密勒补偿电容和调零电阻到底如何工作

kaixuanw

各位前辈，请问对于一个二级放大器（比如folded cascode+diff amp），主极点在输出端，次极点在二级连接处的话，加上密勒电容岂不是将C（1-A）这个大电容加在了次极点上，这不是使得次极点更加低频了吗？使得实现所谓的极点分离，主极点更低频，次极点更高频，还是说加上密勒电容后会将两个极点反过来？另外，调零电阻是如何工作的？求详细解释。。。小弟后天面试亚利桑那的TI，万分着急啊。。。跪谢！！！

690827881

你应该是搞反了，主极点一般在第一级输出，放大器输出端为次极点，然后你就好理解了。
小电阻主要是用来消除前馈零点的影响。。。

690827881

推荐一篇好paper给你，不谢

kaixuanw

回复 2# 690827881

万分感谢！我搞明白了！请问小电阻是不是有两种选择：1，将零点推到高频，消除它的影响，2，用零点cancel第二个极点？

690827881

回复 4# kaixuanw
有三种：1.左移并消除第二级点；2.左移到无穷大，零点消失；3.左移到1.2GBW处，增大PM

kaixuanw

回复 5# 690827881

高手！谢谢！

sweet_julia

实在忍不住吐个槽。。。这是基本analog知识啊 呵呵呵。

没补偿前主极点也可以在output啊，如果load cap 特别大呢？anyway, it doesn't matter...

楼主面试完可以分享点面试题木，感觉TI的analog问题会比较有意思 谢谢啦。

sweet_julia

忽然很想八卦，难道是sensor那个组，让发schematic discuss一下的那个么 hahahahaha

david_reg

回复 1# kaixuanw

米勒补偿只会主极点移到更低频, 使次主极点移到更高频, 不管主极点在补偿前是在第一级输出还是在第二级输出, 都是这样, 它不会使主次极点翻转, 对于补偿前主极点在第二级输出的情况, 米勒补偿的在第一级的等效电路是一个米勒电阻而不是等效电容, 具体详情可以参考这篇paper的第三部分.
111. IMPROVED EQUIVALENT CIRCUIT MODELING
In ths section, we examine the use of small-signal equivalent circuits in frequency compensating two-pole opamps. Our results show that the previous techniques based on Miller-multiplied capacitance models are suitable only for those amplifiers in which the first-stage pole is dominant prior to compensation. In most SC circuits, the internal opamps are transconductance amplifiers whch drive on-chp capacitive loads, and thus for these opamps, the second-stage pole is dominant. Hence, in order to more accurately model the second-stage-dominant pole systems, and to subsequently realize the optimum phase margin condition, we have extended the small-signal model to include resistance effects in addition to Miller capacitance effects.
A.
Our results show that whichever pole is dominant before compensation remains dominant after compensation. Hence, some considerable confusion that has existed previously regarding pole-splitting frequency compensation is eliminated.

[1]
H. C. Yang and D. J. Allstot, “Considerations for fast settling operational amplifiers,” IEEE Trans. Circuits Syst., vol. 37, no. 3, pp. 326–334, 1990.

另一篇关于米勒补偿中的一个零点的文章对米勒补偿有进一步的认识, 实际上米勒补偿不会使极点分离, 补偿后的主次极点都在第一级. 第二级的极点会被一个米勒零点抵消.

[1]
H. C. Yang and D. J. Allstot, “Modified modeling of Miller compensation for two-stage operational amplifiers,” in 1991., IEEE International Sympoisum on Circuits and Systems, 1991, pp. 2557–2560.

The mechanism of Miller compensation for two-stage operational amplifiers is modified. The analysis shows that both dominant and nondominant poles are with the first stage of the opamp after compensation; the second stage pole is canceled by a Miller zero. That is, the poles do not split during the frequency compensation, as previously thought. This improved understanding results in a new conceptual compensation circuit, which improves the performance of the two-stage op amps

kaixuanw

回复 7# sweet_julia


啊啊啊啊啊！你是那个去Intel的TAMU大神吗！世界真小。。。。