14/10/2010

The second stage.
The last lecture we saw how to make a transformer in our circuit to see a lower input impedance of the HF amplifier. Now we have to build this amplifier!
Beacuse we are in high frequency we can't use an operational amplifier and we have to remember and to exploit the propierties of the transistor.
Josè Maria explain us that to better understand the proprierties of the transistor we have to revise the feautures of the diode.
First of all we understand that a diode is made from a semiconductor (silicon), more precisely there are two different region doped P or N joint togheter.
After we have to observe that whenever we have a diode in a circuit, at that time the circuit is no longer linear, and so we saw the curve voltage vs corrent.
In practice we want to work with a linear circuit and therefore we saw how to approssimate a diode with a linear circuit.: we have to distinguish two different stage, one when the voltage is upper than the thereshold voltage and the second when the voltage is lower.
After that is necessary to see what happen when the voltage is moving a little bit from the fixed point used for the approximation. We saw that we have to image that there is a voltage generator and a resistence.
In summary if we have to study a circuit with diodes we have to proceed in two step: in the first step we find the operating point of the diode and in the second step we study what happen with little variation (we also exploit the proprierty of superposition).

After studied the diode we started to see how works a transistor. A transistor is made by 3 regions (normally N-P-N), we observed that the P-region is very very thin and so we can't approssimate the operation of the transistor like simply 2 diodes. The transitor has three terminals: Emitter, Collector and Base.
We saw that a little variations in the voltage of B-E correspond in an high variations in the current C-E, therefore if we insert a resistance we obtain an high variation in its terminal. Therefore we found the amplifiy effect of the diode!