Common base example Active load

figure 1: basic npn common base circuit resistor load (neglecting biasing details). signal applied @ vin, output taken node vout may voltage or current



figure 2: basic npn common base circuit (neglecting biasing details). current source ic represents active load.

in figure 1 load resistor, , current through resistor determined ohm s law as:

i

c


=




v

c
c


−

v

o
u
t




r

c






{\displaystyle i_{c}={\frac {v_{cc}-v_{out}}{r_{c}}}}

.

as consequence of relation, voltage drop across resistor tied current @ q-point. if bias current fixed performance reason, increase in load resistance automatically leads lower voltage vout. in turn lowers voltage drop vcb between collector , base, limiting signal swing @ amplifier output (if output swing larger vcb, transistor driven out of active mode during part of signal cycle).

in contrast, using active load of figure 2, ac impedance of ideal current source infinite regardless of voltage drop vcc - vout, allows large value of vcb. , consequently large output signal swing.