Wienbridge

Wienbridge, also known as the Wien bridge, refers to a frequency-selective electrical network used in audio and signal-processing applications, and most prominently as the core of the Wien bridge oscillator. The network consists of two RC sections arranged in a feedback path: a series RC network and a parallel RC network. When arranged between the output and the noninverting input of an amplifier, the Wien bridge provides a zero phase shift and unity gain at a specific frequency, f0 = 1/(2πRC) (for equal R and C values). At this frequency, the feedback factor is approximately 1/3, meaning the amplifier must supply a gain of about 3 to sustain oscillations.

In the standard Wien bridge oscillator, the amplifier is configured with a gain slightly greater than 3 to start oscillations and a nonlinearity or automatic control element to stabilize the amplitude at a steady sine wave. Historically, one of the first successful implementations used a tungsten lamp as a temperature-dependent resistor to reduce gain as the output grew, producing a stable signal; later designs employed diodes, transistors, or integrated circuit feedback networks for amplitude stabilization. The circuit is valued for producing clean sine waves with low distortion when component values are matched and the stabilizing mechanism functions properly.

Applications of the Wien bridge network include audio frequency oscillators, signal generators, and test equipment where stable, low-distortion sine waves are required. Its simplicity and well-understood behavior make it a common teaching example in analog circuit design. The concept also informs broader discussions of RC phase-shift and feedback networks in oscillators.