Electronic Circuits and Devices
This domain tests how semiconductor devices behave at dc and small signal, and whether you can choose a bias point, draw a load line, and predict gain or switching behavior. Expect diode conduction, BJT regions, MOSFET thresholds, and simple amplifier models.
Core rules
- A silicon PN diode is often modeled as 0.7 V when forward biased; reverse bias gives only leakage until breakdown.
- BJT: the active region needs base-emitter forward and collector-base reverse; saturation means both junctions forward and VCE near 0.2 V.
- MOSFET: check VGS > VTH for inversion and VDS > VGS - VTH for saturation.
- Small-signal models replace devices by gm, rπ, ro; capacitors set high-frequency rolloff.
Worked example
A 5 V source feeds a 1 kΩ resistor and a silicon diode to ground. Assume a 0.7 V drop. KVL gives VR = 5 - 0.7 = 4.3 V, so I = 4.3/1000 = 4.3 mA. The diode's incremental resistance is about nVT/I ≈ 26 mV/4.3 mA ≈ 6 Ω, small versus 1 kΩ, so the 0.7 V assumption is reasonable. If the diode were ideal, current would be 5 mA; the difference matters in precision bias calculations.
Traps
Using 0.7 V for a germanium or Schottky diode; forgetting the temperature coefficient (about -2 mV/°C for silicon); calling a BJT saturated when VCE is 3 V; ignoring the MOSFET body diode; and applying small-signal gain before verifying the dc operating point.