RC oscillator

Linear electronic oscillator circuits, which generate a sinusoidal output signal, are composed of an amplifier and a frequency selective element, a filter.

RC oscillators are a type of feedback oscillator; they consist of an amplifying device, a transistor, vacuum tube, or op-amp, with some of its output energy fed back into its input through a network of resistors and capacitors, an RC network, to achieve positive feedback, causing it to generate an oscillating sinusoidal voltage.

[6] Their lack of bulky inductors also makes them easier to integrate into microelectronic devices.

[6] The amplifier provides gain to compensate for the energy lost as the signal passes through the feedback network, to create sustained oscillations.

In RC oscillator circuits which use a single inverting amplifying device, such as a transistor, tube, or an op amp with the feedback applied to the inverting input, the amplifier provides 180° of the phase shift, so the RC network must provide the other 180°.

Although the frequency can be varied over a small range by adjusting a single circuit element, to tune an RC oscillator over a wide range two or more resistors or capacitors must be varied in unison, requiring them to be ganged together mechanically on the same shaft.

[9] So a much wider frequency range can be covered by a given variable capacitor in an RC oscillator.

Then at the oscillation frequency each RC section contributes 60° phase shift for a total of 180°.

The signal in the C-R-C branch of the Twin-T filter is advanced, in the R-C-R - delayed, so they may cancel one another for frequency

It is possible to stabilize a quadrature oscillator by squaring the sine and cosine outputs, adding them together, (Pythagorean trigonometric identity) subtracting a constant, and applying the difference to a multiplier that adjusts the loop gain around an inverter.

Such circuits have a near-instant amplitude response to the constant input and extremely low distortion.

Similarly, the slightest decrease will cause the sine wave to die out exponentially to zero.

In most ordinary oscillators, the nonlinearity is simply the saturation (clipping) of the amplifier as the amplitude of the sine wave approaches the power supply rails.

The higher gain allows an oscillator to start by exponentially amplifying some ever-present noise.

[12] The net effect is the oscillator amplitude will stabilize when average gain over a cycle is one.

The nonlinearity that reduces the gain may also be more subtle than running into a power supply rail.

So in oscillators that must produce a very low-distortion sine wave, a system that keeps the gain roughly constant during the entire cycle is used.

[15][16] These oscillators exploit the resistance of a tungsten filament of the lamp increases in proportion to its temperature (a thermistor works in a similar fashion).

The lamp both measures the output amplitude and controls the oscillator gain at the same time.