Question

When transistors are used in digital circuits they usually operate in

• A. active region
• B. breakdown region
• C. linear region
• D. saturation and cutoff regions

Answer 1

The correct option is D saturation and cutoff regions

$Answer:-$ D

Transistor in Cutoff/Saturation Mode - Digital Electronics

When both the pn junctions of a transistor is reversed biased we say the transistor is in cut off mode.
When base-emitter junction is forward biased and base-collector junction is reverse biased we say the transistor is in operating in Active mode.
When both the pn junctions are forward biased we say transistor is operating in saturation mode.

Saturation Mode

$I_c>I_c\left(max\right)$

$I_c\left(max\right)=\frac{V_{cc}}{R_c}$
$I_c=\beta I_b$

$I_c=I_c\left(max\right)$

$V_c=I_c\times R_c=V_{cc}$
$V_o=V_{cc}-V_c=0V$
Active Mode
$0<I_c<I_c\left(max\right)$
$I_b=\frac{(V_b-V_{be})}{R_b}$
$I_c=\beta \times I_b$
$V_c=I_c\times R_c$
$V_o=V_{cc}-V_c$

$0V<V_o<V_{cc}$

Cut off region

$V_b<V_{be}$ (Generally $0.7V$)

So, $I_b=0A$

$I_c=0A$
$V_c=I_c\times R_c=0V$
$V_{ce}=V_o=V_{cc}-V_c=V_{cc}$