Amplifiers: Small
signal amplifiers: General principles of operation, classification, distortion,
10 questions on each with explained answers
## General Principles of Operation
**1. Small signal amplifiers operate in which region?**
a) Cutoff
b) Saturation
c) Active/linear region
d) All regions
**Answer: c) Active/linear region** [1]
**Explanation:** Transistor stays in active region where output proportionally follows input without clipping, ensuring waveform fidelity. [1]
**2. Primary purpose of biasing in small signal amps?**
a) Maximize power
b) Set Q-point for linear operation
c) Block AC signals
d) Increase distortion
**Answer: b) Set Q-point for linear operation** [1]
**Explanation:** Q-point centers signal swing in linear portion of transfer curve, preventing distortion from cutoff/saturation. [1]
**3. DC analysis in small signal amps replaces AC sources with?**
a) Short circuits
b) Internal resistances
c) Open circuits
d) Zero voltage
**Answer: b) Internal resistances** [1]
**Explanation:** Establishes quiescent DC operating point before AC small-signal superposition. [1]
**4. Coupling capacitors in small signal amps function as?**
a) DC blocks, AC passes
b) DC passes, AC blocks
c) Voltage doublers
d) Filters
**Answer: a) DC blocks, AC passes** [6]
**Explanation:** Low reactance at signal frequency isolates DC bias between stages. [6]
**5. Small signal model uses?**
a) Large signal nonlinear
b) Linearized parameters around Q-point
c) DC only
d) Time domain only
**Answer: b) Linearized parameters around Q-point** [1]
**Explanation:** Hybrid-pi or T-model approximates behavior for signals << Q-point variations. [1]
**6. Input signals for small signal amps typically range?**
a) Volts
b) Microvolts to millivolts
c) Amps
d) Kilovolts
**Answer: b) Microvolts to millivolts** [1]
**Explanation:** Designed for sensors/audio where signals too weak for direct processing. [1]
**7. Negative feedback in small signal amps improves?**
a) Distortion, reduces bandwidth
b) Linearity, bandwidth, reduces distortion
c) Gain only
d) Power efficiency
**Answer: b) Linearity, bandwidth, reduces distortion** [1]
**Explanation:** Feedback stabilizes gain against variations, trades gain for performance. [1]
**8. AC analysis assumes?**
a) DC sources shorted
b) Superposition around Q-point
c) Nonlinear operation
d) Large signals
**Answer: b) Superposition around Q-point** [1]
**Explanation:** Small AC variations superimposed on DC bias for frequency response. [1]
**9. Transistor choice depends on?**
a) Gain, bandwidth, noise
b) Power only
c) Size
d) Cost alone
**Answer: a) Gain, bandwidth, noise** [2]
**Explanation:** BJT for low noise, FET for high input impedance applications. [2]
**10. Emitter resistor provides?**
a) Positive feedback
b) Negative feedback for stability
c) No feedback
d) Oscillation
**Answer: b) Negative feedback for stability** [5]
**Explanation:** Degenerative feedback linearizes gain, stabilizes against temperature/parameter variations. [5]
## Classification of Amplifiers
**1. Class A amplifiers conduct for?**
a) Half cycle
b) Full 360° cycle
c) 180°
d) None
**Answer: b) Full 360° cycle** [3]
**Explanation:** Transistor active entire input cycle, highest fidelity but lowest efficiency ~25%. [3]
**2. Class B operation uses?**
a) Single transistor full cycle
b) Push-pull, each half cycle
c) Always on
d) Cutoff only
**Answer: b) Push-pull, each half cycle** [3]
**Explanation:** Complementary pair reduces crossover distortion, ~78% theoretical efficiency. [3]
**3. Voltage amplifier classification example?**
a) Common collector
b) Common emitter
c) Emitter follower
d) Buffer
**Answer: b) Common emitter** [10]
**Explanation:** High Av, medium Zi/Zo suits voltage amplification stages. [10]
**4. Class AB improves Class B by?**
a) Increasing distortion
b) Slight forward bias reducing crossover distortion
