Wien Bridge Oscillator Overview
The Wien bridge oscillator, named after Max Wien (1866–1938), is capable of generating a low-distortion sine wave with a constant amplitude, spanning from approximately 15 Hz to 150 kHz. Remarkably, it achieves this functionality using only four operational amplifiers and can be powered by a single 9-volt battery. Unlike typical Wien bridge oscillators, this design stands out by eliminating the necessity for a dual-gang potentiometer for tuning purposes.
Innovative Operating Features
One of the distinctive features of this oscillator is its ability to function without a dual-gang potentiometer for tuning. Operational amplifier IC2b plays a crucial role by creating an artificial ground, enabling the circuit to operate seamlessly from a unipolar supply, such as a 9 V battery or a power pack. Additionally, IC2a serves as the primary amplifier for the oscillator, ensuring smooth and stable operation. To enhance its versatility, the frequency range is divided into four decades, facilitated by a 2-pole, 4-way rotary switch labeled as SW1.
Innovative Compensation Mechanism
In this configuration, only one arm of the Wien network experiences variation. Ordinarily, this change would lead to a shift in positive feedback. However, IC1b intervenes, effectively bootstrapping R2 to counterbalance the positive feedback alteration. This action adjusts the negative feedback sufficiently to sustain oscillation. The resistance of the tuning pot is manipulated linearly, yet this results in a logarithmic frequency change. To achieve a more conventional linear adjustment, a log-taper pot is utilized. Interestingly, the pot is wired so that counterclockwise rotation increases the frequency. Alternatively, an anti-log pot could be used, although these are notoriously challenging to procure.
Amplitude Monitoring and Control
IC1A operates as an integrator, actively monitoring the output signal’s amplitude and driving an LED (D2). Proper positioning of the LED facing the Light Dependent Resistor (LDR) is critical, necessitating shielding from ambient light, often accomplished with heat-shrink tubing. IC1a’s function includes regulating the gain of IC2a, ensuring oscillation persists with minimal distortion.
Performance and Adaptability
The generator achieves a maximum output amplitude of approximately 2 Vp-p when the LED and LDR are closely positioned. Distortion remains below 0.5% in the lowest range, dropping to levels too minimal for the author to measure in higher ranges. Any LDR with a dark resistance surpassing 100 kΩ should suffice. In the absence of an LDR meeting this requirement, increasing R5 until oscillation initiates is a viable solution. The circuit’s prototypes, constructed on breadboards by the author, employed dual and quad opamp packages, both proving equally effective. Schematic and circuit board design files for this project are accessible for download at www.elektor.com/120330.