Microwave Engineering Multiple Choice Questions on “Broadband Transistor Amplifier Design”.
1. High gain is not achievable at microwave frequencies using BJT amplifiers because:
A. device construction
B. complex architecture
C. ports are not matched at high frequencies
D. none of the mentioned
Answer: C
Clarification: At higher frequencies, if higher bandwidth is desired, a compromise on maximum achievable gain is made. But at these higher frequencies, the ports of the amplifier are not matched to 50 Ω.
2. To flatten the gain response of a transistor:
A. biasing current has to be increased
B. input signal level has to increased
C. increase the operational bandwidth
D. give negative feedback to the amplifier
Answer: D
Clarification: Negative feedback can be used to increase the gain response of the transistor, improve the input and output match, and increase the stability of the device.
3. In conventional amplifiers, a flat gain response is achieved at the cost of reduced gain. But this drawback can be overcome by using:
A. balanced amplifiers
B. distributed amplifiers
C. differential amplifiers
D. none of the mentioned
Answer: A
Clarification: In conventional amplifiers, a flat gain response is achieved at the cost of reduced gain. But this drawback can be overcome by using balanced amplifiers. This is overcome by using two 900 couplers to cancel input and output reflections from two identical amplifiers.
4. Bandwidth of balanced amplifier can be an octave or more, but is limited by the bandwidth of the coupler.
A. true
B. false
Answer: A
Clarification: In order to achieve flat gain response, balanced amplifiers use couplers to minimize reflections. But this in turn reduces the bandwidth of the amplifier to the coupler bandwidth.
5. Coupler that is mostly used in balanced amplifiers to achieve the required performance is:
A. branch line coupler
B. wilkinson coupler
C. lange coupler
D. waveguide coupler
Answer: C
Clarification: Lange couplers are broadband couplers and are compact in size. Since the bandwidth of a balanced amplifiers depends on the bandwidth of the coupler used. Lange coupler is thus preferred over couplers.
6. Distributed amplifiers offer very high _________
A. gain
B. bandwidth
C. attenuation
D. none of the mentioned
Answer: B
Clarification: Distributed amplifiers offer very high bandwidth of about 10 decade. But higher gain cannot be achieved using distributed amplifiers and matching at the ports is very important to achieve higher bandwidth.
7. In distributed amplifiers, all the FET stages in the amplifier are connected in series to one another.
A. true
B. false
Answer: B
Clarification: In distributed amplifiers, cascade of N identical FETs have their gates connected to a transmission line having a characteristic impedance of Zg with a spacing of lg while the drains are connected to a transmission line of characteristic impedance Zd, with a spacing ld.
8. ____________ uses balanced input and output, meaning that there are 2 signal lines, with opposite polarity at each port.
A. differential amplifier
B. distributed amplifier
C. balanced amplifier
D. none of the mentioned
Answer: A
Clarification: Differential amplifier uses balanced input and outputs, meaning that there are 2 signal lines, with opposite polarity at each port. It has two input ports and one output port. The difference of the 2 input signals is amplified.
9. A major advantage of differential amplifiers is:
A. high gain
B. low input impedance
C. higher output voltage swing
D. none of the mentioned
Answer: C
Clarification: Differential amplifiers can provide higher voltage swings that are approximately double that obtained with single ended amplifier.
10. Along with a differential amplifier, 1800 hybrid is used both at the input and output.
A. true
B. false
Answer: A
Clarification: A differential amplifier can be constructed using two single-ended amplifiers and 1800 hybrids at the input and output to split and then recombine the signals.
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