26/10/2010

We spent the first our of this morning lesson in the laboratory. We have have to find the curve of amplify of our circuit.
We know from what we study in the theory that we would have bad value in high frequency (more than 1 Mhz) because of the parassite capacitance of the transistor. In the laboratory we found that kind of decrasing curve.
In the previous lecture we saw that we can improve the trend inserting an inductor L to compensate the effect of the parassite capacitance in high frequency, therefore we inserted that inductor and we could find better performance.

In the second part of the lecture Josè Maria explained to us the concept of positive regeneration. The basic idea is that we want to obtain an higher amplify. This idea is patended by Armstrong in 1915 and we have to understand that like in the principle of the electric age the engineers want to obtain the maximum with the minimum, in this case we want the maximum amplify with the minimum number of transistor (at the principle vacuum tube).
The way to obtain this positive regeneration is based on the idea that after we receive the signal we can emitting it with an antenna and then we re-received it.
There are two fundamentals problems: the first one is that we have to re-receive the signal in a constructively way (adding in phase) and the second problem is the saturation.
In practice to build up this "emitting antenna" we have only to add a little solenoid in the same ferrite kernel that we used for the antenna-tuner (paying attention to the phase), and a controlled resistence.
Controlling the resistence we can modify both the sensibily and the selectivity of our circuit (improving both at the same time) and we can avoid the problem of saturation.