30/11/2010

Oscillator with the transistor bipolar.
In the previous lecture we saw how to use a logical CMOS port to obtain an inversor, this tecnic was is called Pierce oscillator thanks to his inventor.

This morning we understood how to realize the oscillator using a bipolar transistor.
The final scheme is very simple because there is very few component, but is not to so easy to understand in depth it, in fact we had to make a lot of preparatory lessons.
We polarized the transistor in the active region simply using two resistor for the base and one for emitter. We understand that with another resistance Rc in the collector we can take the signal in output of the transistor. But what happen if instead of this resistance Rc we insert a "tank" circuit (a parallel of L and C). In DC mode the voltage at the collector would be Vcc and the transistor remain in active region.
It important to note that the range of the output in AC mode would be from 0 to 2 x Vcc, that is around Vcc.
In what frequency we have to put the tank circuit? It's important to note that is not the frenquency of the total oscillator! We need a frequncy below that, because we remembered that the tank circuit is equivalent to a parallel of a C and a R above the oscillating frequency, that is what we need.
Finally we note that we can use a quarz of the cheap one not tuned only for the oscillator frequency that we need, but cut for low frequency, we are sure that with the tank circuit the quartz crystal works in the tone that we need.

After the theory we moved to the laboratory and we can build up our oscillator, was great! Incredible, so few component but all well working.
An important comment is that the tank circuit can vary with temperature weather umidity hit but this doesn't affect the work of the oscillator: now this circuit can perfectly fit in a boat!!!

In the last part of the lecture we start to understand that in some way we have to add a little information in our signal, the most easy way is an audible tone like On-Off.