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QCM- Understanding Oscilator(oscillator circuit) (Principle of oscillator circuit)

In order to understand circuits, one has to understand R (Resistance), C (capacitors), and L (Inductor).
To learn about the electrical properties of these three devices, we need to look up various data. For the moment, let introduce briefly here.

Resistance is a device that flows current proportional to voltage.
However, what we are interested in in in a oscillation circuit is its characteristic of transient conditions. ( When suddenly the pressure is applied )
The figure below shows the voltage current curve for transient conditions of resistance.
The resistive components only carry current proportional to voltage and do not change in time.

The figure below shows the voltage current curve for the transient state of the capacitor. 
Initially, voltage increases from zero, while current flows a lot and gradually decreases.

The figure below shows the voltage current curve for the transient state of the Inductor.
Initially, voltage decreases from maximum, while current flows a little and gradually increases.
In other words, capacitors, voltage, and current tend to reverse over time.

We have previously seen voltage current curves for the transient states( When suddenly the pressure is applied ) of R, L, and C.
As we saw earlier, C and L are used as oscilator devices due to their properties. 
When suddenly turn off the switch after turning on the power, we can find something interesting.
Connect L and C in parallel as shown in the figure below, and then turn off the switch. If this is the case, the power supply is completely disconnected. (Figure below). C then releases the charge that it had due to its nature to store the charge, which is charged to L and then discharged again. And this charge repeats the charge&discharge back to C. As a result, a waveform like a graph is detected on both sides of L and C. The signal gradually decreases because there is no continuous supply of energy.

In the figure below, we changed the capacity of the Inductor of L1. This will change the frequency of the waveform produced as shown in the right-hand picture. In other words, the frequency varies depending on the capacity of L and C.

Let's make some changes to the previous circuit.. 
In the figure below, Q1 is a transistor. It's device that voltage is applied to the control terminal (base) then voltage is amplified according to the gain. In the figure below, voltage is used as the principle of On if applied, Off if not.
In the figure on the right, the voltage at the top(Green) is applied to the control terminal (base). And bottom(Red) is the waveforms from L and C.
As in the above case, waveforms are generated but there is no continuous supply of energy, so you can see them gradually decreasing.

A oscillation circuit is a device that produces a continuous waveform. So how do you generate a continuous waveform?
Wouldn't it be possible if you continue to supply energy and then collect generated waveforms again to turn the transistor on and off?
However, because the signal generated is insufficient to turn the transistor on and off, Let we try to amplify it.

Devices that can amplify voltage include TR (transistor), FET, OPAmp, etc. Use these devices and compose a oscillation circuit by inserting elements such as Feedback and Gain Amplitude etc. 
A schematic of such a principle is the following illustrations frequently appearing in the description of a oscillation circuit.

Let take a look at the Colpitts circuit as shown in the following figure. 
A good combination of TR (Transistor), resistance, and capacitors creates a circuit that can amplify. In the figure below, Amplifier section, A oscillator is made of consisting of a Feedback circuit is constructed that produces a constant frequency by combining well the L and c.
This is known as resonant frequency, because when a feedback is applied, resonance at a constant frequency causes a resonant frequency.

There are many Oscilator circuits designed using the aforementioned principles.
Many of them are as follows.
If you understand the above, the following pictures will be easily understood. The Oscilator is available many type of Oscilator as combinations of R, L and C.