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Today I built another phase-shift oscillator (you can see my full notes here). The circuit is based on those I've found online, and uses the capacitors in a high-pass configuration:

phase-shift osc2

The circuit works, but I'm confused about two things:

  1. I've labeled a resistor RXXX in the schematic, but did not fit it in my circuit. It seems like it should be required to allow the final capacitor to work. But the circuit doesn't work if it's fitted. Why?

  2. What is the purpose of the resistor above RXXX going from the base to the positive supply?

I'm guessing these two questions are probably related?

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2 Answers 2

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A quote from my previous answer given yesterday regarding the phase shift oscillator with series resistors: -

Your transistor circuit with series resistors is self biasing - the DC flows through several resistors back to the base and, due to negative feedback, the collector dc quiescent voltage will be roughly mid-rail on low voltage supplies.

The transistor base must be biased for this to work correctly. R and RXXX are normally regarded as performing this function. Also, either R or RXXX can do the job of the final resistor in the phase shift network. The final resistor in the phase shift network can be tied to any supply rail because the phase shifting it performs is on an AC signal and either supply rail looks the same to AC.

With both resistors fitted, the bias to the base is better defined but maybe both resistors together attenuate the feedback signal too much and cause the circuit to fail to oscillate.

This can possibly be remedied by increasing the collector resistor (also called R to add to the confusion).

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1.) The resistor above RXXX is necessary for proper biasing the base of the transistor (base-emitter voltage Vbe app. 0.65...0.7 volts).

2.) The combined input resistance at the base node (Rcom=R||RXXX|rbe) forms the third C-R highpass which is necessary to allow a 180deg phase shift for the desired frequency. Of coursse, this resistance Rcom can be realized only with R||rbe. However, in many cases it is convenient - with respect to the selection of proper resistor values - to have a third "degree of freedom" provided by a third resistor.

3.) Remember: For an IDEAL phase shift oscillator (opamp based) we have three identical C-R highpass stages with three equal capacitors and three equal resistors. In the above case, for proper fixation of the oscillation frequency the dynamic input resistance rbe must be known (dependent on the DC operating point which is set by R and RXXX).

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