I was wondering whether or not DC voltages can be amplified by op-amps. I thought about a typical Miller op-amp structure (differential pair input) and I suppose it could amplify a DC voltage.
I am slightly confused though, does the open-loop gain of the op-amp as a whole not change when faced with DC input voltages compared to small-signal AC voltages? We always derive our op-amps small-signal gain, DC large-signal gain will be different right?
From the Analog IC Design concepts I have learned, we always used to talk about the small-signal gain of different amplifier stages (common-source, gate, etc.) The DC voltages was just for bias. For example, lets say my amplifier was a MOS common-source stage with a drain resistance, the open loop small-signal gain would be -gmRd. But that's only small-signal, not a DC voltage.
Best Answer
Let's first get this out of the way: the inputs have to be very close to each other in voltage for the op-amp to do anything besides be a comparator. This is because op-amp open loop DC gain is large, in the thousands to tens of thousands, and can vary by an order of magnitude in some cases. More about that here: https://www.analog.com/media/ru/training-seminars/tutorials/MT-044.pdf
That said, an op-amp can be configured with feedback to amplify DC as well as AC, small signals or large. It will nearly perfectly give the gain you're seeking so long as you understand the limits of what the particular op-amp you're using can do.
I think maybe what you're asking is, "is the gain affected by the DC offset of the signal pair?" The answer is yes, somewhat. The specification for this is expressed as the Common Mode Rejection Ratio, or CMRR. More about that here: https://www.elprocus.com/common-mode-rejection-ratio-cmrr-operational-amplifier/
Op-amps also have input offset error which affects DC gain fidelity.