21/10/2010

Trying the transistor.
Today we start the lesson seeing that if we put our information voltage signal in series with the batteries we don' t obtain the gain that we desidered. Therefore in this montage our transistor "only" act a controlled switch.
Josè Maria said to us that we have to put our signal in parallel with the source power hence from base to ground and we have to add a capacitor that doesn't change the circuit in the DC mode.
Using the incremental model and the fasorial representation we arrived to calcute the amplify in this case: in principle we can choose the parameter to obtain the amplify that we want.
But there is a problem, the high we want the amplify the high result the impedence at the input of our amplifier, and remembering the previous lecture this is exactly what we don't want because we need to preserve the sensitivity of the antenna-tuner stage!
The problem of this high input resistance is well know in literature with the name of Miller effect. Fortunately there is a very clever solution to delete the undesidered Miller effect and hence we to replace the Rb (resistence in the base terminal) with a series of two Rb/2 resistence and in the middle a capacitor.

Jose' Maria showed us two other improvement for our circuit. The first is to insert a inductor L to compensate the effect of the parassite capacitance in high frequency, the second improvement is to insert in the emitter terminal a little resistence Re that doesn't afflict the polaritation circuit but that is seen multiplied for Beta in input (is fine for the sensitivity!).

We spent the last thirthy minutes of the lecture in the laboratory. We built the polaritation circuit, we turn on the power and we measured the real parameters of the circuit. We found a Collector voltage of Voq=4.58 V (theorically is 4.5 V) and a Base voltage of Vbq=0.61 V (teorically 0.6 V), with this mesured value we can calculate the various current and also the amplify factor that is A=169,99 that is in line with the theory.