Electronics

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 from Year 12 electricity and the alternating current work in magnetic fields.

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.

Key equations

f₀ = 1/(2π√LC)$f_{0} = 1/(2\pi\sqrt{LC})$f_0 = 1/(2π√(LC))

the resonant frequency of an LC circuit

Q = f₀/bandwidth$Q = f_{0}/\text{bandwidth}$Q = f_0/ bandwidth

the quality factor: how sharp a resonance peak is. This Q is not a charge

f_c = 12πRC$f_{c} = \frac{1}{2\pi RC}$f_c = 1/2πRC

the cut-off frequency of an RC filter

V_out/V_in = -R_f/R_in      1 + R_f/R₁ non-inverting$V_{out}/V_{in} = -R_{f}/R_{in} \qquad 1 + R_{f}/R_{1} \text{ non-inverting}$V_out/V_in = -R_f/R_in 1 + R_f/R_1 non-inverting

closed-loop gain with negative feedback

gain × bandwidth = constant$\text{gain} \times \text{bandwidth} = \text{constant}$gain × bandwidth = constant

what extra closed-loop gain costs in bandwidth

AM bandwidth = 2f_M      FM bandwidth = 2(deviation + f_M)$\text{AM bandwidth} = 2f_{M} \qquad \text{FM bandwidth} = 2(\text{deviation} + f_{M})$AM bandwidth = 2f_M FM bandwidth = 2(deviation + f_M)

the spectrum each modulation scheme occupies

Every equation the electronics notes display is collected, with its conditions, on the electronics equations page.

Common errors

• 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.

Suggested order

The MOSFET lesson and the Zener diode lesson first, since the rest of the analogue work assumes them, then resonant circuits and filters. The ideal op-amp and the comparator before negative feedback amplifiers, and both before summing and difference amplifiers. Digital signal processing and data communication are independent of the analogue half and can be revised on their own.