NPN transistor is set up as amplifier in common emitter mode, output signal voltage obtained across the load resistance of 4 kΩ is 1 volt. Calculate input signal voltage. Assume current amplification fevelar = 250 and base resistance = 1 kΩ:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 1 mV
In a common emitter amplifier the output voltage is across the load in the collector circuit, the input voltage is across the base resistance, and the collector current is \( \beta \) times the base current (current amplification factor \( \beta = 250 \)).
Collector (output) current:
\( \displaystyle I_C = \frac{V_o}{R_L} = \frac{1\,\mathrm{V}}{4\times10^{3}\,\Omega} = 0.25\times10^{-3}\,\mathrm{A} \)
Base (input) current:
\( \displaystyle I_B = \frac{I_C}{\beta} = \frac{0.25\times10^{-3}}{250} = 1\times10^{-6}\,\mathrm{A} \)
Input signal voltage:
\( \displaystyle V_i = I_B R_B = 1\times10^{-6} \times 1\times10^{3} = 1\times10^{-3}\,\mathrm{V} = 1\,\mathrm{mV} \)
Shortcut: Voltage gain \( A_v = \beta R_L/R_B = 250\times4/1 = 1000 \), so \( V_i = 1\,\mathrm{V}/1000 = 1\,\mathrm{mV} \).
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic was removed from the latest NCERT book (2026-27), and it also appears to be out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Exercises – …
