We have met the diode and the transistor. Now we wire them up at speed, push them with feedback, kick them into oscillation, and squeeze real power out of them. This chapter is the analog engineer's playbook: how amplifiers behave at high frequency, how stages cascade, why feedback is the most underrated idea in all of electronics, how oscillators turn DC into clean sine waves, and how power amplifiers move serious wattage to a speaker without melting the heatsink.
In Chapter 1 we built the BJT and the MOSFET as devices, and used the basic h-parameter or hybrid-π model to analyze them at low frequencies. That gave us static gain, but real circuits operate over a wide frequency range, with multiple stages cascaded, with feedback wrapping around them, and sometimes with the transistor pushed hard into nonlinearity. In Chapter 2 we built the network grammar (KVL, KCL, Thevenin, transients). In Chapter 3 we built the language of frequency (Fourier, Laplace, transfer functions, poles, Bode plots). This chapter is where amplifier design lives, and where the intuition built up over the previous chapters cashes in.
The chapter is long because it has to be. Each topic (high-frequency models, multistage analysis, feedback, oscillators, power amplifiers, tuned amplifiers) is its own discipline, and each one reappears in everything you build after this. By the end you should be able to look at any analog schematic, identify the topology, predict its bandwidth, decide whether it is stable, estimate its efficiency, and explain in plain English what every component is doing.