I am designing an op amp that consists of an active loaded single ended output differential amplifier that is connected to a buffer stage and then an output stage.
When the input signal is connected to the +Vin of the differential amp and the negative input is connected to ground, the circuit needs to be designed so that the output of the op amp's DC point is 0V.
Why does this need to be done so that when the op amp is connected for feedback it works correctly?
"Why does this need to be done so that when the op amp is connected for feedback it works correctly?"
Yes - that´s one of the reasons. In general, for each voltage amplifier it is desired to have zero output for zero input. That is the reason we use for opamps - in most cases - two supply voltages (split supply). In this case, the ouput dc voltage can be designed to be zero (for both inputs grounded, ideal case neglecting offset) with the ability to swing positive and negative.
Otherwise, in case of negative feedback - and if the dc output (Q point) is not zero - we would have a continuos current through the feedback path, even if the input is zero. This would cause an unwanted biasing of the input port. More than that, there also would be a dc current through the connected load (if there is no coupling capacitor).
From this, we can conclude that it is always simpler to realize an ac coupled amplifier chain (consisting of several internal capacitive coupled stages) than a dc coupled amplifier.
Because output circuit must be closed to make current flow possible.
Watch this:
"Introduction to the Op Amp Part" by Darryl Morrel (at youtube.com).
