Microwave Engineering Multiple Choice Questions on “Two Port Power Gains”.
1. The power gain G of a two port network is independent of the source impedance of the two port network.
A. True
B. False
Answer: A
Clarification: Power gain G is the ratio of power dissipated in the load ZL to the power delivered to the input of the two port network. This gain is independent of ZS although the characteristic of some active devices is dependent on ZS.
2. __________ is defined as the ratio of power available from the two port network to the power available from the source.
A. Transducer power gain
B. Available power gain
C. Power gain
D. None of the mentioned
Answer: B
Clarification: Available power gain is defined as the ratio of power available from the two port network to the power available from the source. This assumes conjugate matching of both source and the load and depends on ZS, not ZL.
3. Transducer power gain of a two port network is dependent on :
A. ZS and ZL
B. ZS
C. ZL
D. Independent of both the impedances
Answer: A
Clarification: Transducer power gain of a two port network is the ratio of the power delivered to the load to the power available from the source. This depends on both ZS and ZL.
4. For a two port network the voltage reflection coefficient seen looking towards the load, ГS is:
A. (ZS –Z0)/ (ZS –Z0)
B. (ZS +Z0)/ (Z0 – Z0)
C. ZS / (ZS –Z0)
D. Z0/ (ZS –Z0)
Answer: A
Clarification: For a two port network, the reflection coefficient ГS seen looking towards the load is (ZS –Z0)/ (ZS –Z0). Here ZS is the input impedance of the transmission line and Z0 is the characteristic impedance of the transmission line.
5. In a two port network, the source impedance was measured to be 25 Ω and the characteristic impedance of the transmission line was measured to be 50 Ω. Then the reflection coefficient at the source end is:
A. -0.33333
B. -0.1111
C. 0.678
D. 0.2345
Answer: A
Clarification: For a two port network, the reflection coefficient ГS seen looking towards the load is (ZS –Z0)/ (ZS –Z0). Substituting the given values in the above equation, reflection coefficient at the source end is -0.3333.
6. For a unilateral transistor, the S parameter that is zero is:
A. S11
B. S12
C. S21
D. S22
Answer: B
Clarification: In a unilateral transistor power flow occurs only in one direction and hence S12 is sufficiently small and can be ignored. Also for a unilateral transistor the reflection coefficients reduce to Гin=S11 and Гout=S22.
7. Gain of an amplifier is independent of the operating frequency.
A. True
B. False
Answer: B
Clarification: Gain of an amplifier depends on the operating frequency. Gain of a conjugate matched FET amplifier drops off as 1/f2 or 6dB per octave.
8. Gain of a conjugate matched FET amplifier is given by the relation:
A. Rds (fT)2/ 4Ri (f)2
B. 4Ri (f)2/Rds (fT)2
C. Rds/ Ri
D. None of the mentioned
Answer: A
Clarification: Gain of FET amplifier is given by the relation Rds (fT)2/ 4Ri (f)2. Gain depends on the drain to source resistance, input resistance and also on the frequency of operation of the amplifier.
9. When both input and output of an amplifier are matched to zero reflection (in contrast to conjugate matching), the transducer power gain is:
A. │S21│2
B. │S22│2
C. │S12│2
D. |S11│2
Answer: A
Clarification: When both input and output of an amplifier are matched to zero reflection, ГL=0 and ГS=0. This reduces the complex transducer gain equation to the s parameter of the amplifier S21. S21 signifies the power at port 2 due to input applied at port 1.
10. If the load impedance of a two port network is 40 Ω and the characteristic impedance is 50 Ω, then the reflection coefficient of the two port network at the load end is:
A. -0.111
B. -0.333
C. -0.987
D. None of the mentioned
Answer: A
Clarification: Reflection coefficient at the load end of a two port network is given by the ratio (ZL-Z0)/ (ZL+Z0). ZL is the load impedance and Z0 is the characteristic impedance. Substituting, reflection at load end is -0.1111.
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