Current mirror
The Wilson mirror solves the Early effect voltage problem in this design.
The third specification is the minimum voltage drop across the output part of the mirror necessary to make it work properly.
This minimum voltage is dictated by the need to keep the output transistor of the mirror in active mode.
For small-signal analysis the current mirror can be approximated by its equivalent Norton impedance.
However, an ideal current source is unrealistic in several respects: A bipolar transistor can be used as the simplest current-to-current converter but its transfer ratio would highly depend on temperature variations, β tolerances, etc.
To eliminate these undesired disturbances, a current mirror is composed of two cascaded current-to-voltage and voltage-to-current converters placed at the same conditions and having reverse characteristics.
It is not obligatory for them to be linear; the only requirement is their characteristics to be mirrorlike (for example, in the BJT current mirror below, they are logarithmic and exponential).
Thus a current mirror consists of two cascaded equal converters (the first – reversed and the second – direct).
By applying a negative feedback (simply joining the base and collector) the transistor can be "reversed" and it will begin acting as the opposite logarithmic current-to-voltage converter; now it will adjust the "output" base-emitter voltage so as to pass the applied "input" collector current.
The simplest bipolar current mirror (shown in Figure 1) implements this idea.
It consists of two cascaded transistor stages acting accordingly as a reversed and direct voltage-to-current converters.
If Q1 and Q2 are matched, that is, have substantially the same device properties, and if the mirror output voltage is chosen so the collector-base voltage of Q2 is also zero, then the VBE-value set by Q1 results in an emitter current in the matched Q2 that is the same as the emitter current in Q1[citation needed].
Additional matched transistors can be connected to the same base and will supply the same collector current.
The basic current mirror can also be implemented using MOSFET transistors, as shown in Figure 2.
In this setup, the output current IOUT is directly related to IREF, as discussed next.
Thus, by adjusting the ratio of widths of the two transistors, multiples of the reference current can be generated.
Another failure of the equations that proves very significant is the inaccurate dependence upon the channel length L. A significant source of L-dependence stems from λ, as noted by Gray and Meyer, who also note that λ usually must be taken from experimental data.
[5] Due to the wide variation of Vth even within a particular device number discrete versions are problematic.
Although the variation can be somewhat compensated for by using a Source degenerate resistor its value becomes so large that the output resistance suffers (i.e. reduces).
Because they have relatively low compliance voltages, they also are called wide-swing current mirrors.
The operational amplifier is fed the difference in voltages V1 − V2 at the top of the two emitter-leg resistors of value RE.
[nb 2] A small-signal analysis for an op amp with finite gain Av but otherwise ideal is based upon Figure 5 (β, rO and rπ refer to Q2).
can be eliminated, to find:[nb 3] Kirchhoff's voltage law from the test source IX to the ground of RE provides: Substituting for Ib and collecting terms the output resistance Rout is found to be: For a large gain Av ≫ rπ / RE the maximum output resistance obtained with this circuit is a substantial improvement over the basic mirror where Rout = rO.
For Figure 3, a large op amp gain achieves the maximum Rout with only a small RE.
