Practical Approximations
For small-signal analysis the current mirror can be approximated by its equivalent Norton impedance .
In large-signal hand analysis, a current mirror is usually and simply approximated by an ideal current source. However, an ideal current source is unrealistic in several respects:
- it has infinite AC impedance, while a practical mirror has finite impedance
- it provides the same current regardless of voltage, that is, there are no compliance range requirements
- it has no frequency limitations, while a real mirror has limitations due to the parasitic capacitances of the transistors
- the ideal source has no sensitivity to real-world effects like noise, power-supply voltage variations and component tolerances.
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