Electronic Devices and Circuits

Noise

Question 1
Marks : +2 | -2
Pass Ratio : 100%
6-Q.8) An amplifier when used with a source of average noise temperature 60 K, has an average operating noise figure of 5.
Explanation:
Question 2
Marks : +2 | -2
Pass Ratio : 100%
What average operating noise figure results when the amplifier is used with an antenna of temperature 30 K?
9.54 db
10.96 db
11.23 db
12.96 db
Explanation:
The required answer is 1 + 240/30 = 9 or 9.54 db.
Question 3
Marks : +2 | -2
Pass Ratio : 100%
In a receiver the input signal is 100 V, while the internal noise at the input is 10 V. With amplification the output signal is 2 V, while the output noise is 0.4 V. The noise figure of receiver is
2
0.5
0.2
None of the mentioned
Explanation:
NF = (100/10)/(2/0.4) or 2.
Question 4
Marks : +2 | -2
Pass Ratio : 100%
What is the maximum average effective input noise temperature that an amplifier can have if its average standard noise figure is to not exceed 1.7?
203 K
215 K
235 K
255 K
Explanation:
The required answer is 290 x (1.7-1) or 203K.
Question 5
Marks : +2 | -2
Pass Ratio : 100%
A resistor R  1 k is maintained at 17C. The rms noise voltage generated in a bandwidth of 10 kHz is
16 x 10(-14) V
0.4 µV
4 µV
16 x 10(-18) V
Explanation:
v2 = 4kBTR where symbols have their usual meanings, hence v = 0.4 µV.
Question 6
Marks : +2 | -2
Pass Ratio : 100%
A mixer stage has a noise figure of 20 dB. This mixer stage is preceded by an amplifier which has a noise figure of 9 dB and an available power gain of 15 dB. The overall noise figure referred to the input is
11.07
18.23
56.43
97.38
Explanation:
The required answer is 7.94 + (100-1)/31.62 = 11.07.
Question 7
Marks : +2 | -2
Pass Ratio : 100%
A receiver is operated at a temperature of 300 K. The transistor used in the receiver have an average output resistance of 1 k. The Johnson noise voltage for a receiver with a bandwidth of 200 kHz is
1.8 µV
8.4 µV
4.3 µV
12.6 µV
Explanation:
v2 = 4kBTR where symbols have their usual meanings, hence v = 1.8 µV.
Question 8
Marks : +2 | -2
Pass Ratio : 100%
The Te is
70 K
110 K
149 K
240 K
Explanation:
The required answer is 60 (5-1) or 240K.
Question 9
Marks : +2 | -2
Pass Ratio : 100%
A system has three stage cascaded amplifier each stage having a power gain of 10 dB and noise figure of 6 dB. the overall noise figure is
1.38
6.8
4.33
10.43
Explanation:
The gain at each stage is 10db, hence the required answer is 4 + (4-1)/10 + (4-1)/100 or 4.33.
Question 10
Marks : +2 | -2
Pass Ratio : 100%
If the amplifier is sold to engineering public, the noise figure that would be quoted in a catalog is
0.46
0.94
1.83
2.93
Explanation:
The required answer is 1 + (240/290).