Hello can some one tell me why a common mode feedback circuit (CMFB) is required in a fully differential structure and not in single ended structure. A single ended structure also has a DC output which can vary and change the mode of operation of transistors used in a single ended structure but does not require a CMFB

 

Hi,

You should find the answer to that question in any CMOS analog IC design book, e.g. Baker or Razavi. Here, just a short note. Notice that in a single ended you really control every node voltage just by the proper design. Why would you need a CMFB circuit in e.g. 2-stages classical class A opamp: http://www.mtcmos.com/class-a/ ? You can find the explanation of this architecture in the preview here: https://payhip.com/b/5Srt on page 50 (chapter 3.1.1). It should be clear now that no CMFB is needed.

In a fully differential structure CMFB is needed. How can you control output voltages here: http://slideplayer.com/slide/3293189/11/images/3/Folded-Cascode+Op-Amp.jpg if not by using CMFB?