Analog Interview Questions

Islam2014

Today I was having an analog design interview And i asked to plot the output of the following 2 circuits, but it is simple one but it takes me a few minutes to answer it and i want to share it with you guys.
What is the output of this to circuits if i applied a square wave (from 0V to 6V) to it's input?

chris2013

the circuit on the left the output follows the input directly
the one on the right the output follows till the threshold voltage of the NMOS and then decays exponentially to zero

liuqilong8819

小心DB二极管

Islam2014

Reply 3# liuqilong8819


   I'm sorry i don't get what you mean?

liuqilong8819

回复 4# Islam2014


  图二 drain-bulk diode会导通

mssgzw

回复 5# liuqilong8819


   不好意思。从原理图上我只看到电容和FET。真心没看到你说的二极管。请指点。

liuqilong8819

回复 6# mssgzw

你好，
   这个二极管没有在图中明显画出。
   对于NMOS, drain是N type， body或者bulk是P type, 两者组成PN结. 在图二的时候会导通

countersr

小信号时是电容充放电，但是管子比较大电容比较小时，得考虑大信号，怎么分析？解微分方程？
body diode怎么会导通呢？drain电压最小只是0或者是vth

zhangyang_qy

回复 8# countersr


    如图，衬底和drain之间有PN结，如果Drain接负极则PN结导通，drain电位被钳位在一个PN结导通电压。

zhangyang_qy

回复 8# countersr
因为根据电容电压不可突变原理，在信号源突然下降6V的时候，mos管的gate-drain电压也会跟着下降6V，但是由于D-B二极管导通，所以Drain电位被钳位在一个PN结导通电压。