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LM311 - Differential Comparator IC

Pin Configuration

Pin Number

Pin Name




Emitter Output – This is the Emitter pin of the output transistor


Non-Inverting Input (IN+)

The Non-Inverting Pin of the comparator is give a variable  voltage to compare


Inverting Input (IN-)

The Inverting pin is also given a fixed voltage which is compared with the (IN+)


Ground (VCC-)

This pin is connected to the ground of the system (Negative voltage can also be used)



This pin can be used to turn off  the DC-offset voltage



This pin can be used to turn off the output stage



Collector Out – This is the Collector output of the transistor



Provide the operating voltage for the Op-Amp. For LM311 it is upto +15V


LM311 Features and Technical Specifications

  • Wide power supply Range
  1. Singe supply – 5V to 30V
  2. Dual supply – ±2.5V to ±15V
  • Maximum Current from Vcc+: 7.5mA
  • Single supply for two op-amps enables reliable operation
  • Hence capable of driving load up to 50V and 50 mA
  • Can Drive most of the TTL and MOS loads
  • Output can be Isolated from System Ground


LM311 Equivalent ICs

LT111A, NTE922


Alternatives Comparator Op-Amp ICs

LM741, LM358, LM339, LM324


Where to use the LM311 Comparator Op-Amp

The LM311 Differential Comparator Op-Amp is a very old comparator IC from TI that has been used for Voltage comparisons for a long time in electronic designs. Any Op-Amp can be made to work as a voltage comparator, but the LM311 proves itself to be advantages by housing an Output Transistor inside its package. The collector and Emitter pin of this Transistor can also be controlled by the hardware, this makes it’s suitable for many applications. The simplified schematics of the IC is shown below:

LM311 IC Schematic

This transistor can drive loads upto 50V and 50mA which is suitable for driving most of the TTL, MOS and RTL loads. The transistor can also make the Load to be isolated from the system ground, so if you are looking for a Voltage comparator to drive loads of these specifications then this IC might be the right choice for you.


How to use LM311 IC

Like all voltage Comparators the LM311 also has an Inverting Pin and a Non-Inverting Pin. If the voltage at the Non-Inverting Terminal (pin 2) is higher than the Inverting Terminal (pin 2), the output (pin 7) will also be high else the output will be low.

The LM311 can work on a Single supply voltage or in a dual supply mode. For now let’s concentrate on the +5V supply voltage circuit since this is the most used design for digital circuits. In this type, the VCC+ (pin 8) is connected to +5V supply voltage and the VCC (pin 4) is grounded to hold it at 0V potential. A sample circuit is shown below in which the Inverting Terminal is set to 2.5V and the Non-Inverting Terminal voltage is varied using a potentiometer. You can note that the Output voltage stays high when pin 2 has higher voltage than pin 7 and vice versa.

IC LM311 Working

The pins 5 and 6 on the Op-amp are used to set the balance voltage if you want to manually adjust the DC-Offset voltage. Normally these pins are not used since the Input Offset itself is much better controlled. When not in use the pins 5 and 6 should be shorted as shown above. You can also notice that the Collector pin (pin 7) of the transistor is used for output and the emitter pin (pin 1) is grounded; this type of design is called “Collector output Circuit”, however this does not have to be the case always. You can refer the other possible circuits from the LM311 datasheet (given at bottom). An Emitter Output Circuit is shown below.

Emitter Output Circuit


  • Voltage Comparator circuits
  • Can drive Relay, Lamp, Motor Etc
  • Zero Crossing detector
  • Peak voltage Detector
  • High Voltage protection/Warning
  • Oscillator circuits



LM311 IC Dimensions

Component Datasheet

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