Electronic – How to a transistor amplify current in a circuit


At 2:57 of this video, it is said that when we apply a small current to the base of the transistor, a large current is passed through. This is fine, but what bothers me is what he says right after: "If we now manipulate the base current in a certain change, the other current changes proportionally with much higher amplitude"

I just can not understand why it is that variations in the base current should amplify the current through the transistor. If I am understanding correctly, if we apply a current on a transistor, then we simply decrease the size of the depletion region and hence make it more conducting. However when we do this, the transistor didn't really amplify any signal, but rather let more of the current pass through.

So, why is it that we say a transistor is working as an amplifier here? Or am I misunderstanding?

Best Answer

I can understand your doubts - because, in reality, the transistor does NOT amplify the base current. It is true that the collector curent \$I_c\$ is proportional to the base current \$I_b\$ (\$I_c/I_b=\beta\$), but this is a kind of correlation.

(It is really a pity that there are still some books and publications claiming that \$I_b\$ would determine \$I_c\$. Nevertheless, during design and/or analysis of transistor stages we can in many cases treat the transistor as if \$I_b\$ would determine \$I_c\$; this is because the relation \$I_c=\beta I_b\$ does apply - but it is a correlation and does not reflect a causality).

It is not a problem to show and to verify that \$I_b\$ as well as \$I_c\$ are both dependent on the voltage \$V_{be}\$ according to Shockley's equation \$I_e = I_s[\mathrm{exp}(V_{be} / V_t)-1]\$ because the emitter current \$I_e\$ is split into a very small current (\$I_b\$) and a larger current \$I_c\$ (\$I_e = I_b + I_c\$).

Final (summarizing) statement (with respect to the long list of comments):

The following sentence alone cannot explain the transistor principle, but it shows that it is the VOLTAGE which plays the decisive role:

In every conductor/semiconductor, a current (movement of electric charges) can exist under the influence of an electric field only. In electronic circuits, this E-field is generated by an external voltage. A change of the voltage (of the E-field) causes a change of the current. This is true, of course, also for diodes, bipolar transistors (BJTs) and FETs.