Bubba's oscillator, shown in the figure, is another type of phase shift oscillator, but it takes advantage of the quadruple operational amplifiers that can be easily obtained today. Four RC sections provide 45-degree lags per section, with an excellent d / dt which minimizes frequency deviations. The outputs are obtained from alternating sections with low impedance. If the outputs are obtained from each of the operational amplifiers, the signals will have a 45-degree lag. The feedback equation is given below:
When = 1 / RC the equation is reduced for the following formulas:
The gain must be equal to 4 for the oscillation to occur. The test circuit oscillated at 1.76 kHz, a slightly different value than 1.72 kHz when the gain was 4.17. With a low gain A, and a low bias current, the resistor that fixes the Rg gain, does not load the last RC section which leads the circuit to have higher frequency accuracy. Sinusoidal signals with very little distortion can be obtained from the junction of R and Rg. When low distortion signals are required on all outputs, the gain must be distributed across all operational amplifiers. The non-inverting input of the gain amplifier is polarized with 0.5 V in order to set the quiescent output voltage to 2.5 V. The gain distribution requires that the other inputs of the other amplifiers are polarized, but this has no effect about the frequency of the oscillator.
