# Why the offset of source-follower（which connected to the output of opamp as voltage buffer) will limit the op-amp output swing ？

#### ALLENRAZAVISedra

Joined Feb 15, 2024
1
I am currently reading Gray,Hurst's book Analysis and design of analog integrated circuits, chapter 12, the Common-mode feedback circuit. On page 815, he said the following statement for improvement of resistive output loading of CMFB circuit.
The following figure 12.17 is the CMFB circuit after improvement, the previous figure 12.16 is the CMFB circuit before improvement.

"To avoid this resistive output loading, voltage buffers can be added between the op-amp outputs and the Rcs resistors. Source followers are used as buffers in Fig. 12.17. One potential problem is that each source follower introduces a dc offset of VGS between its input and output. To avoid a shift in the CM operating point caused by these offsets, voltage VCM can be buffered by an identical source follower so that the op-amp output voltages and VCM experience equal offsets. However, these offsets limit the op-amp output swing since each source-follower transistor that connects to an op-amp output must remain in the active region over the entire output voltage swing."

1: He said that

To avoid this resistive output loading, voltage buffers can be added between the op-amp outputs and the Rcs resistors. Source followers are used as buffers in Fig. 12.17
Does what he mean is that the previous fig12.16's voltage sensing circuit has a too high output impedance thus we use source follower as a voltage buffer to have a lower output impedance of the sensing circuit?
2: For the second sentence

To avoid a shift in the CM operating point caused by these offsets, voltage VCM can be buffered by an identical source follower so that the op-amp output voltages and VCM experience equal offsets.
Why by buffering VCM to let op-amp output voltages and VCM experience equal offsets could avoid a shift in the CM operating point?

3: As what his last sentence said, why the offset of source-follower（which connected to the output of opamp as voltage buffer) will limit the op-amp output swing ？
Any comment, reference, answer is welcome, and I am really appreciating for anyone who could give me any hint, thank you very much!