What I'm trying to say is when designing op-amp circuits we use the formula:
Vout=Aol(Vinp-Vinm)
where Vout, Vinp and Vinm.
When designing a single-ended differential amplifier at the transistor level, we bias both inputs to the same DC level. If the above formula was correct, the output would be zero
View attachment 172896
For a real op-amp, it senses the small differential change in the input and amplifies this as an output change/swing.
ΔVout = Aol(ΔVinp-ΔVinm) or vout = Aol(vinp-vinm)
We use small letters to denote incremental or ac signals.
However,
ΔVinp-ΔVinm = (Vinp-Vbias) - (Vinm-Vbias) = Vinp - Vinm
So that:
ΔVout = Aol(Vinp-Vinm)
The output swing ,
ΔVout, is not necessarily equal to
Vout. If
Vout starts from a value of zero at the bias point then yes, but that is not strictly true. So I am thinking, shouldnt the formula be:
Vout = Aol(Vinp-Vinm) + Vout,bias
Vout,bias is the output at the dc operating point