A voltage *amplifier* is a special circuit that accepts
an input voltage, and outputs a voltage,
that is proportional to the
input voltage. The proportionality factor is called
the *gain* of the amplifier. If , then the
amplifier actually does *amplify* the input voltage. If
, then the amplifier *attenuates* the input
voltage.

An *operational amplifier* or op-amp is a special
integrated circuit that accepts two input voltages,
and . The op-amp's output is a single voltage
(relative to ground), such that

and such that is a very very large number. In other words, an operational amplifier is an integrated circuit that behaves like a high-gain difference amplifier. It amplifies the difference between two input voltages.

The symbol for an operational amplifier is a triangle that
has two inputs and a single output. This symbol is shown
below in figure 2. The input with a
positive sign is called the non-inverting terminal and the
input with the negative sign is called the inverting
terminal. In addition to the two inputs and single output,
the op-amp must have two *supply voltages*. These are
shown be the two extra lines coming out of the top and
bottom of the triangle in figure 2. The
output voltages generated by the op-amp will be confined to
lie within these two supply voltages. To function properly
the top supply voltage should be at least 7-9 volts and the
bottom supply can be anything less than or equal to 0
volts.

As mentioned above the op-amp is an IC that acts as a
high-gain difference amplifier. The gain is, in fact, very
large, somewhere on the order of . In addition
to this the op-amp circuitry is designed so that the device
has a very high input resistance and very low output
resistance. This means that we can model the op-amp using
a dependent voltage controlled voltage source. A *
dependent* source is a voltage/current source whose value
is a function of some other voltage/current in the circuit.
Your textbook should discuss these idealized circuit
elements in more detail.

Dependent voltage sources are a very good
approximation for the op-amp's behavior. In other words,
the op-amp is a circuit that has been *engineered* to
be well approximated by an idealized circuit element . This
means that we can use op-amp models in a reliable manner to
predict the behavior of op-amp circuits with high
confidence that our analytical predictions will be
duplicated by the physical device. This simple fact makes
the op-amp one of the most useful building blocks in analog
circuit design.

To operate properly, the op-amp must be supplied a voltage
that is larger than the range of differential input
voltages. These other voltages are called *supply
voltages* and they are denoted as
and
in figure 2. In practice there
are two types of op-amps. Double side op-amps have supply
voltage of volts (where is some positive
voltage between 9 and 15 volts). This means that the output
of the op-amp can swing between these positive and negative
supply voltages. A single sided op-amp has a supply voltage
of volts and ground. This means that the output can
only swing from 0 to volts. In our labs we'll be
using a single sided op-amp known as the LM660.

The op-amp you've been supplied with in your kit is a standard single-sided quad op-amp (LM660). By single sided, we mean that the supply voltages are volts and ground (rather than volts). By quad, we mean that there are 4 op-amps on a single chip. The pin-out for the LM660 is shown below in figure 3.