Can't figure out oscillator circuit - Page 1
Forum section: electronics / beginners

author: exit_failure
author_rank: Regular Contributor
timestamp: March 13, 2020, 10:16:03 am
content: While searching for a simple circuit to generate a sine wave, I came across the circuit below. So far I searched for an explanation but probably didn’t enter the right search terms. I also simulated the circuit in Ltspice and the Falstad online circuit simulator to get a better picture of what’s going on, unfortunately to no avail. I would be grateful, if you could point me to the right source for an explanation or maybe if you could explain the circuit to me yourself in some details.Many thanksHarry

author: Mazo
author_rank: Regular Contributor
timestamp: March 13, 2020, 11:07:00 am
content: It is ahttps://en.wikipedia.org/wiki/Phase-shift_oscillatorEssentialy one of the simplest amplifiers one can build(single transistor in CE) with 3 cascaded phase shift networks each providing a ~60deg of phase shift.(assuming equal R's and C's).

author: fcb
author_rank: Super Contributor
timestamp: March 13, 2020, 11:39:46 am
content: Look at the output waveform, then compare it with the voltage waveform at:1. Junction of C4-C1-R32. Junction of C1-C2-R13. Junction of C2-C3-R24. Base of Q1This should give to an idea of what's going on (phase shift of 60oper section at the fOSC).

author: exit_failure
author_rank: Regular Contributor
timestamp: March 13, 2020, 12:43:12 pm
content: Thank you for your answers.So do I understand this correctly: This circuit has has a certain resonant frequency set by the operating point of the transistor and the component values the RC-elements. This frequency is attenuated just as much by the RC-elements as its amplified by the transistor. Also the transistor shifts the signal 180° out of phase. This phase shift is corrected by the RC feedback loop at the resonant frequency. This will lead the circuit to resonate.Unfortunately there are still a couple of things I don’t understand:Where does the “frequency” the circuit resonates at come from? Initial noise when you switch it on maybe?I understand that some frequencies will get attenuated more by the RC-elements tan they get amplified by the transistor and thus will disappear after a certain amount of time. But what about the frequencies that get amplified more than they get attenuated? Shouldn't they overpower the resonant frequency at one point?Also, when I look at Vout the voltage even goes negative. Why is that so?

author: TimFox
author_rank: Super Contributor
timestamp: March 13, 2020, 01:36:22 pm
content: There are two basic forms of this circuit: either series Rs and Cs to ground, or your circuit with series Cs and Rs to ground.Back in the day, one could build a simple oscillator of the series R variety with a single vacuum tube. A large value (maybe 10 Meg) physically large resistor from plate to grid, wrapped in grounded metal foil to make the capacitors. Early in my career, I found an accidental oscillator of this variety (op amp transresistance circuit for a photodiode), built as a chip and wire hybrid on a ceramic substrate, where the layout had a metal layer under the 50 Megohm feedback resistor.Note that in all such circuits, the frequency is that where the total phase shift of the network and amplifier adds up to the required 360 degrees (including the 180 degrees for the inverting amplifier).

author: ArthurDent
author_rank: Super Contributor
timestamp: March 13, 2020, 02:06:08 pm
content: If you have a public address system and get the mike too close to the speakers you will get feedback in the form of that annoying squeal we’re all familiar with. Moving the mike closer or further away from the speakers changes the time delay of the feedback 'circuit' and the frequency of the squeal. Phase shift circuits can be thought of as time delays using RC circuits. By choosing three RC circuits that produce phase shifts totaling 180 degrees (60+60+60) at a frequency, then connecting them to a 180 degree (inverting) amplifier, you have an oscillator with no usual resonate circuit. You could also choose four RC circuits that produce phase shifts totaling 180 degrees (45+45+45+45) and get the same frequency out.As to the output going negative, that is due to the coupling capacitor, C5. If you check the signal on the collector of the transistor it will vary from near zero to basically twice the output amplitude.