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1. Determine the voltage VO in the circuit of FIGURE 1 for: (a) β = 50, VBE = 0.7 V Vs (b) β = 250, VBE = 0.7 V. RB1 RC Comment on the significance of Vo your result. IB IC VB VS 12 V VBE IE RC 1 kΩ...

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1. Determine the voltage VO in the circuit of FIGURE 1 for:
    (a) β = 50, VBE = 0.7 V
    
    
    
    
    
    
    
    
    
    
    Vs
    
    
    
    
    
    
    
    
    
    
    
    
    (b) β = 250, VBE = 0.7 V.
    RB1
    
    
    
    
    
    
    
    RC
    
    
    
    
    
    
    
    
    
    
    Comment on the significance of
    
    
    
    
    
    
    
    
    
    
    
    Vo
    
    
    
    
    
    
    
    
    
    
    
    
    
    your result.
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    IB
    
    
    
    
    IC
    
    
    
    
    VB
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    VS
    12 V
    
    
    
    
    
    VBE
    
    
    
    
    IE
    
    RC
    1 kΩ
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    RB2
    
    
    VE
    
    
    
    
    RE
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    RE
    200 Ω
    
    
    
    
    
    
    
    
    
    
    
    RB1
    15 kΩ
    
    
    
    
    
    
    
    
    
    
    
    0 V
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    RB2
    3.3 kΩ
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
FIG. 1
2. State the effects of negative feedback, when applied in a voltage amplifier, upon:
· the overall amplification
· variations in transistor gain
· non-linearity
· output impedance.
3. Estimate the power developed in the 8 Ω 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.]
    
    
    12 V
    18 kΩ
    1 kΩ
    8.2 kΩ
    
    BC109
    BC109
    
    
    
    Vin
    T3
    T1
    
    
    T2
    
    
    BFY51
    
    47 Ω
    
    
    
    18 kΩ
    
    
    27 Ω
    
    
    Loudspeake
    6.8 kΩ
    470 Ω
    
FIG. 2
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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 cu
ent of the amplifier.
Inverting input
Non-inverting
input
    T16
    T15
    
    T6
    
    
T11
T14
    
    
    T1
    T2
    T13
    
    
    
    
    
    
    
    
    
    
    D2
    T12
    
    
    
    
    
    
    T17
    
    T5
    
    
    
    
    
    
    T7
    
    
    D1
    T
    3
    T4
    T
    8
    
    
    
    
    
    
    
    
    1 kΩ
    50 kΩ 1 kΩ
    
    
Offset null
Comp
Offset null
Comp
+VCC
T9
25 Ω
Output
25 Ω
T10
–VCC
FIG. 3
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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 R1 and R2.
· Build the circuit in PSpice and use it to determine:
(i) the quiescent value of Vout.
(ii) the voltage gain for a 100 mV, 1 kHz , input signal.
· Sketch the small-signal 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 Vout. 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.]
    
    1.5k
    
    R3
    J2N3819
    Vout
    Vin
    12
    
    V1
    10 Meg
    100
    R2
    R1
FIG. 4
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N.B The parameters used by Simetrix for the transistor model can be obtained in the Schematic Command window. Use the function key F11 to reveal this window. But first import the description of the model by selecting from the menu bar;
Simulato
Import Models…/Import direct copy.
Note also that VP = VTO and IDSS = β × VP2 .
________________________________________________________________________________________
COMPONENT DATA
________________________________________________________________________________________
BFY51
________________________________________________________________________________________
ELECTRICAL CHARACTERISTICS BC 109
________________________________________________________________________________________
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2N3819
________________________________________________________________________________________
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1. With reference to the block diagram of FIGURE 1, state the two conditions that must be satisfied to give an oscillatory output.
si – Hso
    si
    –
    G
    so
Hso
H
FIG. 1
2. With reference to the block diagram of FIGURE 1, determine the required value of G to give an oscillatory output if H = –10 dB.
3. FIGURE 2 shows a public address system.
(a) It is found that if the microphone is
ought into proximity of the loudspeaker, the systems will ‘howl’. Carefully explain, making reference to feedback theory why this is so.
(b) Suggest two actions that could be adopted to remedy the ‘howling’.
(c) Measurements show that for a particular a
angement of the equipment and at a particular amplifier setting, the system will howl if 1% of the output power is fed back to the microphone. Estimate the power gain of the P.A. amplifier in decibels.
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FIG. 2
4. FIGURE 3(a) shows the circuit of an Armstrong oscillator (named after its inventor, the American engineer Edwin Armstrong in XXXXXXXXXXHere a transformer is used to couple the output to the input to give feedback. The transformer has a turns ratio of n:1, where n represents the primary winding.
In this particular circuit the transistor’s emitter resistor is bypassed by a large capacitor at a.c. frequencies and its base is biased via the transformer windings.
FIGURE 3(b) represents the a.c. equivalent circuit of the oscillator and (c) its h-parameter equivalent circuit.
(a) Explain the significance of the transformer’s dot notation in relation to the operation of the oscillator.
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(b) It can be shown that the loop-gain of the oscillator at resonance is given by:
    G
    
