Microwave Engineering Multiple Choice Questions on “Heterojunction BJT – 1”.
1. BJTs are bipolar junction transistors. The name bipolar is given because:
A. they are made of n type and p type semiconductor
B. they have holes as charge carriers
C. they have electrons as charge carriers
D. none of the mentioned
Answer: D
Clarification: In bipolar junction transistors, both electrons and holes are charge carriers and both of them together constitute current flow in transistors. Since both carriers result in current, they are called bipolar devices.
2. BJTs are suitable for RF applications because:
A. good performance in terms of frequency
B. power capacity
C. noise characteristics
D. all of the mentioned
Answer: D
Clarification: BJTs designed to operate at certain frequency can be operated over a wide range of frequencies hence offering higher bandwidth. Also they have high power handling capacity and very good noise characteristics.
3. Bipolar junction transistors have _______ 1/f characteristics hence making them suitable for oscillators.
A. high
B. low
C. constant
D. decreasing exponential
Answer: B
Clarification: Bipolar junction transistors have very low 1/f noise. 1/f noise is nothing but thermal noise. Hence BJTs are not very temperature and can be used at high temperature applications as well.
4. Silicon junction transistors are used as amplifiers at frequency range of about:
A. 5-10 MHz
B. 2-10 GHz
C. 40-50 MHz
D. 12-45 GHz
Answer: B
Clarification: Silicon junction transistors have unconditional stability as a two port device at a wide range of frequencies. They are more suitable as amplifiers in the frequency range of about 2-10 GHz. Junction transistors when used as oscillators are used in the frequency range of about 20 GHz.
5. At frequency range of about 2-4 GHz, BJTs are preferred over FETs.
A. true
B. false
Answer: A
Clarification: At about 2-4 GHz frequency range, BJTs have higher gain as compared to FETs, power capacity is high and biasing can be done using a single power supply. Because of these advantages, BJTs are preferred over FETs.
6. One major disadvantage of BJTs over FETs is that:
A. they have low gain
B. they do not have a good noise figure
C. low bandwidth
D. none of the mentioned
Answer: B
Clarification: Bipolar junction transistors are subject to shot noise as well as thermal noise effects, so their noise figure is not as good as that of FET. Noise figure can pose serious problems at high operating frequencies.
7. Bipolar junction transistor is a ________ driven device.
A. current
B. voltage
C. power
D. none of the mentioned
Answer: A
Clarification: Bipolar junction transistor is a current driven device where the collector output current directly depends on the input base current. Base current modulates the collector current of the device.
8. The upper frequency limit of BJT depends on the:
A. collector length in the transistor
B. base length
C. emitter length
D. driving voltage
Answer: B
Clarification: The upper operating frequency limit of a BJT depends on the base length of the transistor. Typical base length of a transistor is in the range of a 0.1 µm. the operating frequency is a few GHz for this base length.
9. In the hybrid –π model of a BJT, the capacitance Cc between the base and collector in the hybrid –π model is ignored.
A. true
B. false
Answer: A
Clarification: The capacitance Cc in the hybrid –π model is small and can be neglected. This has the effect of making the S12 parameter of the BJT equal to zero, implying that the power flows only in one direction through the device.
10. with the increase in the operating frequency of a BJT, the S22 parameter of the transistor:
A. increases
B. decreases
C. remains constant
D. none of the mentioned
Answer: B
Clarification: With increase in the operating frequency of the transistor, S22 parameter of the transistor decreases. S22 parameter signifies the voltage reflected back to port 2. S22 parameter has a value of about 0.93 at 0.1 GHz frequency and 0.33 at 4 GHz frequency.
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