1. An inverting amplifier - Leg two is the input and the output is always reversed or inverted.
2. A Non-inverting amplifier - Leg three is the input and the output is not reversed.

Opposite is a diagram of an INVERTING AMPLIFIER. This means that if the voltage going into the 741 chip is positive, it is negative when it comes out of the 741. In other words it reverses polarity (inverts polarity).
Two resistors are needed to make the 741 work as an amplifier, R1 and R2. In most text books diagrams like this are used to represent the 741.

HOW TO CALCULATE THE 'GAIN'

An operational amplifiers purpose is to amplify a weak signal and this is called the GAIN.

INVERTING AMPLIFIER

GAIN (AV) = -R2 / R1

Example : if R2 is 100 kilo-ohm and R1 is 10 kilo-ohm the gain would be :

-100 / 10 = -10 (Gain AV)

If the input voltage is 0.5v the output voltage would be :

0.5v X -10 = -5v

NON-INVERTING AMPLIFIER

GAIN (AV) = 1+(R2 / R1)

Example : if R2 is 1000 kilo-ohm and R1 is 100 kilo-ohm the gain would be :

1+ (1000/100) = 1 + 10
OR
GAIN (AV) = 11

If the input voltage is 0.5v the output voltage would be :

0.5 X 11 = 5.5v

The polarity of a signal is reversed at the output, pin six.

A negative input becomes a positive output.

A signal applied keeps its polarity at the output, pin six.

A positive input remains a positive output.

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