Analog Electronics

The Current Mirror

Diode-connected Q1 (base-collector shorted) converts the reference current into a VBE; its twin Q2, sharing the same VBE, "mirrors" the same current at its collector. Even if the load resistance changes, the current stays constant.

Reference: Iref = (Vcc − VBE)/R. In the simple mirror, base currents introduce a small error (≈2/β) and the Early effect limits the output resistance; cascode and emitter-degenerated variants improve this.

Current mirrors are the atomic building block of analog design: biasing inside op-amps, active loads, and constant-current sources for LED drivers.

Formulas

Iref = (Vcc − 0.7)/R

Iout ≈ Iref · (1 − 2/β)

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Test Yourself

For Vcc=12 V and R=22k, what is the approximate mirror current?

Answer: 0.51 mA — (12−0.7)/22k ≈ 0.51 mA.

Why is Q1 "diode-connected" (B-C shorted)?

Answer: To convert the reference current into a VBE voltage — Q1 encodes Iref into VBE via the log law; Q2 reproduces the same current from the same VBE.

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