1. What happens to capacitive reactance as the frequency of operation
increases?
ANS it decreases
2. As frequency decreases, how does the reactance of a coil vary?
ANS it decreases
3. what is skin effect, and how does it affect the Q of a coil?
ANS the tendency of electrons flowing in conductor to flow near and on
the after surface of the conductor frequencies in the VHF, CHF microcin
4. What happens to a wire when a ferrite bead is placed around it?
ANS blocks unwanted high frequencies.
5. What is the name given to the widely used coil form that is shaped like
a doughnut?
ANS Toroidal inductor.
6. Describe the current and impedance in a series RLC circuit at resonance
ANS when XL and XC equals, their effects cancel, and the impedence of
the circuit is just that of the resistance; and as they approach one another
the current begins to rise
7. Describe the current and impedance in a parallel RLC circuit at
resonance.
ANS draw no current from the source however, there is a high circulating
current between the inductor and capacitor, storing and transferring
energy between them; has an infinite impedence
8. State in your own words the relationship between Q and the bandwidth
of a tuned circuit.
ANS the bandwidth of a circuit is inversely proportional to Q
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1. What is the gain of an amplifier with an output of 1.5 V and an input of
30 µV?
V out
1.5 V
ANS A v = V → 30 μV → A v =50000V
¿
2. What is the attenuation of a voltage divider like that in Fig. 2-3, where
R1 is 3.3 kΩ and R2 is 5.1 kΩ?
ANS
A 1=
A 1=
R2
5.1 kV
→
→ A1 =0.6071
R1 + R2 3.3 kV +5.1 kV
R2
3.3 kV
→
→ A1 =0.3928
R1 + R2 3.3 kV +5.1 kV
3. What is the overall gain or attenuation of the combination formed by
cascading the circuits described in Problems 1 and 2?
ANS
A r =A 1 × A 2 → 0.6071 ×0.3928 → A r=0.2385
4. Three amplifiers with gains of 15, 22, and 7 are cascaded; the input
voltage is 120 µV. What are the overall gain and the output voltages of
each stage?
A r =A 1 × A 2 × A3 →15 ×22 ×7 → A r =2310
ANS
V out = A r ×V ¿ → 2310× 120 μV →V out =0.2772
V 1= A1 ×V ¿ → 15× 120 μV → V 1=0. 0018
V 2= A2 ×V ¿ → 22× 120 μV → V 2=0. 00264
V 3= A3 ×V ¿ → 7 ×120 μV → V 3=0.00084
5. A piece of communication equipment has two stages of amplification
with gains of 40 and 60 and two loss stages with attenuation factors of
0.03 and 0.075. The output voltage is 2.2 V. What are the overall gain (or
attenuation) and the input voltage?
ANS A= A 1 × A 2 → 40 ×60 → A G =240
A T =A 1 × A2 →0.03 × 0.075→ A T =0.00225
AG=
V out
2.2 V
2.2 V
→240=
→ V ¿=
→ V ¿=9.16 mV
V¿
V¿
240
6. Find the voltage gain or attenuation, in decibels, for each of the circuits
described in Problems 1 through 5.
V out
ANS Problem 1: dB=20 log V → 20 × log ( 50000 ) → dB=93.97
¿
R2
5.1 kV
→
→ A1 =0.6071
R1 + R2 3.3 kV +5.1 kV
Problem 2:
A 1=
Problem 3:
A T =A 1 × A2 →0.6071 ×0.3928 → A T =0.2385
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V out
0.2772V
→ 20 × log
→dB=67.27
V¿
120 μV
Problem 4:
dB=20 log
Problem 5:
A T =A 1 × A2 →0.03 × 0.075→ A T =0.00225
7. A power amplifier has an output of 200 W and an input of 8 W. What is
the power gain in decibels?
Pout
200W
dB=10
log
→ 10 ×log
→ db=13.97
ANS
P¿
8W
8. A power amplifier has a gain of 55 dB. The input power is 600 mW.
What is the output power?
5.5
ANS Pout =P¿ × A p=0.6W × 10 =189.7 kW
9. An amplifier has an output power of 5W. What is this amount of power
in dBm?
5W
ANS Pout ,dBm=10 log 10 0.001W =37 dBm
(
)
10. A communication system has five stages, with gains and attenuations
of 12 dB, -45 dB, 68 dB, -31 dB and 9 dB. What is the overall gain in dB ?
ANS
A T ,dB =A P 1 ,dB + A P 2 ,dB + A P 3 , dB+ A P 4 , dB+ A P 5 ,dB=12−45+ 68−31+9=13 dB
11. What is the reactance of a 7-pF capacitor at 2 GHz ?
−12
C=7− pF → 7 ×10 F
ANS
9
f =2 ×10 Hz
XC=
1
1
→
=11
2 πfC
( 6 .2 8(7 ×10−12)(2× 109 ))
12. What value of capacitance is required to produce 50 Ω of reactance at
450 MHz ?
6
ANS X C =50 Ω, f =450× 10 Hz
XC=
1
1
→50=
2 πfC
( 6.28 ( 7 ×10−12 ) C )
( 1.413 ×1011 ) C=1
−12
C=7.082× 10
13. Calculate inductance reactance of a 0.9-µH coil at 800 MHz?
−6
6
ANS L=0.9 ×10 H , f =800 ×10 Hz
X L=2 π fL → 6.28 ( 800 ×106 ) ( 0.9 ×10−6 )
¿ 4521.6 Ω
14. At what frequency will a 2-µH inductor have a reactance of 300 Ω ?
−6
L=2 ×10 H , X L=300 Ω
ANS
X L=2 π fL
300=6.28 ( 2 ×10−6 ) f
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300
=f
( 1.256× 10−5 )
= 23885350.32 Hz → 23.885 MHz
15. A 2.5 -µH inductor has a resistance of 23 Ω at a frequency of 35 MHz,
What is it Q?
ANS
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