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If the voltage source of the copying current transistor is variable, as in this example while being demonstrated via a Sawtooth wave, the effects are particularly noticeable with respect to the chosen technology: BJT & FET.
Taking a look at the BJT current mirror (blue) it can be seen that despite a variable source voltage the current remains at a constant copy. Specifically the copy is 98.5% efficient. This is due to the BJT being a current based device.
Since the BJT is always on via a constant current, the changing voltage does not equate to a change in current, but rather a change in resistance.
Unlike the BJT, the FET is a voltage based device. Thus when the voltage applied to the FET mirror (green) at the Drain is varying, the current will vary resulting in an unwanted change and an unwanted copy of the supplied current. The range in this case varies from ~42mA to ~81mA copied. This results in a huge deficiency, 80% max, for the FET.
Device selection for an application that has a varying input, such as the Sawtooth Wave, can be critical in instances of current mirroring, as seen here. Despite FETs having a larger voltage operating range and faster switching mechanics, in this application for current mirroring they prove rather inefficient.
The take home here is that for variable voltage fluctuations, such as noise or spikes in a power supply, the current can, and will, maintain a constant state. Device selection (BJT or FET) is very important in how one wants the current copied in addition to the source or signal being used for reference.
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