A common emitter amplifier circuit, built using an npn transistor, is shown in the figure. Its dc current gain is $250, R_C=1 k\Omega$ and $V_{CC}=10 V$. What is the minimum base current for $V_{CE}$ to reach saturation?

JEE Main · 2019 · 08 Apr 2019 · Shift 2 · Semiconductors

A common emitter amplifier circuit, built using an npn transistor, is shown in the figure. Its dc current gain is

250, R_{C} = 1 k Ω

and

V_{C C} = 10 V

. What is the minimum base current for

V_{C E}

to reach saturation?

Answer: (B)

At the edge of saturation

V_{C E} \approx 0

, so the whole supply drops across the collector resistor and the collector current is

i_{C} = \frac{V _{C C}}{R _{C}} = \frac{10}{1000} = 10 mA

. The dc current gain links base and collector currents by

β = \frac{i _{C}}{i _{B}}

, so the smallest base current that drives the transistor to this collector current is

i_{B} = \frac{i _{C}}{β} = \frac{10 mA}{250} = 40 μ A

JEE Main · 2019 · 08 Apr 2019 · Shift 2 · Single correct option