SINGLESTAGE TRANSISTOR AMPLIFIERS

Short notes

Objective questions (Answers are given below)

A CB amplifier has very low input resistance
because
(a) low emitter ac resistance re´ shunts
all other resistances
(b) it handles small input signals
(c) emitter bulk resistance is small
(d) its base is at ac ground.
CE amplifier is characterised by
(a) low voltage gain
(b) moderate power gain
(c) signal phase reversal
(d) very high output impedance.
A CC amplifier has the highest
(a) voltage gain (b) current gain
(c) power gain (d) output impedance.
In a CC amplifier, voltage gain
(a) cannot exceed unity
(b) depends on output impedance
(c) is dependent on input signal
(d) is always constant.
In a class-A amplifier, conduction extends over
360° because Q-point is
(a) located on load line
(b) located near saturation point
(c) centred on load line
(d) located at or near cut-off point.
The circuit efficiency of a class-A ampli- fier
can be increased by using
(a) low dc power input
(b) direct-coupled load
(c) low-rating transistor
(d) transformer-coupled load.
In a class-A amplifier, worst-case condi- tion
occurs with
(a) zero signal input
(b) maximum signal input
(c) high load resistance
(d) transformer coupling.
The output of a class-B amplifier
(a) is distortion-free
(b) consists of positive half-cycle only
(c) is like the output of a full-wave rectifier
(d) comprises short-duration current pulses.
The maximum overall efficiency of a transformercoupled
class-A amplifier is – per cent.
(a) 78.5 (b) 25 (c) 50 (d) 85
A transistor audio amplifier is found to have an
overall efficiency of 70 per cent. Most probably,
it is a ………… amplifier.
(a) class-B push-pull
(b) single-stage class-C
(c) transformer-coupled class-A
(d) direct-coupled class-A
The main purpose of using transformer coupling
in a class-A amplifier is to make it more
(a) distortion-free (b) bulky
(c) costly (d) efficient.
A class-B push-pull amplifier has the main advantage
of being free from
(a) any circuit imbalances
(b) unwanted noise
(c) even-order harmonic distortion
(d) dc magnetic saturation effects.
Crossover distortion occurs in …………….. amplifiers.
(a) push-pull (b) class-A
(c) class-B (d) class AB
The maximum overall efficiency of a class-B
push-pull amplifier cannot exceed – per cent.
(a) 100 (b) 78.5 (c) 50 (d) 85
The circuit of a class B push-pull amplifier is
shown in Fig. 60.50 . If the peak output voltage,
V0 is 16 V, the power drawn from the dc source
would be
(a) 10 W (b) 16 W (c) 20 W (d) 32 W
The dissipation at the collector is zero in the quiescent
state and increases with excitation in the
case of a
(a) class A series-fed amplifier
(b) class A transistor coupled amplifier
(c) class AB amplifier
(d) class B amplifier
Class AB operation is often used in power
(large signal) amplifiers in order to,

(a) get maximum efficiency
(b) remove even harmonics
(c) overcome cross-over distortion
(d) reduce collector dissipation

The main use of a class-C amplifier is
(a) as an RF amplifier
(b) as stereo amplifier
(c) in communication sound equipment
(d) as distortion generator.
If a class C power amplifier has an input signal
with frequency of 200 kHz and the width of
collector current pulses of 0.1μS, then the duty
cycle of the amplifier will be
(a) 1% (b) 2% (c) 10% (d) 20%
The primary cause of linear distortion in amplifiers
is
(a) change of gain with frequency
(b) unequal phase shift in component frequencies
(c) reactances associated with the circuit
and active amplifying element
(d) inherent limitations of the active de
vice.
An amplifier is said to suffer from distortion
when its output is
(a) low
(b) different from its input
(c) noisy
(d) larger than its input.
While discussing amplifier performance, noise
is defined as any kind of unwanted signal in
the output which is
(a) unrelated to the input signal
(b) derived from the input signal
(c) not generated by the amplifier
(d) due to associated circuitry.
An ideal amplifier has
(a) noise figure of less than 1 dB
(b) noise factor of unity
(c) output S/N more than input S/N
(d) noise figure of more than 0 dB.
The decibel is a measure of
(a) power (b) voltage
(c) current (d) power level.
When power output of an amplifier doubles,
the increase in its power level is …………… decibels.
(a) 2 (b) 20 (c) 3 (d) 10
When output power level of a radio recei- ver
increases by 3 dB, its absolute power changes
by a factor of
(a) 2 (b) 10 (c) 1/2 (d) 3.
Zero watt cannot be chosen as zero deci- bel
level because
(a) it is impossible to measure zero watt
(b) it is too small
(c) every power compared with it would
be zero
(d) it would be impossible to define a decibel.
A minus 3 dB point on the gain versus frequency
curve of an amplifier is that point where
(a) signal frequency drops to half the midband
frequency
(b) voltage amplification becomes half of
its maximum value
(c) power falls to half its maximum value
(d) upper cut-off frequency becomes
twice the lower cut-off frequency
The bandwidth of an amplifier may be increased
by
(a) decreasing the capacitance of its by pass
capacitors
(b) minimizing its stray capacitances
(c) increasing input signal frequency
(d) cascading it.
Lower cut-off frequency of an amplifier is primarily
determined by the
(a) interval capacitances of the active device
used
(b) stray capacitance between its wiring
and ground
(c) ac β value of its active devices
(d) capacitances of coupling and bypass
capacitors.
The main reason for the variation of amp- lifier
gain with frequency is
(a) the presence of capacitances, both ex- ternal
and internal
(b) due to interstage transformers
(c) the logarithmic increase in its output power
(d) the Miller effect.
The gain-bandwidth product of an amp- lifier
is given by
(a) f2 – f1 (b) fα – fβ
(c) fT (d) βfα
A Circuit which resonates at 1MHz has a of. Bandwidth between half-power points is
(a) 10 kHz (b) 100 kHz
(c) 10 Hz (d) 100 Hz

(a) 2. (c) 3. (b) 4. (a) 5. (c) 6. (d) 7. (a) 8. (b) 9. (c) 10. (a) 11. (d) 12. (c) 13. (a) 14. (b) 15. (b) 16. (d) 17. (a) 18. (a) 19. (d) 20. (c) 21. (b) 22. (a) 23.(b) 24. (d) 25. (c) 26. (a) 27. (d) 28. (c) 29. (b) 30. (d) 31. (a) 32. (c) 33. (a

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