Transistor as a switch and as an amplifier
Transistors and circuits · Physics
DC circuit
Interactive 3DElectrons drift around the loop — raise the voltage and watch the current obey Ohm's law.
Controls
Drag the sliders or type a value — the simulation updates live.
Study notes
(a) An n-p-n with β = 100 drives an LED (needs 20 mA) from a 5 V supply through R_C. What base current guarantees saturation, and what R_B suits a 5 V logic signal? (b) As a CE amplifier with β = 100, R_C = 2 kΩ, what output results from a 5 mV peak input? Solution: (a) Need I_C = 20 mA; saturation needs I_B > I_C/β = 0.2 mA — use I_B ≈ 1 mA for hard saturation. R_B = (5 − 0.7)/1 mA ≈ 4.3 kΩ (use 4.7 kΩ standard). (b) I_C variation = β × I_B variation; voltage gain |A_v| = β·R_C/r... simply: with r_e' = 25 mV/I_E, if I_E = 1 mA, |A_v| = R_C/r_e' = 2000/25 = 80. V_out = 5 mV × 80 = 400 mV peak (inverted).