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Peaking Current Source


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

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




Copyright 2018 Milind Gupta