Difference Amplifier with Equation Example

Difference Amplifier

An amplifier that amplifies the difference existing between two input signals and provides an output in proportion to this difference is called a difference amplifier. In other words, an amplifier that provides output equal to or in proportion to the difference between two input signals is known as a difference or Subtractor amplifier. A difference amplifier has been shown in figure 8.63.

Figure 8.63 – IC op-amp difference amplifier. With R₃-R₁ and R₄=R₂, the output voltage is V_o= R₂ (V_S2-V_S1)/R₁

A difference amplifier comprises two inputs (i.e. inverting and non-inverting) and like other operational amplifier circuits, a practical amplifier also has negative feedback. As inputs are supplied on inverting and non-inverting terminals of a difference amplifier circuit, therefore this type of amplifier is a combination or collection of both inverting and non–inverting amplifiers. If input terminal 2 is grounded, the circuit operates as an inverting amplifier. Signal V_S2 provides V₄ getting divided parallel to resistor R₃ and R₄ which are received on the non-inverting input of the operational amplifier. If terminal 1 is grounded, R₁ and R₂ operate as feedback components of non- inverting input of an operational amplifier. Thus, voltage V4 amplify as an input provided on a non – inverting amplifier. When V_S2 value is zero, i.e. V_S2 = 0, the value of first output will then be:

V₀₁ = -R₂/R₁ x V_S1

When V_S1 value is zero, then V_R4 and V₀₂ values will be as follows:

V_R4 = R₄/ R₃ +R₄ x V_S2

V₀₂ = R₁+R₂ /R₁ x V_R4

By substituting value of VR4 in the above-mentioned equation,

V₀₂ = R₁+R₂ /R₁ x R₄/R₃x R₄ x V_S2

If R₁ = R₂ = R₃ = R₄, then

V_O2 = R₄/R₁ x V_S2 =R₂/ R₁ x V_S2

The value of output signal in the presence of both the signals will be as follows:

V₀ = V₀₂+V₀₁

= R₂/R₁ x V_S2 – R₂/R₁ x V_S1

=R₂/R₁ x (V_S2-V_S1)

Or V₀ = A_v (V_S2-V_S1)

Here Av means voltage gain and its value is equivalent to R₁ and R₂. It is obvious from the above equation that each input voltage has been multiplied by its input gain and the resulting total output equals to the difference between the two. Suppose that R₁=R₂, then the value of output voltage would be as follows:

V₀= (V_S2 – V_S1) = Difference of the two-input ratio

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