Electronics
What the exam tests
This domain covers semiconductor devices, diodes, transistors, operational amplifiers and logic gates. Questions test device behaviour, circuit calculations and truth tables, including the ideal op-amp model and digital logic combinations.
Core rules
- Intrinsic semiconductors conduct poorly; doping with donor or acceptor impurities makes n-type or p-type material.
- A p-n junction diode conducts in forward bias above its threshold voltage and blocks in reverse bias. It can rectify alternating current.
- A transistor can act as a switch or amplifier; the base-emitter junction is forward biased in the active mode.
- Ideal op-amp rules: infinite input resistance, zero output resistance, and virtual earth between the inputs when negative feedback is present.
- Inverting amplifier gain = −Rf/Rin; non-inverting gain = 1 + Rf/Rin.
- Logic gates: AND outputs 1 only when all inputs are 1; OR outputs 1 when any input is 1; NOT inverts; NAND and NOR are inverted forms.
Worked example
An inverting op-amp has Rin = 10 kΩ, Rf = 100 kΩ and Vin = 0.20 V. The supply is ±12 V. Find Vout. Gain = −Rf/Rin = −100/10 = −10. Vout = −10 × 0.20 = −2.0 V. The negative sign shows phase inversion. Since |−2.0 V| is below the 12 V supply, the op-amp remains in its linear region and does not saturate.
Traps
Assuming an op-amp output can exceed the supply rails; applying virtual earth when there is no negative feedback; mixing transistor terminals; forgetting the diode threshold voltage; writing a truth table with outputs inverted for NAND or NOR.