The peaking current source is shown in the schematic below:
The characteristics of this type of source is that even if
changes a lot
changes very little. So it is useful to generate a constant current from a changing supply where
can be replaced by just a resistor.
changes then the change the change in the Vbe and the drop on the R should be equal and opposite. When that happens the current in the output transistor will remain constant since its Vbe won't change. If
then this condition will be fulfilled. Thus we have:
changes there will be just 1 point in the whole
range where the above relation holds and that is the point where the
curve is flat, it tapers lower on either sides.
A simulation with R=259 ohms causing the peak to be at 100uA is shown below:
The output transistor can be degenerated with a resistor like a Widlar Source to make the output current curve more flat as well.