002909 Electronics
1. A modulation technique that uses two tones that are separated in frequency is
called..
Answer is option A.
(FSK).
2. In order to recover audio from a received single sideband signal, the receiver must..
Answer is option B.
(reinsert the missing carrier).
[refer:- 086044; page 56, paragraph 2 ]
3. Which of the following represents an invalid DTMF tone pair?
Answer is option D.
(1633 and 1477).
[refer:- 086046; page 25, table ‘DTMF TONE FREQUENCIES’].
4. Look at the schematic diagram shown in Figure A-1. This schematic diagram
represents a..
Answer is option B.
(phase-shift oscillator).
Note:- The 3 RC networks yield 60 degree phase shift / network, or a total of 180
degrees phase shift. The op-amp configured as a standard inverter, does the further
180 degree phase shifting.
Inverter configurations in op-amps reduce input impedance from 100s of megaohms
(infinite) value to R2!!
This is because, the op-amp drives its output voltage in such a manner that current flow
in the R1-R2 branch keeps the inverting pin voltage equal to that at the non-inverting
pin. (which is zero). This makes the inverting pin a “virtual ground”. (one that however
does not draw any current to ground via the inverting pin, as both input pins of an opamp have very high input impedance. Though no current is drawn, the (-) pin will closely
follow the (+) pin of the amplifier, establishing a virtual ground. One will however see R2
as the effective resistance to ground as an input impedance of the configuration!! So
one must remember that in real life such inverters if used to buffer potentiometer
signals, will actually load the potentiometer and alter the voltage being measured!!
Many novice engineers, including from Siemens (which I witnessed myself) have been
floored by this defective understanding of “input impedance” of op-amps.
5. In the circuit illustrated in Figure A-1, which of the following are frequencydetermining components?
Answer is option C.
(C1 and R3).
Note:- The other two RC pairs too. We selected C as it was the only representative true
one of the lot. However that does not mean the other 2 remaining RC networks are
useless.
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6. Look at the schematic diagram shown in Figure A-2. In this circuit, the lamp will light
only when..
Answer is option C.
(S2 is closed and S1 is in position A).
Note:- The position C is risky in SCR’s as for low values of currents, pick up signals
may drive the gate and fire the SCR Randomly. Position B is the “safe position”, where
cathode and gate are equipotential, and eliminate false triggering.
7. Which of the following statements about series regulators is correct?
Answer is option D.
(Series regulators are inherently more efficient than shunt regulators because the
current through the shunt regulator doesn’t flow through the load).
Note:- The series regulator is no doubt more efficient than a shunt regulator, manly as
the current flowing in the shunt is NOT load current and is a dissipative waste.
[refer:- 086048; page 48, heading ‘Comparison of Series-Pass and Shunt-Voltage
Regulators’, paragraph 1].
8. Look at the circuit diagram shown in Figure A-3. An electronics technician measures
a voltage of 25.7 V at the collector of Q 2 . Which of the following statements about this
circuit is correct?
Answer is option A.
(The circuit is operating normally).
Note:- The circuit is operating normally because these are precisely the checks.
a) – The base of Q1 should be 0.7 volts higher than the output voltage. Since the output
voltage is 25 Volts, Naturally base of Q1 is at 25.7 Volts. By implication, collector of Q2
is at same voltage, rather it is the one that is holding the output steady at 25 volts,
thanks to –ve feedback by Q2.
Q2 is NOT fully switched on, but always is in a dissipative linear region, ensuring that
base of Q2 remains at 20.7 Volts. The step down ratio of R3-R4 then decide how much
higher the output should be so that after “dividing” that voltage, one still gets 20.7 at
base of Q2.
Load regulation is done then onwards by Q2 and Q1. The two transistors switch partially
ON so that they hold the output voltage steady, at whatever current the output needs.
The more power the load needs, the more power these two transistors dissipate. Ideally
as dictated by the maximum power theorem, 50% of the total power will be eaten up by
these two “regulators”, when delivering maximum power!
THAT rather reflects on efficiency and design power handling safety factors.
9. In the circuit shown in Figure A-3, an electronics technician measures the output
voltage of the regulator circuit at 30 V. Which of the following statements about this
circuit is correct?
Answer is option A.
(A collector-to-emitter short is present in Q1).
Note:- The other 3 conditions listed would not lead to this situation. Truly speaking, if
R1 is to become opened, the zener will still hold its voltage through the emitter of Q2!
