Analog Circuits and Amplifiers
Analog Circuits and Amplifiers
This domain tests biasing, small-signal and power amplifier classes, negative feedback, op-amp configurations, filters and oscillators.
Core rules
- Class A conducts throughout the cycle and is inefficient; class AB reduces crossover distortion with two devices; class D switches for high efficiency.
- Negative feedback sets gain, widens bandwidth, lowers distortion and changes input/output impedance.
- An ideal op-amp has infinite gain and input impedance; practical rules use virtual ground only when negative feedback is present.
- Inverting gain is −Rf/Rin; non-inverting gain is 1 + Rf/Rin.
- Gain-bandwidth product limits closed-loop bandwidth: bandwidth ≈ GBW/closed-loop gain.
- Decouple op-amp supplies close to the pins; otherwise the stage may oscillate.
- Coupling capacitors set low-frequency response; emitter bypass capacitors increase AC gain.
Worked example
A non-inverting op-amp has Rf = 22 kΩ and Rin = 2.2 kΩ, with a 0.2 V input. Gain = 1 + 22/2.2 = 11. Ideal output = 2.2 V. With a 1 MHz gain-bandwidth product, closed-loop bandwidth is about 1 MHz/11 = 91 kHz. Check the supply: on ±12 V rails a 2.2 V output is fine, but a 3 V RMS input would demand 33 V and clip. Also check slew rate if the frequency is high; a 2.2 V peak at 20 kHz needs 0.28 V/µs.
Traps
Swapping inverting and non-inverting inputs; expecting rail-to-rail output from every op-amp; omitting supply decoupling; measuring gain with no load; forgetting that a virtual ground depends on feedback.