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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