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Exam 2 - IC BJT Small Signal Amplifiers

For full credit all calculations and formulas must be shown. All numeric answers should be enclosed in a box or underlined twice. The exam will review course material relating to; bipolar junction transistor integrated circuits, silicon based ICs, emitter, base, and collector properties, small signal IC parameters, and amplifier configurations.



*Assumptions: Silicon integrated circuit BJT at room temperature unless otherwise stated

1. The transconductance of a BJT at room temperature with IC= 1mA is about 38mA/V for a transistor made from?
a. Silicon
b. Germanium
c. Gallium Arsenide
d. All of the above


2. The input resistance rΠ of a common emitter BJT can be calculated from?
a. β+1re
b. βo/gm
c. VT/IB
d. All of the above


3. Which small signal hybrid-pi model parameter increases directly proportional to collector current?
a. Cb
b. fT at low IC
c. gm
d. All of the above


4. Which small signal hybrid-pi parameter is inversely proportional to collector current?
a. rΠ
b. r
c. rO
d. All of the above


5. The Typical value for the feedback resistance of an integrated circuit NPN BJT is?
a. 10rO
b. OrO
c. 10OrO
d. All of the above


6. The small signal input resistance of a common emitter (C.E.) amplifier is?
a. βO+1rΠ
b. rΠO
c. 1/gm
d. All of the above


7. The small signal, open-collector voltage gain of a BJT common emitter amplifier is?
a. gmrO
b. gmrΠ
c. -gmrO
d. βOrΠ/rO


8. The small signal short-circuit current gain of a BJT common emitter amplifier is?
a. αO
b. O+1
c. βO
d. About unity


9. The small signal short-circuit current gain of a BJT common base amplifier is?
a. αO
b. βO+1
c. βO
d. About unity


10. The small signal voltage-gain of a common collector amplifier is?
a. αO
b. βO+1
c. βO
d. About unity


11. Compared to a common emitter amplifier, the magnitude of the open-collector voltage gain of a common base amplifier is about?
a. αO times as big
b. (βO+1) times as big
c. 1/βO times as big
d. None of the above




12. Compared to a common emitter amplifier, the input resistance of a common base amplifier is about?
a. αO times as big
b. (βO+1) times as big
c. (βO+1) times smaller
d. About the same


13. Compared to a common emitter amplifier, the input resistance of a common collector amplifier with a 1k emitter resistor is?
a. About the same
b. Much smaller
c. Much larger
d. About double


14. Compared to a common emitter amplifier, the small signal open-collector voltage gain of a common emitter common base cascode amplifier is?
a. About the same
b. Much larger and same polarity
c. Much larger but opposite polarity
d. None of the above


15. Compared to a common emitter amplifier, the small signal input resistance of a common emitter common base cascode amplifier is?
a. About the same
b. Much larger
c. Much smaller
d. About double


16. Compared to a common emitter amplifier, the small signal short-circuit current gain of a Darlington type amplifier is about?
a. The same
b. βO2 times larger
c. βO times larger
d. βO2 times smaller


17. Compared to a common emitter amplifier, the small signal input resistance of a Darlington type amplifier is about?
a. The same
b. βO times larger
c. βO2 times larger
d. βO times smaller


18. Compared to a common emitter amplifier, the small signal open-collector voltage gain of a Darlingtontype amplifier is about?
a. The same
b. βO times larger
c. βO2 times larger
d. βO times smaller


19. An emitter coupled differential amplifier is often used in integrated analog circuits mainly because it?
a. Has large voltage gain
b. Has large current gain
c. Can be DC coupled
d. Has high input resistance


20. An integrated circuit amplifier has?
a. No coupling capacitors
b. Few load resistors
c. Many active transistors
d. All of the above


21. The β of a BJT decreases at low collector currents because?
a. The base current is larger then ideal
b. The collector current is smaller then ideal
c. Both a and b
d. The base width widens


22. What will the β of a BJT do as it increases with temperature?
a. Increase
b. Decrease
c. Remain the same
d. Approach α


23. The Early Effect is caused by?
a. Base width narrowing with increasing Base-Collector reverse bias
b. Base-Collector depletion region widening with increasing Base-Collector reverse bias
c. Base width narrowing with increasing VCE
d. All of the above


24. The Ebers-Moll model is applicable in the?
a. Forward active region
b. Reverse active region
c. Saturation region
d. All of the above


25. The Ebers-Moll model is applicable for?
a. DC analysis only
b. AC analysis only
c. AC and DC analysis
d. Only with simulations


26. The mobile electron concentration in n-type silicon is?
a. About equal to NA
b. About equal to ND
c. About equal to Ni
d. About equal to PO


27. The Law-of-Mass action states that?
a. PO=NO=Ni
b. NA/ND=1
c. nOPO=ni2
d. nO and PO are unrelated


28. The product of mobility and electric field is?
a. Diffusion constant
b. Drift velocity
c. Breakdown voltage
d. Build-in potential


29. Carriers diffuse as a result of?
a. Electric fields
b. Drift velocity
c. Built-in potential
d. Concentration gradients

Exam 2: Solutions

1. D, All of the above
2. D, All of the above
3. D, All of the above
4. D, All of the above
5. C, 10βOrO
6. B, rΠO
7. C, -gmrO
8. C, βO
9. A, αO
10. D, About unity
11. B, (βO+1) times as big
12. C, (βO+1) times smaller
13. C, Much larger
14.
15. A, About the same
16. C, βO times larger
17. B, βO times larger
18. A, The same
19. C, can be DC coupled
20. D, All of the above
21. A, The base current is larger then ideal
22. A, Increase
23. D, All of the above
24. D, All of the above
25. C, AC and DC analysis
26. B, About equal to ND
27. C, nOPO=ni2
28. B, Drift velocity
29. D, Concentration gradients





Engineering key words: BJT, bi polar junction transistor, circuit potential, breakdown voltage, mass action, diffusion and drift, properties and states, electric fields, electron, holes, n-type, p-type, dope, Eber Model, amplifier saturation, early effect, bias and reverse biasing, common emitter, common base, common collector, open circuit, short circuit, darlington, polarity, cascade amplifier, signal characteristics.



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