Difference Amplifier

Difference Amplifier

The difference amplifier, with four resistors, is a special configuration of the differential op amp where both the inverting and non-inverting inputs are used. It is the basic input stage of all operational amplifiers such as the 741 IC, which has an inverting and non-inverting input. The output voltage is usually a difference function of the input voltages. This type of amplifier also has a feedback resistor connecting the output to the inverting input for providing gain. This enables the op-amp to have a large differential voltage gain, which makes it extremely suitable as a comparator. There is also a resistor from the non-inverting input to the ground. When this resistor is the same value as the feedback resistor, and the two input resistors are the same value, then a special condition formula shown below may be used.

Formula

Formula

When R1 = R2, and Rf = R3, then this formula for the difference amplifier can be used. This is the standard formula GCSE students usually use, where Rf / R1 describes the gain factor. As you can see, the difference between the non-inverting input and the inverting input, multiplies by the gain factor. Therefore, this circuit amplifies the difference of the input voltages.


Gain of 100

Example - Gain=100

If the input resistors R1=R2=15 kΩ, and we require a gain of 100, then we simply use part of the above formula to calculate the gain factor.

For a gain of 100, Rf/15000 = 100, then Rf=1500000, which is 1.5 MΩ, and this is the same value that R3 needs to be as well.