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002909 Electronics
If R4 is opened, even then the base of Q2 will hold at 20.7 Volts, by drawing just
enough current thru resistor R3. If R2 is shorted, even then, R5 will act as a load on the
circuit (shunt bleeder). This R5, will be reflected as (1+beta of Q1) times R5, on the
base side of Q1, (perceived impedance amplification on base circuit due to impedance
insertion in emitter follower). This will prevent runaway current in Q1. If the circuit is
loaded, the load impedance will appear (1 + beta) times its actual value to the base
emitter region, which will again limit the base current. Any impedance inserted as load
in an emitter follower appears roughly beta times more to the base emitter circuit.
10. When a load is applied to the power supply in the circuit shown in Figure A-3, the
output voltage tends to decrease. Which of the following actions in the circuit
counteracts this tendency?
Answer is option A.
(The conduction of Q2 decreases).
Note:- The conduction of Q2 is the main negative feedback part of this regulator.
As the load increases, the voltage momentarily drops a bit at the output.
Q2 amplifies this drop by reducing its collector current, drawn from R1. This now
becomes available to the base of Q1, which then switches it ON a bit more, linearly, so
that the output voltage is restored. It must be noted however that a small residual error
is always retained by this type of regulators, to enable it to take action. The higher the
gains (hfe) of the Q2, the smaller is this residual error. This is part of all
PROPORTIONAL CONTROLLERS. And this one is a proportional control, where
corrective action is proportional to the error.
11. Look at the schematic diagram shown in Figure A-4. This circuit diagram represents
a(n)
Answer is option B.
(frequency synthesizer).
Note:- This is what phase locked loops do, when they generate an output frequency
that is N times greater than the input frequency.
12. In the circuit illustrated in Figure A-4, which of the following factors determines the
accuracy of the output frequency?
Answer is option A.
(The stability of the VCO).
Note:- The VCO if it has drift, then it will give a completely different frequency which will
then be divided and compared in phase. An unstable VCO then could easily give double
the frequency for the same steady reference frequency!!
Though CMOS CD4046 type low power PLLs are now available in the market.
13. For the circuit shown in Figure A-4, what countdown is necessary from the divider
circuit to produce an output frequency of 10 MHz? (Assume a reference frequency of
100 kHz.)
Answer is option B.
(100).
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14. At which of the following points in the circuit shown in Figure A-4 would it be normal
to measure a varying DC voltage?
Answer is option A.
(Point 1).
Note:- Simply because it is the input to the Voltage Controlled Oscillator.
15. Which of the following does a relaxation oscillator depend on for proper operation?
Answer is option C.
(Negative resistance).
Note:- Actually hysteresis!! If it were only to be –ve resistance, the first peak would be
all one would see! The Neon tube would fire, and stay fired! But the firing discharges the
capacitor in the circuit and extinguishes the tube, which can then pass thru the full cycle
again!
16. Look at the schematic diagram and the four waveforms that are shown in Figure A5. Which of the following statements about this circuit is correct?
Answer is option D.
(No combination of closed switches will produce waveform 3 at the output of this circuit).
17. Again, look at the circuit diagram and the four waveforms that are shown in Figure
A-5. Which of the following waveforms will be produced when only S1 is closed?
Answer is option A.
(Waveform 1).
18. In order for a circuit to oscillate, it’s absolutely essential that the circuit have..
Answer is option B.
(regenerative feedback).
Note:- This is so because all circuits dissipate energy. The biggest source of problems
in oscillators is that the moment they produce oscillations, they also dissipate energy in
the reactive and passive elements comprising the “tank / tuned” area, which damps out
oscillations. In such conditions, regenerative feedback that “feeds on itself” helps to
sustain oscillations. Some amount of dampening balances out the regenerative part at
one point and the oscillations stabilize.
19. Look at the block diagram shown in Figure A-6. Which of the following multiplication
factors must take place in the two unmarked stages to obtain the 60 kHz output?
Answer is option D.
(*3 and *2).
20. Look at the circuit diagram and the four waveforms that are shown in Figure A-7.
Which of the four ripple waveforms shown would result if D 2 were to open?
Answer is option B.
(Waveform 2).
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Note:- This configuration fixes the junction of D1 &D2 as the +ve supply rail and ground
as the Other supply rail. Opening of any one diode totally disrupts that arm of the
bridge, resulting in a half-wave rectifier.
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