The current amplifier

A transistor is used above all as a current amplifier: a small current into its input gives a large current at its output.

Activity 11.3

You will need: a 2SD400 (D400) transistor, two 2.5 V torch bulbs, 3 V and 6 V battery holders, six 1.5 V cells, two (push-button) switches, a 10 kΩ volume control and a project board

Switch S₁, the 3 V battery, the volume control V_R and bulb L₁ are in the input (base) circuit; the 6 V battery, S₂ and bulb L₂ in the output (collector) circuit. Connect the batteries with both switches off. With S₁ on, set V_R so L₁ just glows; then try each combination of switches.

Figure to be added

Activity 11.3 — the circuit: base circuit with S₁, 3 V, V_R and L₁; collector circuit with 6 V, S₂ and L₂, on a D400 transistor.

S₁	S₂	Bulb L₁	Bulb L₂
off	off	off	off
on	off	lit, dim	off
off	on	off	off
on	on	lit, dim	lit, bright

The conclusions

Current flows in the output circuit only when current flows in the input circuit.
Even with a supply on the output circuit, no input current means no output current.
A small input current (L₁ dim) gives a large output current (L₂ bright). The input current is the base current I_B; the output current is the collector current I_C.

The transistor turns a small base current I_B into a large collector current I_C: current amplification.