Electronics
Electronics in GATE PH tests semiconductor devices, diodes, bipolar and field-effect transistors, operational amplifiers, feedback, oscillators, digital logic, and basic communication circuits. The exam often uses an ideal-device approximation and expects you to identify the operating region before calculating.
Core rules and concepts
- A diode conducts in forward bias above its cut-in voltage and is approximately an open circuit in reverse bias, apart from leakage and breakdown.
- BJT regions: cutoff, active, and saturation. In the active region, I_C ≈ βI_B and V_BE ≈ 0.7 V for silicon. The load line fixes the operating point.
- An ideal op-amp has infinite open-loop gain, infinite input impedance, and zero output impedance. With negative feedback, the virtual-short condition gives V₊ ≈ V₋.
- Inverting amplifier gain: A_v = -R_f/R_in. Non-inverting gain: A_v = 1 + R_f/R_in.
- Digital logic: verify gate outputs from truth tables; NAND and NOR are universal gates. Oscillators require positive feedback with loop gain of unity at the oscillation frequency.
Worked example
An inverting op-amp has R_in = 2 kΩ and R_f = 10 kΩ. The ideal gain is A_v = -R_f/R_in = -5. For an input of 0.2 V, the output is -1.0 V, provided the op-amp is not saturated and the supply rails exceed 1.0 V in magnitude. The virtual ground at the inverting input keeps the summing junction near 0 V.
Traps that cost marks
- Assuming virtual ground when there is no negative feedback.
- Ignoring op-amp saturation when the calculated output exceeds the supply rails.
- Mixing transistor active and saturation-region equations.
- Forgetting that loading by the next stage changes gain.