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What is a "difference" opamp? Do you mean an opamp circuit with a differential input?
Next time please post a schematic of what you are talking about.
Here is an opamp circuit with a differential input. It uses a positive and negative power supply so its input "bias" voltage is 0V. If the non-inverting voltage is positive then the output goes positive if the inverting input is 0V or is negative. If the inverting input goes positive then the output goes negative if the non-inverting input is 0V or is negative.