Some On-site Interview Questions (© Fuding Ge)

Almost everyone of the following questions was asked to me by an interviewer. Special thanks to Ge Wang at Maxim, Zhitao Jiang at Motorola.

1.If the following inverter biased in the middle of Vdd, what is the small signal gain? (Answer gm X ro)

2. Crossection diagram of the inverter (be able to draw the contact of power supply and ground)

3. From the crossection of the diagram, be able to draw the parasitic BJT leads to latch-up.

4 . How to prevent latch-up (do not forget guard ring, clampping circuits!)

5. Draw the layout of an inverter or NOR/NAND gate.

6. For the following source follower, what is its -3dB bandwidth? How about it stability?

7. In the following figure, if the two resisters are equal, what is its -3dB bandwidth? Compare its stability with that of a source follower.

 

8.For the following circuit, if the input is a rail-to-rail square wave, plot the wave after the inverter and vo.

9.For the following circuits, What is the gain? Using what technology to improve the matching of the input transistors? If the bias current increase, what happens to the gain? (Hit: Decrease!!!) What happens to the bandwidth? Replace the NMOS with npn BJT and PMOS with pnp BJT, answer the above questions.(Now gain remains constant with increasing biasing current!)

10. For the following circuits, answer the questions again. What are the advantages and disadvantages of these two amplifiers?

11. What are the effective resistance from source to drain of the following two transistors? (The value of the resistance is R). Answer: both of them are 1/gm.

12.What is the low frequency gain of the following circuits? The input is the input current Iin. Where does the dominant pole locate? How about the pole at node 1?

13.For the following circuit, the threshold voltage of the transistor is 0.7V. Vb1=1v, Vb2=2v, When Vin change from 5V to 0V, draw the current flow through the transistors VS Vin. (This question was supplied by Wang Ge)

14. For the following circuits, Vdd=5v, tell me what are Vo1 and Vo2 when Vin is 5V, 3V, 2.5V and 0V.

15. For the following circuit, what is the gain of Vout/Vin? Where is the Feedback and what is the function of feedback?

16. For the following circuits, the small signal input is i_in, the small signal output is v_out, what is the small signal output? What is the gain?

17. Figure out the Vout wave form of the following circuits:

Figure 1

Figure 2

Figure 3

Figure 4

Answer Hints: The basic concept to reply these kind of RC (or RLC) network questions is that: for C, it resistance is infinity when frequency=0 and 0 when frequency is infinity. (For L, its resistnace is 0 when frequency=0 while its resitance is infinity when frequency is infinity.

18. For the following circuit, at time 0, the switch switches from A to B, figure out the voltage wave form at B.

Note: this is a "classic" question. It was asked 10 years ago and I was asked this question just recently. You might be asked this question next time...

19. Fo the following circuit, what is the voltage value at A and B? (The Vt of the transistor is 1V).

[Problem 20 and 21 are not posted here.]

Note: Problem 20 and 21 are copied from the book edited by Jim Williams "The Art and Science of Analog Circuit Design", published by Butterworth-Heinemann, 1995. ISBN 0-7506-9505-6, the chapter by Robert Reay, "A new graduate's guide to the analog interview". The chapter has 20 interview questions, but most of them are BJT problems. It may be helpful and interesting to read the chapter and even the wholw book. It is an INTERESTING book, though may be not very useful. It even includes a chapter by Richard P. Feynman (who is that guy? Come on, you should know him!).

Some On-site Interview Questions (© Fuding Ge)