Home > Circuit Bricks > Comparator
UPDATED:.20:58 21 October 2013
FUNCTION BLOCK
DESCRIPTION


  • This is a high precision decision device, it can compare differences in inputs down to a millionth of a volt!
  • It has 2 inputs, "Inverting" (+IP) and "Non-Inverting" (-IP)
  • When Non-Inv. > Inv, the output is on.
  • When Non-Inv. < Inv, the output is off.
CIRCUIT DETAILS
There are a few ways to design comparators, but the most general purpose circuit is shown below.
Circuit Diagram - Basic Comparator
Inputs

The inputs have a very high impedance (high resistance) so no significant current will flow into them.
This can cause problems with some circuits so sometimes a pull-down resistor of about 1M may be required to stablise the circuit.

Output

The output will switch from:

0.9v when off,
(+V - 0.9v) when on.

Circuit Diagram - Comparator with Buffer

Sometimes it can cause problems when the OFF voltage is as much as 0.9v. This is enough to turn on most transistors, (0.7v)

An improved circuit is shown right with a potential divider and a transistor inverter as a buffer to produce correct output voltages.

NOTE: The output is inverted so the inputs have been swapped also so the circuit still functions in the same way.
PACKAGE DETAILS
Integrated Circuit
NPN Transistor

Pin
numbers

When using ICs, it is always best to use an IC socket so the IC can be removed easily if needed.
The 555 timer comes in an 8pin DIL package.
C B E
Written by Phil Townshend - 2008
www.edutek.ltd.uk - Working Electronics For Students & Teachers