Electronics Engineering: Devices, Biasing, and Amplifier Behaviour
Core concepts to master
You must be fluent with the diode (ideal, piecewise-linear, Zener), the BJT and MOSFET in each region of operation, dc load lines and Q-point calculation, and the small-signal models (r_e, hybrid-pi, g_m). Know the three BJT configurations: common emitter inverts and has moderate input impedance, common collector (emitter follower) is a buffer with near-unity gain, common base has low input impedance. Op-amp items assume ideal behaviour: infinite gain, infinite input impedance, zero output impedance, and - only when negative feedback is present - a virtual short between the inputs. Feedback, gain and phase margins, Barkhausen criterion for oscillation, Bode plots, and the decibel definitions all appear.
Traps
- Mixing the two decibel formulas: 20 log for voltage or current ratios, 10 log for power ratios.
- Applying the virtual-ground assumption to a comparator or any open-loop circuit.
- Forgetting that a Zener must be reverse biased with a series current-limiting resistor; check that the load current leaves enough for regulation.
- Confusing the −3 dB cutoff (0.707 of midband gain) with unity gain.
- Assuming a transistor is in active mode: verify by checking that V_CE exceeds about 0.3 V; otherwise it is saturated and the gain formula fails.
Worked example
An inverting amplifier uses Rin = 10 kilohm and Rf = 100 kilohm, driven by 0.2 V. Gain = −Rf/Rin = −10, so the output is −2.0 V. Sanity check: the input current 20 microamperes flows through Rf, giving 2 V - but only if the op-amp rails allow it.
Rule of thumb
For an NPN in the active region: base-emitter forward biased (about 0.7 V), collector-base reverse biased, and I_C = beta·I_B. Solve bias networks with Thevenin reduction of the base divider.