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https://designers-guide.org/forum/YaBB.pl Design >> Analog Design >> some questions about fast settling,zero resistor,PM and ... at high speed opamp https://designers-guide.org/forum/YaBB.pl?num=1249885685 Message started by xwj623 on Aug 9th, 2009, 11:27pm |
Title: some questions about fast settling,zero resistor,PM and ... at high speed opamp Post by xwj623 on Aug 9th, 2009, 11:27pm 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. |
Title: Re: some questions about fast settling,zero resistor,PM and ... at high speed opamp Post by xwj623 on Aug 10th, 2009, 12:26am during PZ analysis, i found that: poles (hertz) real imag -12.5255k 0 -87.0147x -553.516x -87.0147x 553.516x zeros (hertz) real imag -49.5317x 561.919x -49.5317x -561.919x why this -49M zeros happened? when i dont't use the boost amp, pz analysis shows that: poles (hertz) real imag -548.444k 0 -143.312x -434.226x -143.312x 434.226x zeros (hertz) real imag -119.386x -473.816x -119.386x 473.816x also a doublets happened. |
Title: Re: some questions about fast settling,zero resistor,PM and ... at high speed opamp Post by raja.cedt on Aug 10th, 2009, 1:30am hi, i think you are following that paper completely....i feel these are assumption that pap 1.Boosted gain amplifier gain is more. 2. Amplifier which is used for the boosting is completely dominate pole compensated......they neglected pole zero doublet formed due to active pmos load. 3.Main amplifier don't have any zero But in your design Two stage opamp and miller compensated.....so there is zero but you have compensate that very carefully........so in this type situation better use indirect compensation (check baker book). And loop PM is 30 means there is lot of chance for oscilations , so decrease boosted amp BW or increase 2nd dominate pole in the main amp. Ans to your 4th question is it create very high frequency zero provided output pole is dominate...so nominally you can ignore that. Can you post schematic? Thanks, Rajasekhar. |
Title: Re: some questions about fast settling,zero resistor,PM and ... at high speed opamp Post by xwj623 on Aug 10th, 2009, 3:04am raja.cedt wrote on Aug 10th, 2009, 1:30am:
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Title: Re: some questions about fast settling,zero resistor,PM and ... at high speed opamp Post by raja.cedt on Aug 10th, 2009, 3:39am hi, you see gain plot in the pap,there he gave ugb of boosted amp is grater than 3db of the main amp..this is needed only in case we have high gain in boosted amp..but any how this is not needed in this discussion its two pole system.... to get feel break the loop and see.. ya thats what i am telling you will zero's in simulation because they are coming because of the active load indirect compensation eliminates zero, better use cascode regarding miller effect if o/p pole is dominate then by the time cgd is active o/p impedance would have reduced.........but better to investigate this through simulation.... Bottom line is try to increase loop PM, i think problem may solved. Thanks, Rajasekhar. |
Title: Re: some questions about fast settling,zero resistor,PM and ... at high speed opamp Post by nobody on Aug 10th, 2009, 4:20am Few questions for xwj623. What are 1st gm of your input pair and 2nd gm of your second stage ? I guess gm2 is your gm of input pair. |
Title: Re: some questions about fast settling,zero resistor,PM and ... at high speed opamp Post by xwj623 on Aug 10th, 2009, 4:41am why only high gain boosted amp need its gbw is bigger than 3db or colsed loop gbw? active load,do you mean a mosfet biased by a dc voltage or a regulated cascode mosfet? i biased the cascode amp by dc voltage, but the zero seems remain the same. To increase PM, the settling is becoming worse,for the ringing and overshoot is very small, linear settling is almost flatten,but the slow settling(which showed degraed in the waveform) is remains the same. raja.cedt wrote on Aug 10th, 2009, 3:39am:
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Title: Re: some questions about fast settling,zero resistor,PM and ... at high speed opamp Post by xwj623 on Aug 10th, 2009, 4:43am nobody wrote on Aug 10th, 2009, 4:20am:
gm1 is gm of the first stage, or the input differential pair. gm2 is gm of the second output stage,or the class A output stage. |
Title: Re: some questions about fast settling,zero resistor,PM and ... at high speed opamp Post by nobody on Aug 10th, 2009, 8:52am I calculate the GBW based on gm1/(2pi*Cc) derived from a two-stage amplifier with a miiler compensation and assume Cc=1.2p to include parasitic caps from mosfets. I will get GBW=57MHz if it is a single pole system. I am not convinced by your gm1. Maybe now it is more clear. GBW=0.43m/(6.28*1.2p)=57MHz |
Title: Re: some questions about fast settling,zero resistor,PM and ... at high speed opamp Post by xwj623 on Aug 10th, 2009, 6:29pm nobody wrote on Aug 10th, 2009, 8:52am:
sorry i have mistaken the I with gm. you should replace the gm with I in the first post. |
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