what is the difference between opamp & differential amplifier?
Well, you can build a differential amplifier by using two op-amps.
From a practial point of view the difference is simply that a differential amplifier has TWO inputs and amplifies the DIFFERENCE between two signal; an op-amp configured as e.g. a standard non-inverting amplified will amplify a signal with respect to ground (i.e it is a single-ended amplifier).
An op-amp is a just a high gain high input impedance differential amplifier. I guess the main distinction is that an op-amp is never really intended to be operated in an open loop mode whereas an amplifier described as a "differential amplifier" may be intended for either open-loop or closed loop operation. Specifically an op-amp is usually intended for operation in a circuit configuration where the overall characteristics are primarily determined by the feedback network and therefore largely independent of the exact op-amp characteristics.
BTW, a differential amplifier with specified input impedance and gain can be constructed from an op-amp with a suitable arrangement of external resistors.
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