    1
    
    hfe
    R′
    
    
    
    
    
    
    
    
    
    
    n
    
    
    
    
    Vloop gain
    
    
    h
    L
    
    
    
    ie
    
    
    
    
    
    
where RL′ is the effective resistive load on the transistor, i.e.:
RL′  RL
hoe
n 2  R1
R2 
Estimate the required value of turns-ratio if:
R1 = 4.7 kΩ, R2 = 24 kΩ, RL = 2.7 kΩ, hfe = 250, hoe = 10–5 S, hie = 4 kΩ
+VCC
    R2
    CL
    
    RL
    
    
    
    L
    1:n
    Vo
    1:n
    
    Vi
    Vo
    
    
    Vi
C1 R1
    
    
    RE
    
    
    CE
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    (b)
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
(a)
FIGS. 3(a) and 3(b)
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    RL
    
    CL
    
    
    i
    
    
    
    
    
    n:1
    R1
R2
    Cin
    hie
    Vin
    
    hoe
    Co
    Vo
    L
    
    
    
    
    hfe i
    
    
    
    
    
    
    
    
    
    
    
    
    
(c)
FIG. 3(c)
5. FIGURE 4 shows another variation the Armstrong oscillator. A transformer with two secondary windings has been use, one to give feedback and one to give the oscillator’s output.
Write a short report [two to three pages] on an investigation into the operation and performance of this circuit1.
The report should em
ace, as far as you are able, the following themes:
(a) Why the output is taken via the transformer rather than directly off the collector of the transistor.
(b) The agreement between the measured and calculated quiescent voltages on the three terminals of the transistor.
________________________________________________________________________________________
1The circuit model is available in the module’s Learning Materials on Blackboard. In this simulation two extra components, C5/R5, have been added to the left of C2. The added capacitor ca
ies a small initial voltage to act as the necessary noise required to ‘kick start’ the oscillator. The default run time is from 100 to 102 milliseconds. The transformer has been formed from three mutually coupled inductors, rather than using the transformer model. This has been done because the parameter ‘inductance’ can be swept for an inductor in an a.c. analysis. This facility is not available in the transformer model.
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(c) The agreement between the measured and calculated frequency of oscillation.
(d) The shape of the output waveform in the first 10 milliseconds of start-up.
(e) The given L1:L2 ratio is not necessarily the optimum value to give a good sinusoidal output. [A Fourier probe on the output will give a spectral response]. Try to devise an experiment to find the optimum ratio*. Express the ratio as a turns ratio.
The report should include copies of any graphical responses produced in the investigation.
*This can be done by performing an AC sweep on the inductance parameter L2. However a sweep cannot be performed without a voltage source in the circuit. For the purposes of this analysis a small voltage source can be inserted into the feedback loop as shown in the second version of the circuit on Blackboard.
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2.7k
18k
R1
10u
Probe2-NODE
R3
    
    Q2
    C3
    
    10m
    C2
    BC 109
    
    
    L3
    
    
    C4
    L1
    5
    1u
    
    
    
    
    
    
    1
    10m
    V1
    
    
    
    
    
    
    
    
    
    1m
    8.2k
    1k
    C1
    
    L2
    R4
    R2
    10u
    
    
FIG. 4
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Answered 642 days After Jan 27, 2020

Solution

Swapnil answered on Oct 30 2021
122 Votes
1
    
    2
    Gain without feedback = A
Gain after feedback = A/ 1+ Aβ (β = feedback factor)
    A
    The overall amplification: Overall amplification decrease
    B
    Variations in transistor gain:

1
So, Gain = A/Aβ
Gain = 1/β (Gain only depends on β)
So the variation basically gains the transistor that cannot affect the overall transistor gain.
    C
    Non Linearity:
Non linearity reduces upon applying negative feedback
    D
    Output Impedance:
Input and output impedance will also improve by a factor of 1+Aβ.
    3
    Speaker impedance = 8 Ω
Voltage of input signal = 100mv
Input...
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