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Revision as of 16:38, 30 July 2011

Need to add a schematic (or several) -- I'll get to it shortly Madhu

link broken

the link to the other schematic, supposedly in spanish, is broken. —Preceding unsigned comment added by 172.213.91.54 (talk) 20:16, 14 April 2008 (UTC)

An oscillation circuit in which a balanced bridge is used as a feedback network is the Wien bridge oscillator shows in fig. —Preceding unsigned comment added by 117.98.64.51 (talk) 18:18, 11 May 2009 (UTC)

Is explanation of operation of light bulb gain control correct?

I think the explanation of the cause of the nonlinear resistance of the light bulb is misleading. The actual cause of the nonlinearity is that the power dissipation in the filament is proportional to the square of the current P = iR. I would think that the large nonlinear increase in power radiated away from the filament with temperature due to the Stefan-Boltzman law would tend to reduce the temperature rise of the filament, thus reducing the nonlinearity. --Chetvorno 17:30, 10 July 2009 (UTC)

How do RC oscillators produce sine wave?

(copied from Electronic oscillator talk page)

... I will add to this discussion all RC oscillators (e.g., Wien bridge) that are a big challenge for human imagination. Why? Just because it is too hard for a mere mortal:) to imagine how the humble RC circuit can produce sine wave, how it can act as a "resonator" at all. Three years ago I managed to reveal how the more sophisticated LC circuit does this magic. Then I began thinking about how the humbler RC circuit could do it... and this was a big challenge for my imagination. Here are my intuitive achievements about the most general (philosophical:) idea behind RC oscillators. I have used, as usual, a figurative and colorful language to picture the circuit operation.

RC oscillators stay between relaxation and harmonic (LC) oscillators; they possess properties from the both. Like relaxation oscillators, they have only one storing element (capacitor) that continuously charges and discharges; it stores only one kind of energy (electric) that is wasted. Like LC oscillators, the storing element is connected in a positive feedback loop to sustain the oscillations; they produce "rounded", "smooth", sine waves... Let's see why and how.

Simply speaking, both the relaxation and RC oscillator consist of a voltage source (an amplifier) driving a capacitor through a resistor. To make the voltage across the capacitor wiggle, this source has somehow to change its polarity at the peaks of the halfwaves.

• In a relaxation oscillator, the amplifier output voltage stays constant (maximum or minimum, at one of the supply rails) until the capacitor charges/discharges. When voltage drop across the capacitor reaches the peak, the amplifier switches sharply (helped by the accelerating positive feedback) this voltage from the current to the other rail. As a result of this voltage jump, the shape of the relaxation oscillation is peaked, angular, not sine...

• In an RC oscillator (Wien bridge oscillator is a good example), the storing element (the grounded capacitor in the figure on the right) is connected in the positive feedback loop. (IMO) the loop gain has to be close to but yet a little more than unity. At these conditions, the amplifier output voltage is constantly a little higher than the voltage drop across the capacitor and the latter continuously charges. The capacitor voltage tries to reach the amplifier voltage that continuously shuns up because of the positive feedback. Figuratively speaking, the capacitor is "self-charging"; it "pulls up" itself (with the help of the supplied amplifier) like Baron Münchhausen escaping from a swamp by pulling himself up by his own hair:) If the loop gain was exactly unity, the amplifier output voltage would be equal to the voltage drop across the capacitor... no current, no voltage change, no wave... Another impressive analogy is a cage equipped with "antiweight". Imagine you are in the cage but some "joker" has increased slightly the antiweight and, of course, loosed the brakes:) As a result, to your great surprise, you will begin lift up just like the voltage across the capacitor... If the antiweight was equal to the cage weight, you will stay immovable.

When the capacitor voltage approaches the positive supply rail, the amplifier begins saturating; the loop gain begins decreasing and the voltage change looses its nerve. Finally, at the top of the halfwave, the amplifier does not amplify at all and the voltage stops changing; thus the upper sine peak. Now the grounded capacitor begins discharging through the parallel connected resistor (note there is no charging current from the amplifier output since the upper capacitor impedes it). The positive feedback helps this process as above (now the joker has decreased slightly the antiweight and you travel down:) Its voltage begins decreasing trying to reach the amplifier voltage that continuously shuns down. When the capacitor voltage approaches the negative rail, the amplifier begins saturating; the loop gain begins decreasing and the voltage slows its change. At the bottom of the halfwave, the amplifier does not amplify and the voltage stops changing; thus the bottom sine peak.

As a final conclusion, RC oscillations arise because of the slight positive feedback with dynamic loop gain (between peaks it is bigger than one; at the peaks it is exactly one). The shape of an RC oscillation is smooth (sinusoidal) since at the peaks of the halfwaves the amplifier output is saturated and does not change its voltage (just like an LC oscillator).

These were only my insights. I would be glad if you share and enrich them... Circuit dreamer (talk, contribs, email) 20:22, 28 July 2011 (UTC)

All you need to make a sine wave are an LC and an excitation source. If you solved a few differential equations you would discover sinusoids are very common. Tape an LED to a bicycle wheel and watch it's locus as the wheel rolls - another sinusoid! Zen-in (talk) 05:31, 29 July 2011 (UTC)

Well, LC oscillator is clear but RC one is the problem. The big question is (was:), "How does a sine wave conceive in the humble RC circuit?" Differential equations will show sinusoids in detail but will not answer this question. The LED example is very attractive; I will carry out it to explain to my grandson what a sinusoide is:) Circuit dreamer (talk, contribs, email) 06:16, 29 July 2011 (UTC)

A sine wave is the time-wise representation of a single frequency. If you just have one RC you can only have one frequency. Another question though is: If you haven't been able to grasp this basic concept then why are you editing this page? Zen-in (talk) 16:38, 30 July 2011 (UTC)