S4-1.1 BJT Biasing & Amplifiers

Work through these

• Operating point, bias stability and stability factors

  The operating point is the steady DC voltage and current a transistor sits at before any signal arrives, and stability factors measure how far that point drifts when temperature or the device itself varies. Every amplifier result later in this subject assumes the point stayed where you put it.

• Fixed bias, collector-base bias, emitter bias, voltage-divider bias

  Four ways to set that operating point, from the crude single resistor to the voltage divider most real circuits use. Comparing them is the work: each buys stability at some cost in components or gain, and voltage-divider bias is the one worth knowing cold.

• Thermal runaway: what it is and how bias design prevents it

  A transistor that warms up passes more current, which warms it further, and left alone the cycle destroys the device. This is why bias networks carry the resistors they do, which turns out to be a design decision rather than a formality.

• CB, CE, CC amplifier analysis using h-parameters (approximate and exact)

  The h-parameter model turns a transistor into a small box of four numbers, so all three amplifier arrangements can be analysed with ordinary circuit algebra. The exact analysis is what examinations ask for; the approximate one is what working engineers reach for.

• Compare the three configurations: gain, impedance, use cases

  Common base, common emitter and common collector trade voltage gain, current gain and impedance against each other differently, which is why real circuits chain them. Knowing which to pick for a job outlasts any of the derivations behind them.

• Miller's theorem and an application circuit

  Miller's theorem explains why a capacitance bridging input and output looks far larger from the input side than its printed value says. It is the reason amplifier bandwidth collapses at high frequency long before the transistor itself runs out.

  Falstad Circuit Simulator · Reference

• Frequency response of BJT amplifiers

  An amplifier's gain is a function of frequency rather than a single number: coupling capacitors kill the low end and device capacitances kill the high end. The midband between them is the only region where the gain figures from earlier items hold.

• Lab: build or simulate a voltage-divider-biased CE amplifier and measure its gain and bandwidth

  The practical check on everything above. Build or simulate the circuit, measure what it actually does, and compare against your hand analysis; where the two disagree, the disagreement is the finding worth chasing.