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Electronics

The option unit for signals and the circuits that handle them: diodes and transistors doing real jobs, resonant circuits that pick one frequency out of many, op-amps that amplify, add and subtract, and the logic, counting and data links that carry information as numbers.

Year 13 · 6 topics · AQA option unit.

What electronics covers

One of the five AQA option units, and you sit exactly one option, so take this only if it is the one your centre teaches. It covers signals and the circuits that handle them: switching a load, picking one frequency out of many, amplifying and combining voltages, and handling information as numbers. It assumes the potential divider and alternating current work from Year 12 electricity.

The main ideas

The MOSFET as a switch: the drain characteristic, the threshold voltage, and driving a load from a logic output that supplies almost no current.
The zener diode with its series resistor holding an output steady, and what a photodiode and a Hall effect sensor each measure.
LC resonance, series against parallel, the Q factor with bandwidth, and RC filters with their cut-off frequency.
The ideal operational amplifier, saturation, and the comparator built from it.
Negative feedback, the inverting amplifier and its virtual earth, the non-inverting amplifier, and the gain-bandwidth product.
The summing amplifier as a weighted adder and a digital to analogue converter, and the difference amplifier rejecting interference common to both inputs.
Logic gates, the half-adder, the D-type flip-flop, counters and astable timing, then modulation, sampling, propagation modes and multiplexing.

The equations it turns on

$f_{0} = 1/(2\pi\sqrt{LC})$: the resonant frequency of an LC circuit
$Q = f_{0}/\text{bandwidth}$: the quality factor: how sharp a resonance peak is. This Q is not a charge
$f_{c} = \frac{1}{2\pi RC}$: the cut-off frequency of an RC filter
$V_{out}/V_{in} = -R_{f}/R_{in} \qquad 1 + R_{f}/R_{1} \text{ non-inverting}$: closed-loop gain with negative feedback
$\text{gain} \times \text{bandwidth} = \text{constant}$: what extra closed-loop gain costs in bandwidth
$\text{AM bandwidth} = 2f_{M} \qquad \text{FM bandwidth} = 2(\text{deviation} + f_{M})$: the spectrum each modulation scheme occupies

Where it usually goes wrong

Every op-amp gain equation holds only while the output stays inside the supply rails. Work out the ideal output, then check it against the saturation voltage, because questions push it past deliberately.
A tuned circuit passes a band of frequencies, not one. Added resistance lowers the peak and widens the band, and the Q factor reports both at once.
The MOSFET gate takes no steady current, which is why a logic output rated at a milliamp can switch several amps through a load.
In a summing amplifier each input has its own gain, the feedback resistor divided by that input's own resistor, so the inputs are not simply added.

Where to start

Discrete semiconductor devices first, since the rest of the analogue work assumes them, then resonant circuits and filters. Operational amplifiers before summing and difference amplifiers. Digital signal processing and data communication are independent of the analogue half and can be revised on their own.