Electronic Devices and Circuits
Electronic Devices and Circuits tests whether you can move between physical device behaviour and the biasing, small-signal and frequency-response models used in later analog design. Expect questions on PN junctions, BJT and MOSFET operating regions, bias stability, hybrid-pi and T models, and feedback.
Core rules
- Diode: I = Is(exp(V/(nVT))−1); VT ≈ 26 mV at 300 K and rises with temperature. Reverse saturation current roughly doubles every 10 °C.
- BJT: active region requires BE forward, BC reverse; IC = βIB, gm = IC/VT, rπ = β/gm. Saturation and cut-off are switch states, not amplifier states.
- MOSFET: triode when VDS < VGS−VTH; saturation when VDS ≥ VGS−VTH in long-channel models. gm = 2ID/(VGS−VTH) = sqrt(2μnCox(W/L)ID).
- Bias networks must stabilise IC against β and VBE spread; emitter degeneration trades gain for stability.
- Coupling and bypass capacitors set low-frequency poles; device capacitances set high-frequency poles; gain–bandwidth product is roughly constant for a given amplifier.
Worked example
A silicon diode with Is = 1 pA, n = 1, conducts at VD = 0.6 V. At 300 K, VT = 26 mV, so VD/VT = 23.08. exp(23.08) ≈ 1.06×10^10. Then I ≈ 10^-12 × 1.06×10^10 ≈ 10.6 mA. The −1 term is negligible. If temperature rises, VT increases and Is rises faster; the same current needs a lower forward voltage, roughly −2 mV/°C.
Traps
- Using 26 mV at every temperature.
- Treating a BJT in saturation as an amplifier.
- Ignoring Early effect, which gives finite output resistance ro.
- Forgetting body effect, which raises VTH when source is not at body potential.
- Mixing DC bias and small-signal quantities.