The Wien-Bridge Oscillator

 

The Wien-bridge oscillator is a commonly used low-frequency oscillator. This circuit achieves regenerative feedback by introducing no phase shift (0°) in the positive feedback path. As shown in Figure 18-4, there are two RC circuits in the positive feedback path (output to noninverting input).



FIGURE 18-4 Wien-bridge oscillator.
Thecircuit forms a low-pass filter, and the circuit forms a high-pass filter. Both RC filters have the same cutoff frequency (). Combined, they create a band-pass filter. As you know, a band-pass filter has no phase shift in its pass-band. As shown in Figure 18.7 of the text, the circuit oscillates at the intersection of the high-pass and low-pass response curves. It is common to see trimmer potentiometers added in series with and . They are used to fine-tune the circuit’s operating frequency.
The negative feedback path is from the output to the inverting input of the op-amp. Note the differences from a normal negative feedback circuit. Two diodes have been added in parallel with , as well as the potentiometer labeled . The potentiometer is used to control the of the circuit. The diodes also limit the closed-loop voltage gain of the circuit. If the output signal tries to exceed a predetermined value by more than 0.7 V, then the diodes conduct and limit signal amplitude. The diodes are essentially used as clippers.
Earlier we said that the Wien-bridge oscillator is a common low-frequency oscillator. As frequency increases, the propagation delay of the op-amp can begin to introduce a phase shift, which causes the circuit to stop oscillating. Propagation delay is the time required for the signal to pass through a component (in this case the op-amp). Most Wien-bridge oscillators are limited to frequencies below 1 MHz. Refer to Figure 18.9 of the text for a summary of Wien-bridge oscillator characteristics.