Basic and Digital Electronics
Basic and Digital Electronics
This domain tests diodes, BJTs, FETs, op-amps, logic gates, Boolean algebra, flip-flops and data converters. Expect both circuit calculations and truth-table reasoning.
Core rules
- A silicon diode drops about 0.7 V in forward conduction; germanium about 0.3 V. A Zener diode regulates in reverse breakdown.
- A BJT is in active mode when the base-emitter junction is forward biased and the base-collector junction is reverse biased. In saturation it behaves as a closed switch; in cutoff, as an open switch.
- An ideal op-amp has infinite open-loop gain and input impedance, and zero output impedance. Virtual short at the inputs applies only when negative feedback is present.
- Inverting amplifier gain = −Rf/Rin; non-inverting gain = 1 + Rf/Rin. Output cannot exceed the supply rails.
- NAND and NOR are universal gates. De Morgan: (A·B)' = A' + B'; (A + B)' = A'·B'.
- A modulus-n counter needs at least ceil(log2 n) flip-flops. An n-bit ADC has resolution Vref/2^n.
Worked example
An inverting op-amp has Rin = 10 kΩ, Rf = 100 kΩ and Vin = 0.2 V. Gain = −100/10 = −10. Output = −10 × 0.2 = −2 V. With ±12 V supplies the op-amp remains linear, so the result is valid.
Traps
Do not apply virtual ground without negative feedback. Do not ignore supply-rail saturation. Do not confuse asynchronous clear with clocked operation. Do not count flip-flops by n rather than by the required modulus.