S4-1.5 Oscillators & Power Amplifiers

What this is and why it exists

This unit holds two endpoints of the amplifier story. Feed an amplifier's output back to its input in phase and it stops amplifying and starts generating — every radio, clock and synthesiser depends on that controlled self-excitation. And when an amplifier must drive a speaker or a transmitter rather than the next small-signal stage, fidelity gives way to efficiency as the ruling concern, which is what the amplifier classes are about.

The vocabulary

• Barkhausen conditions — sustained oscillation needs loop gain of exactly one and loop phase shift of zero (or a whole multiple of 360 degrees) at the oscillation frequency.
• RC phase-shift oscillator — three RC sections contribute 180 degrees to complete the loop around an inverting amplifier; an audio-frequency choice.
• Hartley and Colpitts — LC oscillators distinguished by their tank: Hartley taps an inductor, Colpitts taps a pair of capacitors; radio-frequency workhorses.
• Crystal oscillator — a quartz resonator replaces the LC tank; its extraordinarily sharp resonance gives clock-grade frequency stability.
• Class A / B / AB — how much of each signal cycle the output device conducts: all of it, half of it, slightly more than half.
• Crossover distortion — the glitch where a class B push-pull pair hands over, while both devices are off near zero.
• Conversion efficiency — AC signal power delivered to the load divided by DC power drawn from the supply.

The mental model

An oscillator is a child on a swing. The amplifier is the push, the frequency-selective network is the swing's natural period, and the Barkhausen conditions say: push in time with the swing (zero net phase) and push hard enough to replace friction (loop gain of one). Push harder than that and the amplitude grows until the amplifier saturates — real oscillators start with gain above one and let the device's own limits settle the level. The tank's quality factor decides purity: RC networks are gentle slopes, LC tanks are sharper, and quartz is so sharp that the frequency barely moves with temperature or supply — which is why every microcontroller board carries a crystal.

Power amplifier class is where the heat goes. Class A conducts always: the output is faithful and the supply is drained continuously, with a hard ceiling of twenty-five percent efficiency resistively coupled and fifty percent transformer-coupled — the rest is heat in the transistor. Class B lets two devices conduct alternate half-cycles: idle dissipation vanishes and the ceiling rises to about seventy-eight percent, but the handover moment produces crossover distortion because neither device conducts until its base-emitter voltage is reached. Class AB spends a small standing current to bias both devices barely on, buying the handover smoothness of A at nearly the efficiency of B — the standard compromise in audio output stages. The heat that remains must leave through a heat sink, sized from the dissipation and the thermal resistances in the path from junction to air. Tuned amplifiers close the unit: an LC load makes a power stage selective, amplifying one frequency band while rejecting the rest — the shape of every transmitter output.

What you should now be able to explain or do

State both Barkhausen conditions and identify which network supplies the phase in each named oscillator. Choose between RC, LC and crystal oscillators for a stated frequency and stability need. Compute efficiency for class A and class B stages and explain where the lost power goes. Explain crossover distortion and how class AB removes it.

Check yourself