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Kerala Plus Two Electronics (DHSE) study materials
Kerala Plus Two Electronics (DHSE) study resources

Kerala Plus Two Electronics (DHSE): Amplifiers and Oscillators

Kerala Plus Two Electronics (DHSE) · Work through your exam, one topic at a time.

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Amplifiers and Oscillators

Amplifiers and Oscillators tests BJT amplifier configurations, gain and decibel calculations, power amplifier classes, feedback and the Barkhausen criterion. Numerical questions usually involve gain, dB conversion or identifying the oscillator type from its feedback network.

Core concepts

  • CE amplifier: high voltage and current gain, 180° phase inversion, moderate input and output impedance. CB: high voltage gain but current gain ≈α (<1), no inversion, low input impedance. CC emitter follower: voltage gain below 1, high current gain, no inversion, high input and low output impedance, used as a buffer.
  • Voltage gain in decibels: Av(dB) = 20 log(|Av|); power gain uses 10 log(P2/P1).
  • Power classes: class A conducts for 360°, maximum efficiency 25% with resistive load and 50% with transformer coupling; class B conducts for 180°, efficiency 78.5%, but suffers crossover distortion; class AB reduces crossover distortion; class C is highly efficient and used in tuned RF stages.
  • Negative feedback: reduces gain but stabilises it, widens bandwidth, reduces distortion and noise, and changes input/output impedance.
  • Oscillator: positive feedback with loop gain ≥1 and total phase shift of 360° (0°). RC phase-shift uses three 60° sections plus 180° from the transistor; Wien bridge uses lead-lag network; Hartley uses a tapped inductor; Colpitts uses split capacitors; crystal oscillators have very high Q.

Worked example

A CE amplifier has IE = 1 mA, Rc = 2.2 kΩ and a bypassed emitter resistor. The internal emitter resistance is re' = 26 mV / 1 mA = 26 Ω. Voltage gain is Av = −Rc/re' = −2200/26 ≈ −84.6. The magnitude is about 85, so Av(dB) = 20 log 85 ≈ 38.6 dB. The minus sign means phase inversion, not loss.

Traps that cost marks

  • Using 10 log for voltage gain instead of 20 log.
  • Saying an oscillator needs negative feedback; it needs positive feedback.
  • Swapping Hartley (tapped inductor) and Colpitts (split capacitor).
  • Quoting class B efficiency as 100%; the theoretical maximum is 78.5%.
  • Forgetting that bypassing RE increases AC gain but does not change DC bias.