Electronic Circuits and Amplifiers
What this domain tests
This domain covers biasing, amplifier configurations, gain, feedback, oscillators, and operational-amplifier circuits. The exam favours load-line reasoning, phase relationships, and the effect of feedback on gain and bandwidth.
Core concepts
- The quiescent point (Q-point) sets DC collector current and collector-emitter voltage. The DC load line joins VCC/RC on the current axis and VCC on the voltage axis.
- Common-emitter (CE) gives voltage gain and a 180° phase shift. Common-base (CB) has low input impedance and current gain below unity. Common-collector (CC) has voltage gain near unity and high current gain, so it is a buffer.
- Class A conducts for the full cycle and has low efficiency. Class B conducts for half a cycle and is about 78.5% efficient in push-pull. Class AB reduces crossover distortion.
- Negative feedback reduces voltage gain but improves bandwidth, input/output impedance control, distortion, and stability.
- For an ideal op-amp with negative feedback, the input terminals are at nearly the same potential (virtual short) and input current is nearly zero.
Worked example
A CE amplifier has VCC = 12 V, RC = 2 kohm, IC = 2 mA, and beta = 100. VCE = VCC - IC RC = 12 - (2 mA × 2 kohm) = 8 V. The base current is IB = IC/beta = 20 microampere.
Traps
A bypass capacitor increases AC gain without changing the DC Q-point. Coupling capacitors block DC but pass AC. The virtual-short rule applies only with negative feedback, not to an open-loop comparator.
Key points
- CE: voltage gain plus 180° phase inversion.
- CC: buffer with voltage gain ≈ 1.
- Class B push-pull efficiency ≈ 78.5%.
- Negative feedback trades gain for bandwidth and linearity.
- Op-amp virtual short requires negative feedback.