inverting input.
LM311 pins connection
Output stage of LM 311 is different from a typical Op Amp line 741. This modification enhances the flexibility of LM311. 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.