Electronic Devices and Circuits
This domain covers semiconductor physics, diode and transistor operation, biasing and small-signal models. Expect numericals on diode conduction, BJT and MOSFET bias points, and hybrid-π parameters derived from the quiescent current.
Key rules
- Diode: ID = IS(e^(VD/VT) − 1); a conducting silicon diode is taken as 0.7 V; a Zener regulates while its current stays between IZ(min) and IZ(max).
- BJT regions: active (BE forward, BC reverse), saturation (both forward), cut-off (both reverse). In the active region IC = βIB and VCE = VCC − ICRC − IERE.
- MOSFET: in saturation ID = ½k′(W/L)(VGS − VT)², valid when VDS ≥ VGS − VT.
- Small signal: gm = IC/VT ≈ 38.5 mS at 1 mA with VT = 26 mV, and rπ = β/gm.
Worked example
Fixed bias with VCC = 12 V, RB = 470 kΩ, RC = 2.2 kΩ, β = 100, VBE = 0.7 V. IB = (12 − 0.7)/470 kΩ = 24.0 µA; IC = 100 × 24.0 µA = 2.4 mA. VCE = 12 − (2.4 mA)(2.2 kΩ) = 12 − 5.28 = 6.72 V. Because VCE is well above 0.3 V, the device is genuinely in the active region and the assumed β is valid. The transconductance is gm = 2.4 mA/26 mV ≈ 92 mS.
Traps
Assuming β holds when the device is actually saturated - always check VCE first; omitting the drop across RE; using 0.7 V for a germanium diode; and mixing depletion-mode and enhancement-mode MOSFET equations.