Oscillators

An oscillator generates a periodic signal with no external input — it's a circuit that sustains itself. The feedback loop that makes an amplifier oscillate is usually something you want to avoid, but oscillators are the intentional version: controlled, stable, and useful. Clock signals, audio tones, RF carriers, PWM references — they all come from oscillators.

The Feedback Condition

An oscillator needs two things: sufficient loop gain to sustain oscillation (gain × feedback ≥ 1), and a phase shift around the loop of exactly 360° at the oscillation frequency. The Barkhausen criterion formalises this. Satisfy both conditions at one frequency and the circuit oscillates at that frequency.

RC Oscillators

Use resistors and capacitors for the frequency-setting network. Good for audio frequencies. The frequency is set by RC time constants.

Phase Shift Oscillator

Three RC stages, each contributing 60° of phase shift at the oscillation frequency, plus the 180° from an inverting amplifier. Total: 360°.

f = 1 / (2π × RC × √6)

Example: R=10kΩ, C=10nF
f = 1 / (2π × 10,000 × 0.00000001 × 2.449)
f ≈ 649 Hz

Wien Bridge Oscillator

A non-inverting amplifier with a Wien network (series RC + parallel RC) in the feedback. Clean sine wave output, widely used in audio test equipment. The classic HP 200A audio oscillator used a Wien bridge.

f = 1 / (2πRC)

Requires automatic gain control (AGC) to maintain stable amplitude — without it, the oscillation either dies or clips. The original HP design used a small incandescent lamp as a thermistor for AGC.

LC Oscillators

Use an inductor-capacitor tank circuit for frequency selection. Better frequency stability and better performance at RF frequencies than RC oscillators.

f = 1 / (2π√(LC))

Colpitts Oscillator

A transistor or FET amplifier with an LC tank circuit, where the capacitor is split into two series capacitors and the feedback is taken from the junction between them. Standard topology for RF oscillators from a few MHz to hundreds of MHz.

Hartley Oscillator

Similar to Colpitts but with a tapped inductor instead of split capacitor. Less common now but historically important in AM radio receivers.

Crystal Oscillators

A quartz crystal is a piezoelectric resonator — it has a very high Q factor (10,000 to 100,000, compared to 100–200 for LC circuits). This means the oscillation frequency is extremely stable and precise. Temperature-compensated crystal oscillators (TCXOs) and oven-controlled oscillators (OCXOs) take stability further.

Crystal frequency: set by the crystal's physical dimensions, specified in the part number
Stability: ±20ppm to ±0.1ppm depending on grade

Virtually all digital systems use crystal oscillators as clock sources — microcontrollers, microprocessors, Ethernet PHYs, GPS receivers. The 32.768kHz crystal is a special case: it's used as a real-time clock source because 2^15 = 32768, so 15 binary divisions gives exactly 1Hz.

Relaxation Oscillators

A capacitor charges through a resistor until it hits a threshold, then rapidly discharges, then repeats. The output is a sawtooth or square wave depending on how you measure.

The 555 timer in astable mode is the classic relaxation oscillator:

f = 1.44 / ((R_A + 2×R_B) × C)
Duty cycle = (R_A + R_B) / (R_A + 2×R_B)

Simple, robust, works from a wide supply voltage range, needs no external gain stage. Good for audio tones, LED blinkers, PWM generation, and wherever precise frequency isn't critical.

VCOs — Voltage-Controlled Oscillators

A VCO's frequency is controlled by a voltage input — higher voltage, higher frequency (or lower, depending on design). The essential component of PLLs (phase-locked loops), which are used in frequency synthesis, clock recovery, FM demodulation, and motor control. The 555's timing resistor can be replaced with a transistor-based circuit to make a simple VCO.