The figure shows the open loop configuration of differential amplifier in which the input signals Vin1 and Vin2 are applied to the positive and negative input terminals since the op-amp amplifies the difference between the two input signals,this configuration is called differential amplifier.Vin1 and Vin2 could be either ac or dc voltages.The source resistances Rin1 and Rin2 are negligible compared to the input resistance Ri.The voltage drop across the resistors can be assumed to be zero,which then implies that V1=Vin and V2=Vin2.The the output voltage Vo=A(Vin1-Vin2).

The output voltage is equal to the voltage gain A times the difference between the two input voltage.The polarity of the output voltage is dependent on the polarity of the input difference voltage(Vin1-Vin2).In open loop configuration gain A is commonly referred to an open loop gain.

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