c) Full class A
d) Efficiency drop
**Answer: b) Slight forward bias reducing crossover distortion** [3]
**Explanation:** Quiescent current prevents gap at zero crossing. [3]
**5. Power amplifiers classified by?**
a) Conduction angle
b) Gain only
c) Frequency
d) Load
**Answer: a) Conduction angle** [3]
**Explanation:** Class A 360°, B 180°, AB >180°<360°, C <180°. [3]
**6. Common base amplifier classifies as?**
a) Voltage amp
b) High frequency, low Zi
c) Buffer
d) High power
**Answer: b) High frequency, low Zi** [11]
**Explanation:** Low Miller capacitance suits RF stages. [11]
**7. Class C efficiency?**
a) Low
b) Highest >90%
c) 25%
d) 50%
**Answer: b) Highest >90%** [10]
**Explanation:** Narrow conduction tuned loads, high distortion unsuitable audio. [10]
**8. Buffer amplifier is?**
a) Common emitter
b) Common collector/unity gain
c) High gain
d) Inverting
**Answer: b) Common collector/unity gain** [11]
**Explanation:** High Zi low Zo impedance matching. [11]
**9. RC coupled amplifiers classified as?**
a) Power
b) Voltage, interstage coupling
c) RF
d) Audio only
**Answer: b) Voltage, interstage coupling** [6]
**Explanation:** Capacitors couple stages, resistors bias. [6]
**10. Transformer coupled advantage?**
a) DC coupling
b) Impedance matching, no capacitors
c) Low frequency response
d) Distortion free
**Answer: b) Impedance matching, no capacitors** [3]
**Explanation:** Wide bandwidth via turns ratio. [3]
## Distortion in Amplifiers
**1. Harmonic distortion caused by?**
a) Linear operation
b) Nonlinear transfer function
c) Perfect Q-point
d) Feedback
**Answer: b) Nonlinear transfer function** [1]
**Explanation:** Generates harmonics (2f,3f) from fundamental when signal large. [1]
**2. Crossover distortion occurs in?**
a) Class A
b) Class B/AB push-pull
c) Single ended
d) FET only
**Answer: b) Class B/AB push-pull** [3]
**Explanation:** Gap near zero when both devices off. [3]
**3. Total harmonic distortion (THD) measures?**
a) DC component
b) Ratio of harmonics to fundamental
c) Gain
d) Bandwidth
**Answer: b) Ratio of harmonics to fundamental** [1]
**Explanation:** Lower THD indicates cleaner amplification. [1]
**4. Negative feedback reduces distortion by?**
a) Increasing gain
b) Linearizing response, sampling output
c) Adding harmonics
d) No effect
**Answer: b) Linearizing response, sampling output** [1]
**Explanation:** Compares output to input, corrects errors. [1]
**5. Clipping distortion from?**
a) Small signals
b) Overdrive into saturation/cutoff
c) Perfect bias
d) Feedback
**Answer: b) Overdrive into saturation/cutoff** [1]
**Explanation:** Flattens waveform peaks asymmetrically. [1]
**6. Intermodulation distortion from?**
a) Single tone
b) Multiple frequencies mixing nonlinearly
c) DC bias
d) Linear gain
**Answer: b) Multiple frequencies mixing nonlinearly** [7]
**Explanation:** Produces sum/difference products f1±f2. [7]
**7. Phase distortion affects?**
a) Amplitude
b) Frequency response non-flat phase
c) Harmonics only
d) DC
**Answer: b) Frequency response non-flat phase** [7]
**Explanation:** Transient smearing in non-minimum phase systems. [7]
**8. Emitter degeneration reduces?**
a) Gain only
b) Distortion via negative feedback
c) Bandwidth
d) Input Z
**Answer: b) Distortion via negative feedback** [5]
**Explanation:** Re feedback stabilizes against nonlinearities. [5]
**9. Thermal distortion from?**
a) Constant temp
b) Junction heating varying parameters
c) Cold operation
d) Feedback
**Answer: b) Junction heating varying parameters** [2]
**Explanation:** Self-heating shifts bias during large signals. [2]
**10. Lowest distortion amplifier class?**
a) Class C
b) Class A
c) Class B
d) Class D
**Answer: b) Class A** [3]
**Explanation:** Continuous conduction avoids crossover/clipping issues. [3]
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