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'.