yschuang said:This is how i get the relation between GBW and 2nd pole, F2nd
GBW=gm1/(2pi*Cc)
F2nd=gm2/(2pi*CL)=3GBW=3*gm1/(2pi*Cc)
gm2/CL=3gm1/Cc
yschuang said:This is how i get the relation between GBW and 2nd pole, F2nd
GBW=gm1/(2pi*Cc)
F2nd=gm2/(2pi*CL)=3GBW=3*gm1/(2pi*Cc)
gm2/CL=3gm1/Cc
yschuang said:Hello, kingmaker
The formulas I got can be found in many textbooks. The way I get the 2nd pole is
short the input and output @ high frequency, or F2nd. The impedacne is 1/gm2 and the capacitance is around Cl+Cgs2. Then you can get F2nd=gm2/(2pi*Cl) if Cl is larger than Cgs2. For more details, you can go through slides named "Analog Design Essentials". Read Chapter 5. You can use free mirror to download that material.
yschuang said:The link still works. Try agian or just search for "analog deisgn essentials by sansen". Put key words "analog and design and essentials and sansen"
yschuang said:Hi,kingmaker
I think Cc=0.5Cl is an initial and reasonable assumption. It is ok to have Cc=0.3 or 0.4Cl. However, from the formula that gm2/gm1=4Cl/Cc, as you set Cc less than half Cl, you will burn more power. Let's assume Cc=0.5Cl, then gm2/gm1=8. If Cc=0.25Cl, then gm2/gm1=16, which means you will burn more power in the second stage. By the way, there is another assumption about Cc. In the slide, Cc is around 3.3 times Cn1, which is Cn1/Cc=0.3. So you can not set the value of Cc too small and there will be no miller effect I think.
kingmaker said:yschuang said:Hi,kingmaker
I think Cc=0.5Cl is an initial and reasonable assumption. It is ok to have Cc=0.3 or 0.4Cl. However, from the formula that gm2/gm1=4Cl/Cc, as you set Cc less than half Cl, you will burn more power. Let's assume Cc=0.5Cl, then gm2/gm1=8. If Cc=0.25Cl, then gm2/gm1=16, which means you will burn more power in the second stage. By the way, there is another assumption about Cc. In the slide, Cc is around 3.3 times Cn1, which is Cn1/Cc=0.3. So you can not set the value of Cc too small and there will be no miller effect I think.
i see... got your point. thanks again.
1 question come to my mind. what is the relationship between dominant pole and GBW(fT)? And also, 2 stage op-amp same as miller cmos OTA?
by the way, do you any recommended book to read regarding the Feedback?
yschuang said:Hi, kingmaker
I think you do not have to go through all details about feedback and there is no one good book that covers all feedback theories. Sorry I can not help you about books on feedback. Analog Design Essentils is good because it tells you how to pick the value of Cc or gm2. You will see more details in chapter 6, which tells you a systematic way to design an opamp. The rest is how to simulate an opamp. You can take a look at the following link.
https://www.designers-guide.org/Forum/YaBB.pl?num=1155668476
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