Audio systems: the audio chain, voltage and power amplification, gain in decibels, amplifier classes (A, B and AB), crossover distortion, and bandwidth.

RMS voltage (up to 3 marks): from $P = \dfrac{V_\text{rms}^2}{R}$, $V_\text{rms} = \sqrt{P R} = \sqrt{8.0 \times 8.0} = \sqrt{64} = 8.0\ \text{V}$.

RMS current (up to 2 marks): $I_\text{rms} = \dfrac{V_\text{rms}}{R} = \dfrac{8.0}{8.0} = 1.0\ \text{A}$ (or $I_\text{rms} = \sqrt{\frac{P}{R}} = \sqrt{\frac{8.0}{8.0}} = 1.0\ \text{A}$).

Markers reward $V_\text{rms} = 8.0\ \text{V}$ using $\sqrt{PR}$, and $I_\text{rms} = 1.0\ \text{A}$.

Class A (up to 2 marks): the output transistor conducts for the whole signal cycle. This gives low distortion (the signal is reproduced faithfully) but poor efficiency (typically under 30 per cent) because the transistor dissipates power even with no signal.

Class B (up to 2 marks): two transistors each conduct for one half of the cycle (push-pull). This is much more efficient (up to about 78 per cent) because each transistor is off for half the time, but it suffers crossover distortion at the point where one transistor hands over to the other (both are briefly off near zero, distorting the waveform).

Class AB (up to 2 marks): the transistors are biased slightly on so that each conducts a little more than half the cycle and they overlap at the handover. This removes the crossover distortion while keeping most of the Class B efficiency, so it is the standard for audio output stages.

Markers reward Class A (low distortion, low efficiency), Class B (efficient but crossover distortion), and Class AB (slight bias removes crossover distortion while staying efficient).