Part 1:
Question 1. FIGURE 1 shows the block diagram of a superhetrodyne radio receiver. In a test a 20∝V signal was fed from the aerial into the first stage of the receiver, a radio frequency amplifier. This signal is the passed through several stages of the receiver to eventually appear at the input to the AM detector. For the AM detector to work satisfactory it requires a minimum signal level of 3dB(mW). Calculate the required minimum gain of the fifth intermediate amplifier given the data in TABLE 1.
Stage

Gain

RF amplifier

+10 dB

Mixer

7 dB

BPF

1 dB

IF1

+15 dB

IF2

+15 dB

IF3

+20 dB

IF4

+10 dB

TABLE 1
FIG. 1
Question 2. As a rule of thumb, the perceived noise from a source falls by 6 dBA with every doubling of the distance from the source.
If a 1 MW wind turbine (FIGURE 2) produces 103 dBA of noise within one metre of its base, estimate the distance from the turbine at which the noise can be reckoned to have fallen below the background noise level of 40 dBA.
FIG. 2
Question 3. A simplified model of ADC noise refers the noise to a noisy input source resistance Rn while assuming the rest of the signal path to be noiseless. FIGURE 3 represents a particular 18bit ADC that has a 10 V input voltage range. The ADC has a bandwidth of 1 MHz.
Calculate the maximum value of Rn if the resolution of the ADC is not to be adversely affected by thermal noise. Assume the ADC operates at 25°C.
[N.b. The voltage resolution of an ADC is equal to its overall voltage measurement range divided by the number of discrete values possible on its output.]
FIG 3.
Question 4. The specification for a 4way TV antenna amplifier [FIGURE 4] is given in TABLE 2, below.
Bandwidth

40862 MHz

Gain

20 dB

Noise Figure

6 dB

Max. Output

85 dBµV

Input Impedance

75 ?

Output Impedance

75 ?

TABLE 2
FIG. 4
(a) Determine the voltage required on the input of the amplifier to give the maximum output of 85 dB∝V.
(b) If the signal level from the aerial is 5 dBmV and the input noise level is 20 dB∝V, calculate the signaltonoise ratio on the output of the amplifier.
Question 5. A voltage amplifier ideally should have the inputoutput relationship of v_{o} = 100v_{i} but in practice the relationship is v_{o} = v_{i}(98 + 2v_{i}).
Calculate the %age second harmonic distortion present in the amplifier's output for a sinusoidal input of 10 mV r.m.s.
Question 6.
You may find the following relationship helpful in answering this question:
For a lossless line:
The characteristic impedance is given by Z_{o} = √L/C and the velocity of propagation by v_{p} = 1/√LC
where L and C are, respectively, the line's inductance and capacitance per metre length.
A transmission line is formed by two identical parallel tracks in a printed circuit board, as depicted in FIGURE 5. The line has a length of 50 mm and all line terminations are of 70 ?. The line can be assumed to be lossless.
(a) Use equations (4) and (5) from the lesson to calculate the magnitude of the NEXT and FEXT voltages generated in the victim conductor when the source voltage, Vin, in the aggressor conductor is a voltage step of 2 V with a rise time of 100 ps.
[Note that the V_{s} voltage at the input to the aggressor conductor is
V_{s} = V_{in}. R_{in}/(R_{in} + Z)
(b) Sketch the NEXT and FEXT waveforms.
The relevant parameters for the line are listed in TABLE 3.
FIG. 5
Parameter

Value

Line inductance

400 nH per m

Line capacitance

80 pF per m

Mutual inductance

80 nH per m

Mutual capacitance

10 pF per m

TABLE 3
Part 2:
Question 1. Determine the voltage V_{O} in the circuit of FIGURE 1 for:
(a) β = 50, V_{BE} = 0.7 V Vs
(b) β = 250, V_{BE} = 0.7 V.
Comment on the significance of your result.
V_{S} 
12 V 
R_{C} 
1 k? 
R_{E} 
200 ? 
R_{B1} 
15 k? 
R_{B2} 
3.3 k? 
FIG. 1
Question 2. State the effects of negative feedback, when applied in a voltage amplifier, upon:
 the overall amplification
 variations in transistor gain
 nonlinearity
 output impedance.
Question 3. Estimate the power developed in the 8 ? speaker of the circuit of FIGURE 2 for a 1 kHz sinusoidal input signal of 100 mV peak. All capacitors may be assumed to act as a short circuit at the frequency of operation.
Compare your estimate with that derived from a PSpice simulation.
[A Simetrix version of the circuit can be downloaded from the module's Learning Materials on BlackBoard.]
FIG. 2
Question 4. FIGURE 3 shows the circuit of a multistage amplifier. Identify the stages and describe the operation and principle features of the amplifier. You should also make an estimate of the maximum output current of the amplifier.
FIG. 3
Question 5. FIGURE 4 shows an amplifier circuit. operation and performance of the circuit. should:
Write a short report on the In completing the report you
 Explain the operation of the circuit and in particular the role of resistors R_{1} and R_{2}.
 Build the circuit in PSpice and use it to determine:
(i) the quiescent value of V_{out}.
(ii) the voltage gain for a 100 mV, 1 kHz , input signal.
 Sketch the smallsignal equivalent circuit of the amplifier and use it to estimate the voltage gain. Compare your answer with that of (ii) above.
 Attempt to calculate the quiescent value of V_{out}. Compare your answer with that given by the PSpice model. Try to explain any discrepancies.
[Hint : Apply the appropriate equation (1 or 2) of Lesson 4.]
FIG. 4