infact a bjt is called as current controlled device.
output current (Ic) is controlled by input cuurent(Ib) by a factor β
consider a base resistor baising circuit (Re = 0)
i/p side:
Ib = (Vcc - Vbe) / Rb
Ib is a function of Rb in math terms
in this equation, both Vcc n Vbe are constants. only Rb can be varied
i.e., Ib depends on value of Rb (Ib descreases with increase in value of Rb)
o/p side:
Vce = Vcc - Ic.Rc
Vcc is a function of Rc
Vcc is constant as we said earlier, Ic = βIb. indicates current in output circuit is not at all depends on the component values (circuit configuration) of the output circuit.
** Thus output circuit current is independent of output circuit variables and depends only on input current.
back to o/p eq.
only variable in output eq. is Rc
implies Vce(Collecter to Emitter Voltage) increases with decrese in Rc
final notes:
*** more the value of Rb --> less Ib --> less Ic --> more Vce
these are voltage amplifiers (Resistances in KΩ, Ic in mA)
*** less Rb --> more Ib --> more Ic --> less Vce
these are power amplifiers (Resistances in Ω, Ic in A)