the Colpitts oscillator
/ KOL-pits /
The Colpitts oscillator is a workhorse for making sine waves at radio frequencies, from hundreds of kilohertz up into the hundreds of megahertz, far higher than RC types comfortably reach. Its heart is an LC resonant tank: an inductor and capacitors that, like a struck bell or a child on a swing, naturally want to ring at one particular frequency. The amplifier's only job is to tap a little energy back into the tank each cycle to replace what is lost, keeping the bell ringing forever instead of dying away.
What distinguishes a Colpitts is how the feedback is tapped: the tank uses two capacitors in series across the inductor, and the junction between them is the feedback tap. Think of it as a capacitive voltage divider that picks off a fraction of the tank's swing to feed back to the transistor or op-amp in the right phase. The oscillation frequency is f equals 1 over (2 times pi times the square root of (L times C_series)), where C_series is the series combination of the two capacitors, C1 times C2 over (C1 plus C2). For example L equal to 1 microhenry with two 100 picofarad capacitors gives C_series of 50 picofarad and f near 22 MHz.
Colpitts is popular because the capacitive divider is easy to build and behaves well at high frequency, where stray capacitances can be absorbed into the two tank capacitors rather than spoiling things. Its frequency stability is far better than an RC oscillator's, set mostly by stable L and C values and by the tank's quality factor Q, the sharpness of its resonance. For the very best stability you replace the LC tank with a quartz crystal, giving the Colpitts crystal oscillator that clocks countless microcontrollers. Its close cousin, the Hartley, does the same job but taps a divided inductor instead of divided capacitors.
Choose L equal to 1 microhenry and two capacitors of 100 picofarad each. Their series value is (100 times 100) over (100 plus 100) equals 50 picofarad. Then f equals 1 over (2 times pi times the square root of (1 microhenry times 50 picofarad)), which is about 22 MHz.
An LC tank rings at its resonance; the capacitive tap feeds back just enough energy to sustain it.
An LC oscillator's frequency drifts with temperature and component aging far more than a crystal's, since L and C values shift; do not expect quartz-grade precision from a bare LC tank. A higher tank Q gives a cleaner, more stable tone, which is why low-loss inductors matter at RF.