# how an oscillator works?

Discussion in 'General Electronics Chat' started by xljin2014, Nov 11, 2014.

1. ### xljin2014 Thread Starter New Member

Nov 11, 2014
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hi, everybody,

in my opinion after sometime of study, an oscillator is kinda a tank circuit (rc or lc) plus a possitive feedback circuit(which supply the lost energy of the tank circuit), but in a perticular circuit i often get lost. so my question is: can anybody give examples(the more the better) to explain how the oscillators work? (where is the tank circuit and how the feedback circuit supply the lost energy?)

thx a lot!

2. ### WBahn Moderator

Mar 31, 2012
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5,758
There are many kinds of oscillators with different ways for how they oscillate.

One of the simplest is to take an odd number of digital inverters and connect them in a ring. This is known, not too surprisingly, as a ring oscillator. Draw this up, say with three inverters, and see if you can figure out how it works and what parameter determines the frequency of oscillation. Then think about how you might add an R and C to this so that you could have better control over setting the frequency of oscillation.

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3. ### xljin2014 Thread Starter New Member

Nov 11, 2014
15
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i feel that the ring oscillator can still be thought of a tank circuit with a possitive feedback, the tank circuit is due to the parasitic capacitors,

now i need to analys different oscillator circuits and i need help

thx again

Last edited: Nov 11, 2014
4. ### wayneh Expert

Sep 9, 2010
13,627
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A 555 astable does not use an energy storage to oscillate per se. It uses it for timing, but the energy does not "ring".

5. ### xljin2014 Thread Starter New Member

Nov 11, 2014
15
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if a 555 circuit given, you may see there is tank circuit, a capacitor for instance.

another case other than possitive feedback maybe "negative resistance", that i not know much about,

ty.

6. ### WBahn Moderator

Mar 31, 2012
20,247
5,758
An RC timing circuit is not a tank circuit. The idea of a tank circuit is one in which energy is shuttled back and forth between two reactive elements, one inductive and one capacitive.

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7. ### xljin2014 Thread Starter New Member

Nov 11, 2014
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when the capacitor is charged, it tends to discharge, which i feel one the real reason of oscillation?

8. ### crutschow Expert

Mar 14, 2008
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A tank is normally considered to be a LC circuit, which can be used to form a sine-wave oscillator but oscillators can be built without a tank.
There are two basic types of oscillators, sine-wave and digital (square-wave).
RC digital relaxation oscillators, such as a 555 are very common.
Another type of sine-wave oscillator is a phase-shift oscillator which typically uses a series of RC elements to generate a 180 degree phase shift for positive feedback at the oscillation frequency (combined with a 180 degree shift in the amplifier).
The ring oscillator, already mentioned, is a digital oscillator where the sum of the delays in each of the digital gates determines the oscillation (1/2 of the) period.

All oscillators are designed with some sort of positive feedback at a specific frequency which keeps the oscillation going at that frequency. The overall positive feedback gain has to be a least 1 to generate oscillations.

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9. ### xljin2014 Thread Starter New Member

Nov 11, 2014
15
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at least with a capacitor, ( lc, or rc),

and what about negative resistor oscillator? can it be clarified as possitive feedback oscillator?

Last edited: Nov 12, 2014
10. ### crutschow Expert

Mar 14, 2008
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Not sure if it's defined as a positive feedback oscillator or not. Basically the negative resistance cancels the positive (lossy) resistance so that the tank circuit can keep oscillating. Normally an oscillating tank voltage will slowly decay due to resistive losses. In effect negative resistance eliminates the resistive loss so the tank keeps oscillating.

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11. ### xljin2014 Thread Starter New Member

Nov 11, 2014
15
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ty so much,

need to think it over through examples