S4-1.5 Oscillators & Power Amplifiers

Work through these

• Positive feedback and conditions for sinusoidal oscillation

  An oscillator is an amplifier deliberately fed back on itself in phase, and the Barkhausen conditions say when that produces a clean sine wave rather than noise or silence. Both conditions matter: loop gain of one, and total phase shift back to zero.

• RC phase-shift oscillator

  The RC phase-shift oscillator builds its feedback delay out of resistors and capacitors alone, with no inductor anywhere. It is the cheapest oscillator to understand and the standard first worked example.

• LC oscillators: Hartley and Colpitts

  Hartley and Colpitts do the same job with a tuned circuit, and they differ only in whether the tank is split across inductors or capacitors. Telling them apart on a circuit diagram is the practical skill here.

• Crystal oscillator

  A quartz crystal behaves as an extremely selective tuned circuit, which is why clocks and radios use one instead of coils and capacitors. This is where frequency accuracy stops being an approximation.

• Classes of operation and harmonic distortion

  Amplifier classes describe how much of the input waveform the device actually conducts for, and the price of conducting less is distortion. The harmonic content is how that distortion is measured rather than described.

• Class A resistive-coupled and transformer-coupled amplifiers

  Class A conducts through the whole cycle, which is faithful and wasteful; the transformer-coupled version claws some efficiency back. Working the numbers once shows exactly where the wasted power goes.

• Class B push-pull and complementary symmetry; Class AB operation

  Class B splits the waveform between two devices and hits far better efficiency, at the cost of distortion right where the two hand over. Class AB pays a little idle current to remove that crossover problem, which is why almost all audio output stages are Class AB.

• Power dissipation and efficiency calculations; heat sinks

  The efficiency calculations are ordinary algebra, but the physical question behind them is not: power that does not reach the load becomes heat, and heat has to leave through a sink. This item is where amplifier design meets a thermal budget.

• Introduction to tuned amplifiers

  A tuned amplifier deliberately amplifies one narrow band of frequencies and rejects the rest, which is how a radio receiver picks one station. It closes the analog sequence by combining amplification with selectivity.

• Lab: simulate a Colpitts oscillator and verify oscillation frequency

  Simulate a Colpitts oscillator and check that it starts and settles at the frequency the tank formula predicts. An oscillator that refuses to start is the most instructive failure in this unit.