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# fully differential amplifier training video questions.

In part 2 of fully differential amplifier training video, the following are said

“Look closely at equation 3, and you will notice that each half of the FDA acts as a single-ended op-amp in an inverting configuration.”

It means that we can split equation 3 to: Vout-=-(Vid+)*(Rf/Rg). Vout+-=-(Vid-)*(Rf/Rg). And then in the video it's said that Vin_cm=0.

But how can split the equation like that? It's not resonable

• Hi Howard,

What about splitting the equation in this way seems unreasonable? Keep in mind that this makes no mention of the function of the Vocm pin, which is going to shift each amplifier's output accordingly to satisfy the commanded common-mode voltage. If we look at a simple example though where the input common-mode matches the output common-mode and there is no shift, then you can see that each side really does behave like an inverting amplifier.

Regards,

Zak Kaye
Precision Amplifiers Applications

• In reply to Zak Kaye:

Thank you for your answer.

From mathematical view it's not reasonable, just like you can not get "x=a" and "y=b" from "x-y=a-b".

The simulation is okay and can get the result, but the derivation process is not convincing.
• In reply to Howard Zou:

Howard,

I understand your point and the way it is presented may be a bit roundabout. Even though mathematically speaking you cannot assume x=a and y=b from x-y = a-b, in this case this ends up being true, and I think the observation was made to point that out for a functional understanding of an FDA rather than to offer a strict mathematical proof that the equation can be split. Thanks for the feedback!

Regards,

Zak Kaye
Precision Amplifiers Applications

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