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What are the differences between a differential amplifier and an op-amp? Both have two inputs. How do they differ from each other?

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A differential amplifier traditionally has differential outputs: -

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Opamps almost without exclusion have a single output. However, over the years the distinction has become a bit cloudy. For instance, this is also called a differential amplifier: -

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Basically it's an op-amp with a differential input configuration.

Then there is the difference amplifier that I would say is the more correct way to describe the op-amp circuit above. There is also the Instrumentation amplifier (really just a difference amplifier): -

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I view a differential amplifier as having

1) current source

2) two matched transistors (or tubes) having 2 outputs that are transconductors

3) most likely one or two current-to-voltage converters, such as resistors

Most opamps have 1 and 2, with lots of additional circuitry to implement the (3), provide frequency-response shaping to make easy the use in feedback loops, the addition of short-circuit protection, etc.

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I'm a digital designer, not an analog one, but I took plenty of analog classes in school, so hopefully this still applies...

A differential amplifier is any amplifier that responds to the difference of two signals. Therefore, all op amps are differential amplifiers. An op amp is a differential amplifier that has high gain, high input impedance, and low output impedance. It can have a single ended output, or differential output.

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