FYI, here is an attempt I did this morning.  I didn't bother to beautify the schematic but you should be able to see the operating points.

huber wrote on Feb 1^st, 2006, 9:16am:


Hello Marc and Huber,

Thanks both of you commenting on the circuit, espeically Marc's illustration example. I see why Marc picked fT rather than Id in sizing. What is your rule of thumb in picking fT?

I can use VCVS to force the CMFB work, i.e., make Voc~1.8V. In this case, Huber's comment is definitely right. However, since the circuit is using SC CMFB, there is not too much adjust on the SC CMFB network except the transistor sizes (CMFB consists of just ideal switches). I am not using transistor CMFB. This comes back Cri's comment, which he also suggested to use VCVS in CMFB. But I ended up still needing to re-size the trans after returning SC CMFB. Did I do something inefficiently?

Regards,
Steven

I made it work but not completely understand why previously the circuit didn't work: I picked Vcm=1.5 in SC CMFB previously, which didn't work. Now if picking Vcm=-1.5, everything works perfectly, the sizing, phase margin, and gain. I don't know why yet. Someone can provide a clue?

By the way, since now the GBW is only about 700MHz. Can someone provide reference on how to further improve GBW? Marc's fT selection is obviously a good way. Other ways?

Thanks

Hi, someone asked me a question about gm/Id and fT.  Hopefully my answer is helpful to others, and if I am wrong in my understanding, please correct me!

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Re: cascode tail current source.  The cascode current source is just used to get a higher output resistance, and thus performs as a better current source.  The downside is it limits the minimum input swing of the amp; it requires a larger voltage drop than a simple source  to get it looking like a current source.

gm/id is about efficiency...you want to get a lot of current change, i.e. high gm, at the lowest bias Id possible.  The best efficiency occurs at around subthreshold.  However, at subthreshold the overdrive voltage Vov=Vgs-Vth is small, and so fT is small.  People are interested into weak/moderate inversion design because both the gm/Id and fT aren't that bad.  For high-speed design you just pick a big fT knowing that gm/Id will be low.

fT is the point where short-circuit current amplification stops happening...i.e. id/ig=1  I think of it as a figure of merit...the higher the fT, the faster your circuit can potentially be.  In a real circuit you want some gain and have a load so fT isn't a reachable operating frequency.  It isn't related to the model 'falling apart'.

HI steven, Marc and all,

        I am still kind of confused about gm/id, ft .  Just like Marc said: "the higher the fT, the faster your circuit can potentially be." we may tweak the transistor to make the ft big, does that mean the speed of the transistor is fast? from my understanding, "potentially faster" doesn't really mean high performance?

thanks

What's the meaning of "fT"?
Thanks.