19/10/2010

Putting transistor in the active region.
After seeing another time how the transistor works we want to stress the fact that the nobel idea in the transistor is that little variations in the input imply big variations in the resistence in the output because there is a strong sensitivity in the voltage Base-Emitter.
Transistor comes from the words transfer + resistor that means that we can transfer the same current from an high resistence to a resistence that we can choose.

If we want to obtain the ransistor effect we have to put the transistor in the active region which means that we need a positive voltage both for the Collector-Base and for Base-Emitter.

If we want to check if the transistor in a circuit is in active region we have to check if at leat one of these current are positive: emitter, collector or base.

In the class we saw some proposals about how put a transistor in the active zone, in the simpliest cases we found some problem about the instability of the circuit due to temperature, the dependence from the value Beta of different transistor or the need for 2 batteries to aliment the circuit.
The last example that we have seen is made with only one batteries, with four resistance and it doesn't depend from the different value Beta of transistor (Note that also if the model of the transistor is the same we can find very different value of the parameter Beta).

In the last part of the lecture we developed an incremental model for study circuit with transistors and we start talking about the possibility of insert our voltage signal in parallel with the voltage Base-Emitter.