Why lc oscillators oscillate

This oscillator is invented in the year by an American engineer namely Edwin H. The main feature of this oscillator is that the feedback of the active device can be taken from a voltage divider that can be made with two capacitors that are connected in series across the inductor. The Colpitts oscillator is an alteration of the Colpitts oscillator. This oscillator can be designed by adding an extra capacitor that can be connected in series with the inductor in the oscillator circuit.

This extra capacitor can be made changeable in changeable frequency applications. The frequency generated by the LC oscillator circuit completely depends on the capacitor and inductor values as well as their resonance condition. The LC oscillator frequency can be expressed as. From the above frequency equation, we can conclude that if either capacitor or inductor is reduced, then the frequency will increase. To maintain the oscillations in an LC circuit, we have to change all the energy which is lost in every oscillation at a stable level.

So, the energy changed should be equivalent to the energy lost throughout every cycle. The simplest method to replace this lost energy is to get the output from the LC oscillator circuit, amplify it, and then feed it back again into the LC circuit. This process can be achieved through a voltage amplifier with a FET, bipolar transistor, or operational amplifier like its active device.

If the loop gain is large, then the waveform will be distorted. Thanks for putting the circuit on line however I have tried to simulate this in LTSpice and I get a frequency 1. In this case from v AC 1. I was thinking using the right capacitor would do the trick. Do you have any suggestions? Yes capacitors values can eb changed for getting the desired frequency, or you can also modify the following circuit for the same:. I have an air coil with these characteristics calculated in Coil32 One-layer coil calculation.

Hence I calculated that I need a 7 pf cap to resonate in the coils self resonance. Voltage applied is 9V from a battery. Further calculations I use available calculators, not calculations derived from formulas give me these values. The coils power use, based on its resistance, would be Would this give an amount of oscillations before all power is used up? Dear Sol, Calculating physically can be a quite time consuming, since there are many conversions required in the process….

As I did explain, I do use online calculators. None of them give an indication how long the oscillation will be alive. Everybody says, if it were an idealistic case, the oscillation will go on forever, but resistance in the circuit will kill the oscillation after some time. If my assumption of cycles is right, it would mean some ms? That would probably to short to obtain any useful operation. OK, go it, but 22 mA is not a very high current for a 9V PP3 battery with mAh capacity, it should be able to sustain it for quite sometime….

I came to that conclusion imagining cycles at a frequency of ca. The 22 mA I got by using Ohms law. But I might be far off there. I understand that you focus on how long the battery will last. That was not my question. I wanted to find out how long a one charge of the LC tank would last.

So how long will the oscillating last before dies out, or needs reloading. If the 7pF is connected across the supply rail and you are asking how long the charge inside the 7pF can sustain the oscillation in the LC circuit once the battery power is cut off, then that will need some calculation? The power gain and voltage gain of the common-base configuration are high enough to give satisfactory operation in an oscillator circuit.

The wide range between the input resistance and the output resistance make impedance matching slightly harder to achieve in the common-base circuit than in the common-emitter circuit. An advantage of the common-base configuration is that it exhibits better high-frequency response than does the common-emitter configuration. Since there is no phase reversal between the input and output circuits of a common-base configuration, the feedback network does not need to provide a phase shift.

The common-emitter configuration has high power gain and is used in low-frequency applications. For the energy which is fed back from the output to be in phase with the energy at the input, the feedback network of a common-emitter oscillator must provide a phase shift of approximately degrees. An advantage of the common-emitter configuration is that the medium resistance range of the input and output simplifies the job of impedance matching.

All Rights Reserved. The current, in turn, creates a magnetic field in the inductor. The net effect of this process is a transfer of energy from the capacitor, with its diminishing electric field, to the inductor, with its increasing magnetic field. In Figure b , the capacitor is completely discharged and all the energy is stored in the magnetic field of the inductor. At this instant, the current is at its maximum value and the energy in the inductor is.

Since there is no resistance in the circuit, no energy is lost through Joule heating; thus, the maximum energy stored in the capacitor is equal to the maximum energy stored at a later time in the inductor:. At an arbitrary time when the capacitor charge is q t and the current is i t , the total energy U in the circuit is given by.

After reaching its maximum the current i t continues to transport charge between the capacitor plates, thereby recharging the capacitor. Since the inductor resists a change in current, current continues to flow, even though the capacitor is discharged. This continued current causes the capacitor to charge with opposite polarity.

The electric field of the capacitor increases while the magnetic field of the inductor diminishes, and the overall effect is a transfer of energy from the inductor back to the capacitor. From the law of energy conservation, the maximum charge that the capacitor re-acquires is However, as Figure c shows, the capacitor plates are charged opposite to what they were initially.

When fully charged, the capacitor once again transfers its energy to the inductor until it is again completely discharged, as shown in Figure d. Then, in the last part of this cyclic process, energy flows back to the capacitor, and the initial state of the circuit is restored. We have followed the circuit through one complete cycle. Its electromagnetic oscillations are analogous to the mechanical oscillations of a mass at the end of a spring.

In this latter case, energy is transferred back and forth between the mass, which has kinetic energy , and the spring, which has potential energy. With the absence of friction in the mass-spring system, the oscillations would continue indefinitely. Similarly, the oscillations of an LC circuit with no resistance would continue forever if undisturbed; however, this ideal zero-resistance LC circuit is not practical, and any LC circuit will have at least a small resistance, which will radiate and lose energy over time.