In April 2014, the Peking University microelectronics re-examination was held in the Micronano Electronics Building. At that time, one question was drawn during the interview.
I drew a question during the interview:
What is a small signal?
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What I answered is less than a thermal voltage Vt.
The other teacher (it seems to be Mr. Lu Wengao) asked:
What is thermal voltage
I answer: kt/q
Q: How big is this thermal voltage
My answer: 26mV
Finally the teacher said that the small signal should be infinitesimal.
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(My answer at the time was actually for bjt, because there are diodes everywhere in bjt. The formula for diodes is:
id=Is*(e^Vd/Vt-1)
According to the equivalent infinitesimal formula of advanced mathematics:
When x is infinite, we have:
a^x-1=xlna
So substituting the above diode formula has id=Is*(e^Vd/Vt-1) =Is*Vd/Vt (in this case, Vd is less than Vt)
But obviously I didn't consider the situation of MOS tube, so the teacher's answer is more rigorous. )
Then the teacher asked:
What kind of work do you do?
I said:
Copy film, reverse (reverse) engineering.
The teacher said:
Copy film?
I said: yes
Then discuss with two teachers:
We also recruited such a student in the first two years.
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Some can't remember in the middle.
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Teacher Li Qian (sitting on the far right side opposite me, wearing a hundred black) asked:
Why did you take the postgraduate entrance examination (after work) after (work)?
I replied:
I don’t want to copy other people’s circuits all the time.