# The circuit

The peaking current source is shown in the schematic below:# Application

The characteristics of this type of source is that even if`$I_{IN}$`

changes a lot `$I_{OUT}$`

changes very little. So it is useful to generate a constant current from a changing supply where `$I_{IN}$`

can be replaced by just a resistor.# Design

If`$I_{IN}$`

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 `$ΔI_{IN}R={ΔI_{IN}}/g_m$`

then this condition will be fulfilled. Thus we have:Since

`$I_{IN}$`

changes there will be just 1 point in the whole `$I_{IN}$`

range where the above relation holds and that is the point where the `$I_{OUT}$`

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: