Analog and Digital Circuits
Analog and Digital Circuits is examined through rectifiers, amplifiers, op-amps, logic gates, flip-flops, counters and number systems.
What the exam tests
Questions cover rectifiers and filters, transistor amplifiers, op-amp configurations, logic gates, flip-flops, counters, number systems and Boolean simplification. You should calculate gain, identify gate outputs, convert binary/hex, and recognise linear versus switching operation.
Core rules
- Half-wave rectifier ripple frequency = supply frequency; full-wave and bridge = twice supply frequency. A larger filter capacitor reduces ripple but increases surge current.
- Ideal op-amp: infinite open-loop gain and input impedance, zero output impedance. Inverting gain = −Rf/Rin; non-inverting gain = 1 + Rf/Rin. Output cannot exceed supply rails.
- AND = all inputs 1; OR = any input 1; XOR = inputs different; NAND and NOR are universal gates.
- De Morgan: (A·B)' = A' + B'; (A + B)' = A'·B'.
- Binary to hex: group four bits from the right. A modulus-N counter has N states; a 4-bit ripple counter divides by 16.
Worked example
An inverting op-amp has Rin = 10 kΩ, Rf = 100 kΩ and Vin = 0.2 V, with ±12 V rails. Gain = −100k/10k = −10. Vout = −10 × 0.2 = −2.0 V. The negative sign means phase inversion, not a fault. If Vin were 2 V, the ideal result would be −20 V, but the op-amp would clip near −12 V, so the linear formula no longer applies. The reasoning is: calculate gain, multiply, then check against rails.
Traps that cost marks
Dropping the minus sign in inverting gain; applying the non-inverting formula to an inverting stage; expecting an op-amp to swing beyond its supply; using half-wave ripple frequency for a bridge; confusing latch and edge-triggered flip-flops; simplifying Boolean expressions by ordinary algebra instead of Boolean identities.