Posted on Feb 6, 2014

A tone generator that runs on as little as 1. 5 Vdc, using the Sallen-Key configuration. And finally, I`ll apply the tone generator to implement a phantom powered signal source for testing balanced microphone inputs. Textbooks and web pages usually show the Wien Bridge configuration when describing single-opamp audio oscillators. The Wien Bridge circuit shown in sketch form below will oscillate at a

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frequency equal to 1/(2 Oscillation will be self-starting when overall gain is greater than 3, so we want (R3/R4 + 1) > 3, or R3/R4 > 2. Once oscillation begins, additional circuit elements must be brought into play to maintain a constant output level with low harmonic distortion. Tuning this kind of oscillator is complicated by the fact that the amount of gain required to maintain stable oscillation depends on how closely the tuning resistors and capacitors are matched. This is why the output level of an RC (resistance-capacitance) oscillator bounces around so much when setting the frequency. If you need a tunable single-opamp RC oscillator, then you need to take special care to be sure that R`s and C`s remain equal as they are varied. Here I`ll diverge from standard practice and introduce a new circuit for RC oscillators. Based on the Sallen-Key equal-component-value second-order LP (low-pass) filter, my new circuit is nothing more than an LP filter with the input grounded. Because it is second-order, this circuit has the capacity for sustained oscillation built in, which designers normally try to avoid. Frequency is the same as for the Wien Bridge oscillator, which is 1/(2 RC) where R = R1 = R2, and C = C1 = C2. The condition for oscillation is also the same as for the Wien Bridge configuration, which is an overall gain greater than 3. Actually, most operating parameters of this new circuit are similar to the Wien Bridge...

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