Operational Amplifier as Differential Amplifier
Operational Amplifier as Differential Amplifier
Derivation of the equation for the output voltage considering that all 4 resistors R1 to R4 are of different values:
Case: VIN1 = 0 and thus Vi- = 0:
Calculation of Vi+ via voltage divider:
For the partial output voltage VOUT+ you get:
Eq. (1) in Eq. (2) results in
Case: VIN2 = 0 and therefore Vi+ = 0:
Bringing together equations (3) and (4):
If the input voltage in both inputs should be acquired in the same way:
The symmetry of the resistors must be right, i.e. the resistance ratios R1=R2 and R3=R4 must apply so that the operational amplifier, as a differential amplifier, acquires the same voltages at its inputs in the same way:
Exercise:
In the following circuit you see a differential amplifier using an OP-amp. Calculate the resistor R4.
Operational Amplifier as Differential Amplifier