# The circuit [top]

The peaking current source is shown in the schematic below:

# Application [top]

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 [top]

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:
\$\$R=V_T/I_{IN}\$\$

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:

# Extensions [top]

The output transistor can be degenerated with a resistor like a Widlar Source to make the output current curve more flat as well.