what is the application for the three kinds of bias circuit?

Hi all ,There are three kinds of bias circuit in the book of Razavi,they are :temperature indpendent bias circuit,PTAT current generation circuit and constant Gm circuit.But which one should be chosen when desing a circut? what is the rule？I used to design a temperature indpendent bias circuit for my circuits but now I am cofused ,thanks.

I would say it depends on how great of a bias generator you are looking for. If you want just a supply independent use just a GM reference (without the SC for the R so page 381 sec 11.3). If you would need a temperature independent generator use a bandgap. If all depends on how precise of a bias generator you need! If its for industry I would say they always use a first or second order bandgap. I have even heard of a third order for really specific applications. There also could be circuits which don't need a precise bias so a gm reference can be used locally instead of wiring over the entire chip from the bandgap.

If its a school project, just use a gm reference, its fun to build, easy and works very well! I think its a better idea then wasting a pin to pump on chip a 100uA current(or whatever you need) for you circuit to work, which could have noise or leakage current in it!!
Hope this helps
Jgk

afujian said:

... which one should be chosen when desing a circut? what is the rule

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There's no rule; this depends on the circuit's requirement, as Jgk explained above.
Occasionally I use a PTAT source to counteract the speed deterioration at high temperatures (due to mobility degradation).

I used to use OpAmp as temperature compensation circuits, i.e. some constant current circuits for last stage PA.

yes erikl,it is that I was confused when I found some one use a PTAT current generator for biasing,could you give a example for the application of the PTAT current circuit?

---------- Post added at 13:28 ---------- Previous post was at 13:26 ----------

hi jpk,thanks for the detailed explanation，I would rather use a temperature independent circuit?

afujian said:

... could you give a example for the application of the PTAT current circuit?

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See my explanation above:

... PTAT source to counteract the speed deterioration at high temperatures ...

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OTAs, OpAmps and other amplifying circuits operated with temperature-independent bias current show lower UGB at higher temperatures. If you are ready to spend more current at higher temperatures by using a PTAT bias current source, you can counteract this tendency and keep the UGB high even at high temperatures.

thank you.