17.1: Theory Overview

The Wien bridge is a four element resistor-capacitor network that can be thought of as a combination of lead and lag networks. As such, it attenuates very high and very low frequencies. At its critical frequency, where the magnitude of \(X_c\) equals \(R\), the bridge voltage produces no phase shift and exhibits a modest signal loss of 1/3. An op amp with a voltage gain of 3 may be used to overcome this loss, and as long it produces no additional phase shift, the system can produce stable oscillation at the critical frequency. A non-inverting amplifier is ideally suited to this task. The gain needs to be slightly greater than 3 to begin oscillation and should fall back to 3 to maintain oscillation. The gain variation may be achieved through the use of limiting diodes in the negative feedback network.

---