AN3846| Application Note
Maxim/Dallas > App Notes > DIGITAL POTENTIOMETERS Keywords: Wien Bridge , Digital Potentiometer, digital pot, oscillator
May 24, 2006
APPLICATION NOTE 3846
Analysis of a Digitally Controlled Wien-Bridge Oscillator
Of all the low-frequency oscillator configurations, the Wien bridge is the easiest to use. It is reliable, uses standard components, gives a good sine wave, and is fairly immune to the type of op amp around which it is designed. A Wien bridge can, however, be misunderstood and oversimplified, leading to designer frustration. This article describes the theory and practicalities of using a Wien-bridge oscillator, and how to make the circuit more stable and more factory-/user-flexible.
The Wien-Bridge Circuit
The circuit of a standard Wien-bridge oscillator is shown in Figure 1.
Figure 1. Standard Wein-Bridge oscillator circuit. A circuit oscillates if, at a given frequency, it has greater than unity gain and zero phase shift from the input. First, the phase shift needs to be considered. In Figure 1, R1 and C1 produce a positive phase-shifted current with respect to the output voltage. When this phase-shifted current meets R2 and C2, these latter components produce a negative phase-shifted voltage. At a specific frequency, the phase shift caused by R1 and C1 is offset by an equal and opposite phase shift caused by R2 and C2, so the net phase shift is zero. When this occurs, the circuit is in danger of oscillating. Next we consider the transfer function of the network comprised of R1, C1, R2, and C2. Because the op amp's output impedance is low and its impedance at both inputs is very high, deriving the transfer function of the Wien network (the resistive divider made up of R1 and C1 on the top and R2 and C2 on the bottom) is relatively straightforward. Thus, the transfer function is
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