page_title: Current mirror
section_title: Extensions and complications
hierarchical_section_title: Current mirror / Circuit realizations of current mirrors / Basic BJT current mirror / Extensions and complications
caption_attribution_description: Simple MOSFET mirror
context_page_description: A current mirror is a circuit designed to copy a current through one active device by controlling the current in another active device of a circuit, keeping the output current constant regardless of loading. The current being "copied" can be, and sometimes is, a varying signal current. Conceptually, an ideal current mirror is simply an ideal inverting current amplifier that reverses the current direction as well. Or it can consist of a current-controlled current source. The current mirror is used to provide bias currents and active loads to circuits. It can also be used to model a more realistic current source.
  The circuit topology covered here is one that appears in many monolithic ICs. It is a Widlar mirror without an emitter degeneration resistor in the follower transistor. This topology can only be done in an IC, as the matching has to be extremely close and cannot be achieved with discretes.
  Another topology is the Wilson current mirror. The Wilson mirror solves the Early effect voltage problem in this design.
context_section_description: When Q₂ has VCB > 0 V, the transistors no longer are matched. In particular, their β-values differ due to the Early effect, with
  where VA is the Early voltage and β₀ is the transistor β for VCB = 0 V. Besides the difference due to the Early effect, the transistor β-values will differ because the β₀-values depend on current, and the two transistors now carry different currents (see Gummel-Poon model).
  Further, Q₂ may get substantially hotter than Q₁ due to the associated higher power dissipation. To maintain matching, the temperature of the transistors must be nearly the same. In integrated circuits and transistor arrays where both transistors are on the same die, this is easy to achieve. But if the two transistors are widely separated, the precision of the current mirror is compromised.
  Additional matched transistors can be connected to the same base and will supply the same collector current. In other words, the right half of the circuit can be duplicated several times with various resistor values replacing R₂ on each. Note, however, that each additional right-half transistor "steals" a bit of collector current from Q₁ due to the non-zero base currents of the right-half transistors. This will result in a small reduction in the programmed current.
  See also an example of a mirror with emitter degeneration to increase mirror resistance.
  For the simple mirror shown in the diagram, typical values of  will yield a current match of 1% or better.
caption: Figure 2: An n-channel MOSFET current mirror with a resistor to set the reference current IREF; VDD is the supply voltage
exif: {}