Checking Current Mirror Accuracy Across Output Voltage

A two-transistor current mirror can match its target current at one output voltage and depart from it elsewhere. Device matching is only one contributor. The output transistor must remain in a suitable operating region, and its current can change as collector voltage changes. Validate the range the load will actually impose.

Define the error being measured

The Analog Devices chapter on current mirrors discusses base-current error, compliance voltage, and the effect of collector voltage on current. These mechanisms explain why a nominal one-to-one mirror is not an ideal current source.

State whether accuracy is measured against the external reference-branch current, the reference transistor’s collector current, or a calibrated target. These quantities differ when base currents are included in the reference branch.

Sweep the intended output range

Hold the reference arrangement fixed and vary output voltage with a controlled load or source. Measure output current and reference-branch current simultaneously where possible. Record transistor voltage and dissipation at every point.

Identify the low-voltage region where the mirror no longer maintains the required current. Define usable compliance from the application’s allowed error rather than assuming that every voltage above a nominal saturation value is equally accurate.

Separate voltage effects from heating

At higher output voltage, the output transistor may dissipate more power than the reference transistor. Compare short measurements with thermally settled measurements to reveal changes associated with temperature imbalance.

For a hypothetical 1 mA mirror, moving the output transistor from 1 V to 10 V changes its dissipation from approximately 1 mW to 10 mW. Whether that matters depends on the devices and thermal arrangement, but the two test points do not impose the same heating.

Compare implementation choices fairly

If evaluating separate transistors against a matched pair, retain the same reference current, output range, and thermal conditions. A matched pair can improve matching without removing every error caused by finite gain or unequal collector voltages.

Check resistor tolerance and source stability before attributing the entire observed error to the transistors. Measure the reference current rather than assuming that a nominal resistor establishes an exact value.

Repeat at relevant temperature and supply conditions, including the load’s startup voltage trajectory. A mirror may spend part of startup outside its accurate range even if the settled load voltage is acceptable.

Save current versus output-voltage plots, reference-current measurements, temperature conditions, and the defined usable range. The completed review should identify how much current error the load sees throughout operation and distinguish calibration at one point from performance over the complete voltage and thermal envelope.