Differential Amplifiers

An ideal differential amplifier is a circuit which amplifies the difference between two inputs, i.e., \(V_{out} = A_d \left( {V_1 - V_2 } \right)\) where \(A_d\) is the differential-mode gain and \(V_1\) and \(V_2\) are inputs. In practice, however, \(V_{out} = A_d \left( {V_1 - V_2 } \right) + A_c \left( {\frac{V_1 + V_2 }{2}} \right)\) where \(A_c\) the common-mode gain of the amplifier. The common-mode rejection ratio (CMRR), usually defined as the ratio between differential-mode gain and common-mode gain, indicates the ability of the amplifier to accurately cancel voltages that are common to both inputs. In this chapter you will see how differential amplifiers can be implemented with Op Amps. You will learn how to measure the \(A_d\) , \(A_c\) and CMRR as well.