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RLC oscillator circuit

  1. May 11, 2012 #1
    Hello I am trying to understand how exactly an oscillator circuit work and how to build one.

    So I was palying around in falstad and made this:

    the following is link. tried to get around spam filter so.. write without spaces
    h t t p s : / / w w w . f a l s t a d . c o m / c i r c u i t / #

    and add:


    As it is visible from circuit simulator, only a capacitor, inductor and resistor is used to create non-ending sinus waveform from DC.

    Question: would this actually work in real circuit? And if so, then why every example of oscillator circuit online includes at least one transistor and few more capacitors and inductors?

    thank you.
  2. jcsd
  3. May 11, 2012 #2


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    Despite what you simulator is showing you, this doesn't actually happen.

    What does happen is that the capacitor charges up to the same DC voltage as the supply and then all further activity stops.

    At best you might get a slight "ringing" or damped oscillation when power is first applied but this reduces in amplitude to zero, usually after just a few cycles.

    You do need a transistor, a FET, an integrated circuit or even a tunnel diode to produced sustained oscillation.
  4. May 11, 2012 #3
    It seems this website does not have correct electric circuit simulation algorithm. An RLC circuit produces damped oscillations, or an aperiodic puls, depending on the value of the resistance.
  5. May 11, 2012 #4
    You can use a positive feedback (so output and feedback input are in phase) gain of greater than 1 to overcome your resistive losses. The oscillation will grow til its held back down by nonlinearities of your amplifier.
  6. May 11, 2012 #5
    Or an additional element with negative differential resistance.
  7. May 12, 2012 #6
    Thank you for explaining that. it makes sense that capacitor would only charge till the voltage of Voltage source. I'm trying to understand oscillator schemes. For example Hartley oscillator. Understanding part does not go that well.

    For example, in Hartley oscillator. the middle of two inductors is connected to ground. Why? how does it change the circuit? Also I tried changing values of capacitors and inductors in circuit simulator. once i did, the oscilliation broke down. So apparently they must be in right proportions. But how?

    Does anyone know where I could find materials about oscilator online?
  8. May 12, 2012 #7

    I understand how feedback in phase could do the trick. but getting that feedback is the hard part. I guess, a good way to do it is to somehow connect transistor base to the oscillations in LC. But not sure how.
  9. May 12, 2012 #8
    Usually, in most types of oscillators, the feedback is achieved by potentiometrically dividing the voltage, but, instead of Ohmic resistors, one uses reactive elements, so that the Barkhausen stability criterion is satisfied for a particular (carrier) frequency.

    The schematics given are deformed so that this basic scheme may seems obscured, but, it should be possible, through topologically equivalent deformations (i.e. only elongating and twisting the wires without breaking them, or reconnecting them if they cross), to bring it to the textbook example.
  10. May 12, 2012 #9


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    Have a look at the links in this post:

    The center tap of a Hartley oscillator is not grounded. Notice that this is a single coil with a tapping on it, so the two halves are magnetically coupled together. The tap is usually one third of the way up from the grounded end.

    Feedback in this oscillator is from the emitter via the coil (which is a step-up transformer) back to the base of a transistor or to the gate of a FET.
    Tuning the coil with a capacitor ensures that this feedback only happens at one frequency, so that is where the circuit oscillates.

    However if you build this circuit, you will need an oscilloscope to view the output, or a receiver in the same room so that you can listen to it. the receiver will not need to be connected to the circuit as there is always some radiation.
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