xwj623
|
Hi,all I designed a 600M~800M GBW two stage opamp. the first stage is rail to rail input,gain boosting folded cascode structure, the second stage is normaly class A output. the compenstation method is miller compenstation, that is a resistor in series a cap.
After simulation,the main OTA GBW and Total opamp GBW is about 600M~800M,PM is about 40~70,which can be adjust by compensation. The boosting opamp GBW is at the order of 1.4G, i didn't postion the boost opamp GBW near Total opamp GBW for not degrade the settling,while the boosting loop is stable,for a PM about 30~40. gm2=3m,gm1=0.43m,Cl=2.5p,Cc=1p.
But now, i find that the settling is not as good as i expect,it seems there is a doubltes in total opamp GBW, or to say there is a zero in GBW,the settling waveform is in the attached fig.the green waveform is Rc=100ohm, while blue waveform with no resistor but Cc is 100f bigger.
we can see that point A, B are degrade by a slow settling, i am confused by this degraed phenomena. I think this transition point is not the point from slew to linear settling,the waveform didn't seems that.
i think about the following questions,am i right? 1. because the GBW is big, about 600M~800M, so the RHP zero caused by zero resistor through miller compensation is easily enter the GBW region or 3x GBW region, and caused a degraded settling,even thouth the Rc is very small. But if we don't use a zero resistor,is PM is poor, and need a big Cc and big I2. 2. there is still a degraed settling even though i didn't use a zero resistor showed by the blue waveform at point A. What caused this and how to solve it? 3.it seems there is a zero is in GBW? 4.Is the Cgd of input differential pairs caused the zero?
i am confused much and didn't how to avoid this degrade. thanks.
|