为什么是PTAT电流源？

hym4063

原帖由 JTR6907 于 2008-12-3 13:22 发表

                               
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很多模块都需要一个PTAT电流去补偿另一个IPTAT电流，比如VGA里

运放里需要吗？

hym4063

原帖由 tiantsmart 于 2008-12-3 13:53 发表

                               
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一般情况下,不需要基准电流源吧,除非是对电流要求高的啊

感觉补偿一下也不是特复杂，何乐而不为呢

waliley78751

原帖由 hym4063 于 2008-12-3 14:54 发表

                               
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谢谢！
不过我觉得比较牵强
温度上升 电路的性能是变好还是变坏？
   温度升高，迁移率增大，我觉得性能还会变好啊！~

Well, I sort of remember a rough principle in devices: when temperature goes up, everything is worse.
Actually mobility goes down when temperature goes up. It is not very hard to explain this because lattice scattering increases with temperature. Please refer to the attachment. It takes me a while to find this diagram.

Take a look at the approximation of gm, gm~M*Cox*(W/L)*(Vgs-VT), when temperature increases, M decreases, and the increase in bias current cause a increase in (Vgs-Vt) term, which compensates for the decrease of mobility. Of course it is a rough argument, but it does work for many designers.

As to your another argument, yes, it is not hard to achieve a nearly flat current output with respect to temperature by summing a PTAT and an I-PTAT curernt sources.

[ 本帖最后由 waliley78751 于 2008-12-3 15:47 编辑 ]

hym4063

原帖由 waliley78751 于 2008-12-3 15:45 发表

                               
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Well, I sort of remember a rough principle in devices: when temperature goes up, everything is worse.
Actually mobility goes down when temperature goes up. It is not very hard to explain this bec ...

OK thank you!
that is to say it is just the decreasing mobility leads to the poorer perfomance...?
Can the temperature coefficient of Vth compensate the decreasing mobility?
the diagram is from M.Sze's book....?

waliley78751

原帖由 hym4063 于 2008-12-3 17:38 发表

                               
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OK thank you!
that is to say it is just the decreasing mobility leads to the poorer perfomance...?
Can the temperature coefficient of Vth compensate the decreasing mobility?
the diagram is from ...

Partially right. I won't put an emphasis on Vt variation w.r.t. temperature since I would take care of (Vgs-Vt) term.
It is just an experience talk to use PTAT to bias your circuit. There definitely exists other applications that may require other type of current sources (temperature-independent or even I-PTAT) to obtain optimal performance. Right?

By the way, it is not from S.M. Sze's book, but I believe they are pretty much the same. Q_Q

[ 本帖最后由 waliley78751 于 2008-12-3 20:01 编辑 ]

guang3000

原帖由 hym4063 于 2008-12-3 14:54 发表

                               
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谢谢！
不过我觉得比较牵强
温度上升 电路的性能是变好还是变坏？
   温度升高，迁移率增大，我觉得性能还会变好啊！~

怎么我记得温度上升，晶格散射加强，载流子平均自由程下降，迁移率下降？

ives

迁移率先升高，后下降。常温时应该是下降的了，我记得是。现在手里没有书，查不到。

leonthomas

这个说得有道理。

smilelangjun

温度上升，迁移率下降，op的gain减低，
ptat电流可以弥补迁移率下降的不足

lakeoffire

It is kind of ambiguous to define the circuit performance is good or bad. More often, you want the circuit meets your specs while keeping power, die size, etc as small as possible. Therefore, depending on the target of your circuits, you sometimes want PTAT bias, but sometimes want others like constant-gm, bandgap.

BTW, high temp is not always "worse" than low temp, especially in deep sub-micron CMOS such as 45nm process. There is something called "inverse effect"

原帖由 waliley78751 于 2008-12-2 12:18 发表

                               
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My experience is: because circuit performances worsen when temperature goes up, and thus designers prefer higher bias current in high-temperature environment to maintain roughly the same performances  ...