Transistor as an Oscillator

An electronic oscillator converts DC energy into AC energy of a chosen (usually high) frequency, ranging from a few Hz to several MHz -- unlike an amplifier, an oscillator needs NO external AC input signal at all; its output is entirely SELF-SUSTAINED. Oscillators are broadly sinusoidal (generating a pure sine wave of constant amplitude and frequency) or non-sinusoidal (generating square, ramp/sawtooth or triangular waveforms). Sinusoidal oscillations are further either damped (amplitude decreasing over time, due to resistive energy loss) or undamped (amplitude staying constant). A transistor oscillator circuit combines three parts: a TANK circuit (an inductor L and capacitor C in parallel, which stores energy alternately in L and C to produce natural oscillations of frequency $f=1/(2\pi\sqrt{LC})$, per the LC-oscillation physics covered in Volume 1's Section 4.9.1), a transistor AMPLIFIER (which amplifies the tank circuit's own signal, using it as the AC input source instead of an external signal), and a FEEDBACK NETWORK (which routes a portion of the amplified output back to the tank circuit, in phase with the existing oscillation, to replace the energy that resistive losses in the tank's real components would otherwise dissipate each cycle -- without this positive feedback the tank circuit's own oscillations would simply damp out). For SUSTAINED oscillation, the Barkhausen conditions must both hold: the total loop phase shift around the amplifier-plus-feedback loop must be $0^\circ$ or an integer multiple of $2\pi$, and the loop gain must be exactly unity, $A\beta=1$ (where $A$ is the amplifier's voltage gain and $\beta$ here is the feedback network's feedback ratio -- the fraction of output fed back to the input -- NOT the transistor current gain of 9.4.3.3). Different tank-circuit designs give different named oscillator circuits -- Hartley, Colpitts, phase-shift and crystal oscillators are named examples -- and oscillators find use generating periodic waveforms, RF carriers, audio tones, digital clock signals, and TV/CRO sweep circuits. …