I'm trying to understand how oscillators work and am currently working with an RC phase shift oscillator:
$$R_1, R_2, R_3 = 200 \Omega$$
$$C_1, C_2, C_3 = 4.7\text{nF}$$
one of the issues I'm having is that one source states that \$R_{\text{fb}} = 29R\$ for sustained oscillation yet when I set the resistor to \$5.8\text{k}\Omega\$ I get a flat line in nV.
What am I missing out?
Answer
There are three problems.
1 - The biggest is that you have no excitation. That is, your simulator will provide a perfectly balanced set of operating points, and the circuit will have no reason to start oscillating. Try adding a circuit to provide a brief pulse into the - input just after startup. Adding a noise generator to the input to simulate real op amp noise performance will do the trick, too.
2 - Assuming you are doing transient analysis, a perfect gain (your 29) will cause an oscillation which never grows larger. Try a feedback value of 40 times, rather than 29, and you'll see the oscillation grow fairly rapidly. Start with a simulation time of at least 50 msec, then tailor as necessary. Once you see this, start reducing the gain ratio, and you'll see the startup time gradually increase.
3 - As is true of any open-loop oscillator, if you have enough gain to sustain oscillation, you almost certainly have too much gain, and the amplitude of oscillation will grow exponentially until the amp starts clipping. Real oscillators require some sort of automatic gain control in order to provide a low-distortion waveform.
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