AllenD
Member level 5
(fully)-differential circuit tail current cource and CMFB
Hi Team
I have a question about differential circuit design and its common mode rejection.
Assuming a simple differential circuit: nmos tail current source(M1) with nmos diff pair(M2 M3) as the input stage and 2 diode connected pmos(M4 M5) as load. The gates of the nmos are the 2 input and drains of the pmos are the 2 output.
To my understanding so far
1.The tail current source is definitely provide common mode rejection.
2. I also need to add CMFB to increase common mode rejection.
3. If I instead , use a current mirror as the load, I don't need the CMFB because the current mirror itself is going to provide common mode rejection.
Q1. If my "statement 1" is correct, then the common mode is rejected by the tail current source, why do I need CMFB to reject common mode further?
Q2 The way I see CMFB is to define/regulate the output DC voltage. But if you consider a single input single output common source amplifier, the output DC voltage is undefined just like a fully differential common source amplifier without CMFB. Why nobody is talking about such feedback for a single input single output common source amplifier?
Q3 Is the tail current is for input common mode rejection and CMFB for output common mode rejection?
Thanks
Allen
Hi Team
I have a question about differential circuit design and its common mode rejection.
Assuming a simple differential circuit: nmos tail current source(M1) with nmos diff pair(M2 M3) as the input stage and 2 diode connected pmos(M4 M5) as load. The gates of the nmos are the 2 input and drains of the pmos are the 2 output.
To my understanding so far
1.The tail current source is definitely provide common mode rejection.
2. I also need to add CMFB to increase common mode rejection.
3. If I instead , use a current mirror as the load, I don't need the CMFB because the current mirror itself is going to provide common mode rejection.
Q1. If my "statement 1" is correct, then the common mode is rejected by the tail current source, why do I need CMFB to reject common mode further?
Q2 The way I see CMFB is to define/regulate the output DC voltage. But if you consider a single input single output common source amplifier, the output DC voltage is undefined just like a fully differential common source amplifier without CMFB. Why nobody is talking about such feedback for a single input single output common source amplifier?
Q3 Is the tail current is for input common mode rejection and CMFB for output common mode rejection?
Thanks
Allen