Comparators

Why Interface Circuits?

        Computers don't exist in a vacuum.  They have to interface with the world in many ways.  You can sit at a keyboard and type and you're using a computer interface.  You click a mouse button and that's another interface.

        Measurement devices often need to communicate with computers also.

• A computer control system needs temperature values in order to compute whether a valve should be opened.

• A computer control system needs to be able to set a voltage in order to control the speed of a motor.

These kinds of needs - and many others - show why interface circuits are necessary.

        You'll need to use a comparator, sooner or later.  Here's what you'll need to learn about comparators.

• Given a need to develop a "zero-one" signal that depends upon whether an analog signal has exceed a limit,

• Be able to use a comparator to generate the desired signal.

• Be able to wire the comparator correctly - for an LM339.

What Is A Comparator?

        A comparator is the simplest circuit that moves signals between the analog and digital worlds.  What does a comparator do?

• Simply put, a comparator compares two analog signals and produces a one bit digital signal.  The symbol for a comparator is shown below.

• The comparator output satisfies the following rules:

• When V₊ is larger than V_- the output bit is 1.

• When V₊ is smaller than V_- the output bit is 0

        Just to give you an idea of how a comparator works, here is a simulation of using a comparator.  Set the voltages on the control panel to adjust the voltage inputs to the comparator.

What About Real Comparators?

        Real comparators may work like the one in the simulation, but there are sometimes other considerations.  For example, a common comparator is the LM339, which come on a chip with four comparators.  The four comparators are all open collector outputs.  We need to discuss that.

• Here's a diagram of the output circuitry showing how the comparator is connected to the output transistor, and how the collector of the transistor is connected to the output terminal on the chip.

        In this situation, you don't need to know a lot about transistors (although it's a good idea to learn that if you don't know it!).  What you need to know is that, in this situation, the transistor acts like a switch.  A transistor doesn't always act that way, but it does in this situation.

• When the output of the comparator is a 1, current flows from the comparator through the base of the transistor, out the emitter to ground, as shown.

• When that current flows, the transistor acts like a switch that permits current to flow from the collect to the emitter to ground.

        The way you connect the comparator is to put your load between five volts and the collector connection on the chip - like this.

Finally, if you want to use an LM339, you'll need this pin-out.