Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Question about common mode feedback

Status
Not open for further replies.

samster19

Newbie
Joined
May 15, 2017
Messages
5
Helped
0
Reputation
0
Reaction score
0
Trophy points
1
Activity points
73
Untitled.png


Hi, in the image above (taken from Design of CMOS Analog Design Circuits by Razavi), a high gain amplifier is used to provide common mode sense and feeback by controlling the current through M3 and M4. The idea is that if both Vout1 and Vout2 rise, then Ve rises, and the current through M3 and M4 rises, which then brings down the output common mode level.

Now I just want to know if my understanding of why exactly this brings the common mode level down is correct. Please correct me if my explanation below is wrong.

Raising Ve will raise the current through M3 and M4, but the transistors stacked on top are biased for a different current (their gate voltages are set). To satisfy KCL at Vout1 and Vout2, the drain voltages move down so that the same currents flow through the cascode PMOS and NMOS transistors and M3 and M4.

In other words, the voltages rise or fall to favor equal currents throughout the branch except for the top PMOS transistors, above the folding node.

I hope my explanation makes sense. Do correct me if I am wrong.
 

Raising Ve will raise the current through M3 and M4, but the transistors stacked on top are biased for a different current (their gate voltages are set). To satisfy KCL at Vout1 and Vout2, the drain voltages move down so that the same currents flow through the cascode PMOS and NMOS transistors and M3 and M4.
Verbose explanation of simple cascode stages. Saying the transistor "are biased for a different current" unnecessarily complicates things. The cascode transistor passes the current driven to it's source to the drain. The same with PMOS cascode stages.
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top