Optical Communications Multiple Choice Questions on “FET Pre – Amplifiers”.
1. ____________ is the lowest noise amplifier device.
a) Silicon FET
b) Amplifier-A
c) Attenuator
d) Resonator-B
Answer: a
Explanation: FET operates by controlling the current flow with an electric field produced by an applied voltage on the gate of the device. Silicon FET is fabricated for low noise devices. It is the lowest noise amplifier device available.
2. FET device has extremely high input impedance greater than _________
a) 107 Ohms and less than 108
b) 106 Ohms and less than 107
c) 1014 Ohms
d) 1023 Ohms
Answer: c
Explanation: FET operation involves the applied voltage on the gate of the device. The gate draws virtually no current, except for leakage, giving the device extremely high input impedance.
3. The properties of a bipolar transistor are superior to the FET.
a) True
b) False
Answer: b
Explanation: bipolar transistor operates by controlling the current flow with an electric field produced with a base current. The properties of a bipolar transistor are limited by its high trans-conductance than the FET.
4. Bipolar transistor is more useful amplifying device than FET at frequencies _____________
a) Above 1000 MHz
b) Equal to 1 MHz
c) Below 25 MHz
d) Above 25 MHz
Answer: d
Explanation: In FETs, the current gain drops to values near unity at frequencies above 25MHz. The trans-conductance is fixed with decreasing input impedance. Therefore, bipolar transistor is more useful amplifying device at frequencies above 25MHz.
5. High-performance microwave FETs are fabricated from ___________
a) Silicon
b) Germanium
c) Gallium arsenide
d) Zinc
Answer: c
Explanation: Since the mid- 1970s, the development of high-performance microwave FETs found its way. These FETs are fabricated from gallium arsenide and are called as GaAs metal Schottky field effect transistors (MESFETs).
6. Gallium arsenide MESFETs are advantageous than Silicon FETs.
a) True
b) False
Answer: a
Explanation: Gallium arsenide MESFETs are Schottky barrier devices. They operate with both low noise and high gain at microwave frequencies (GHz). Silicon FETs cannot operate with wide bands.
7. The PIN-FET hybrid receivers are a combination of ______________
a) Hybrid resistances and capacitances
b) Pin photodiode and low noise amplifier (GaAs MESFETs)
c) P-N photodiode and low noise amplifier (GaAs MESFETs)
d) Attenuator and low noise amplifier (GaAs MESFETs)
Answer: b
Explanation: The PIN-FET or p-i-n/FET receiver utilizes a p-i-n photodiode along with a low noise preamplifier (GaAs MESFETs). It is fabricated using thick-film integrated circuit technology. This hybrid integration reduces the stray capacitance to negligible levels.
8. PIN-FET hybrid receiver is designed for use at a transmission rate of _____________
a) 130 Mbits-1
b) 110 Mbits-1
c) 120 Mbits-1
d) 140 Mbits-1
Answer: d
Explanation: At 140 Mbits-1, the performance of PIN-FET hybrid receiver is found to be comparable to germanium and alloy APD receivers. A digital equalizer is necessary as the high-impedance front end effectively integrates the signal at 140 Mbits-1.
9. It is difficult to achieve higher transmission rates using conventional __________
a) Voltage amplifier
b) Waveguide Structures
c) PIN-FET or APD receivers
d) MESFET
Answer: c
Explanation: It is difficult to achieve higher transmission rates due to limitations in their gain bandwidth products. Also, the trade-off between the multiplication factor requirement and the bandwidth limits the performance of conventional receivers.
10. Which receiver can be fabricated using PIN-FET hybrid approach?
a) Trans-impedance front end receiver
b) Gallium arsenide receiver
c) High-impedance front-end
d) Low-impedance front-end
Answer: a
Explanation: Trans-impedance front-end receivers are fabricated using the PIN-FET hybrid approach. An example of such receivers consists of a GaAs MESFET and two complementary bipolar microwave transistors.
11. A silicon p-i-n photodiode utilized with the amplifier and the receiver is designed to accept data at a rate of ___________
a) 276Mbits-1
b) 274 Mbits-1
c) 278Mbits-1
d) 302Mbits-1
Answer: b
Explanation: A silicon p-i-n photodiode is used with the low-noise preamplifier. This preamplifier is based on a GaAs MESFET. Thus, a receiver using p-i-n photodiode accepts a data rate of 274 Mbits-1 giving a sensitivity around -35dBm.
12. What is usually required by FETs to optimize the figure of merit?
a) Attenuation of barrier
b) Matching with the depletion region
c) Dispersion of the gate region
d) Matching with the detector
Answer: d
Explanation: Total capacitance is given by Ct = Cd + Ca. The figure of merit is optimized when Cd=Ca. This requires FETs to be matched with the detectors. This requires FETs to be matched with the detectors. This procedure is usually not welcomed by the device and is not permitted in current optical receiver design.
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