In an opamp configurated as a unity-gain buffer, output voltage amplitude and input voltage amplitude should ideally be equal. However should the output DC level be equal to input DC level as well?
Here is the configuration of the unity-gain buffer.
source: http://www.learningaboutelectronics.com/Articles/Unity-gain-buffer
Ideally yes.
If you look closely, you may find it's some small but roughly constant voltage away from the input. This is known as the "input offset voltage" and can be up to 5 or 10mV in some opamps, 1mV in others, or microvolts in "chopper stabilised" opamps where DC accuracy really matters. (Check datasheets for the details)
Some opamps have offset trimming inputs which allow you to add a pot and trim that offset out.
If your amplifier is configured with gain, that input offset is multiplied by that gain, so many high gain amplifier circuits take care to reduce the gain to 1 at DC.
If the following circuit is what you are referring to, then yes. The only limitation, which applies in both the AC and DC cases is that the voltage at the output cannot exceed the rail-voltages powering the op-amp.
Yes. That configuration is a unity-gain follower: the output will track the input for an ‘ideal’ op-amp.
A real op-amp has a limited gain, in the 10K to 1 million range. There will be a small difference because of that - the gain error. A gain of 10^6 op-amp wired as unity gain will have an error of one microvolt per volt.
More about that here: How is op amp output not zero if inputs have the same voltage?
