ImpulsE041993, I have already explained the fact that the Tank Circuit is covered on Talking Electronics website via the link provided. Here is the explanation:
The TANK CIRCUIT.
This is the coil and capacitor in the collector of the oscillator transistor.
Even though these two components are classified as "passive" - and do not amplify (as separate items), when they are connected in parallel, they form a circuit known as a TANK CIRCUIT and actually produce a waveform that is larger than the supplied voltage. These two components set the frequency of the circuit and when they are operating correctly, the output will be a maximum.
In the circuit provided, the inductor is printed on the circuit board. This type of inductor has very poor immunity to surrounding capacitance and the circuit will drift when the board is touched. In addition, this type of inductor has very poor capability of returning magnetic flux to keep the circuit oscillating and thus it will produce a very low “Q” value.
When power is applied, the oscillator starts to operate due to the 2k2 bias resistor on the base. The actual voltage on the base is created by the load from the previous transistor plus the current taken by the RF stage.
The next point to note is the base is held rigid by the 1n. This capacitor has a very low impedance at 100MHz, so it holds the base very firm.
Now we come to understanding how an NPN transistor "turns on."
It can be turned on in two ways. The emitter can be held rigid and the base can be raised to 0.6v and if the voltage is raised slightly more and the base is fed with current, the transistor will conduct and current will flow in the collector-emitter circuit.
The other way to turn on an NPN transistor is to hold the base firm and lower the emitter voltage. Once the emitter is lower than the base by 0.65v, the transistor turns on and if the emitter is lowered slightly more, the transistor turns on more.
This may be difficult to visualise, but this is occurring in the oscillator stage and is called a COMMON BASE configuration.
The transistor turns ON and current flows through the collector-emitter. This current also flows through the inductor printed on the board.
The coil does not get energy instantly because it resists any quick flow of current into it due to magnetic flux generated by the current creating a “back-voltage.” In addition, the voltage across the 12p capacitor takes time to increase.
We are only talking about milli-microseconds, but this is the “time-values” for the circuit.
As the 12p charges, the voltage on the collector decreases and this voltage is passed to the emitter via the 2p7.
We have already explained the fact that the transistor is turned on in this arrangement by decreasing the voltage on the emitter.
The transistor gets turned on more and more. The 12p charges and the inductor produces maximum flux.
Up to this point in time the magnetic flux is called EXPANDING FLUX and this expanding flux produces a voltage across it that allows the 12p to charge.
Eventually the transistor cannot be turned on any more. The 12p is charged and the inductor has maximum current flowing through it.
But this current is not increasing and the voltage produced by the inductor no-longer increases.
Some of the energy from the inductor gets lost to the surroundings in the form of electromagnetic radiation and the voltage across the 12p reduces. This voltage-reduction is transferred to the emitter via the 2p7 and the transistor starts to turn off.
The current through the inductor reduces and this causes the magnetic flux to start and collapse and produce a voltage across it OF OPPOSITE POLARITY. This has the effect of firstly discharging the capacitor and then starting to charge it in the reverse direction.
As soon as the magnetic flux starts to collapse and produce a “reverse voltage,” this voltage is passed to the emitter via the 2p7 and the transistor starts to get turned off.
Very soon the transistor is completely turned off and it can be considered to be OUT OF CIRCUIT.
We now have just a coil and capacitor with the coil delivering energy to the capacitor in the reverse direction.
Now, here is one of the amazing things of an inductor.
The collapsing magnetic flux will produce a voltage (in the opposite direction) that is higher than the original supplying-voltage.
The size of this voltage is (as a percentage) is due to the quality of the inductor and this is known as the “Q” value.
The value of “Q” can be as high as 10, 100 or even 1,000 but in this case the quality of a printed coil is very low.
Since there is no transistor in the circuit, the voltage produced by the inductor is passed to the capacitor. However a wire is connected to the capacitor (called the antenna) and since this voltage is increasing very quickly, some of the energy flows through the wire and escapes to the surroundings as magnetic radiation.
The inductor does not have a lot of energy and eventually the voltage it delivers to the capacitor starts to decrease.
This voltage is monitored by the 2p7 and the voltage on the emitter starts to rise and the transistor starts to turn on again.
It’s as simple as that.