Current mirror

Transistors and thyristors · Advanced · about 20 minutes · part of Electronics Engineering

Q1 is wired as a diode: its collector current sets a V_BE. Q2 shares that V_BE, so, being identical and at the same temperature, it carries the same current, almost whatever its collector voltage. Chips use it everywhere to copy bias currents.

Open this lesson in the labFree, in your browser, no account needed.

How it works

Try it on the bench

1. Set R_load to 12 kΩ: Q2 saturates and the mirror stops copying.
2. Set the mismatch to 5 mV: I_out is 21 % high, which is why mirrors need matched transistors on one die.

Parts on the bench

• Bench power supply
• Matched transistor pair
• 2 × Resistor
• Digital multimeter

In the course

Electronics Engineering, module “Transistor amplifiers”.

More on transistors and thyristors

• BJT current gain (β)
• Transistor as a switch
• Emitter follower (buffer)
• MOSFET low-side switch
• Inductive load and the flyback diode
• Darlington pair
• H-bridge motor driver
• Push-pull output stage