Hi

Every text tells you about the maximum limit of the collector/drain current of a BJT or FET but none tells you about the minimum current you can run through it. I suppose this woul be down to signal-to-ratio i.e. a typical small signal BJT can take 100mA through it but could you run 1uA through it?

Br

Edward

 

You can operate transistors at extremely low current levels, well down into the nanoamp region. There are no hard transitions between levels, it is a continuous curve. At some point your signal becomes swamped by noise, but there are lots of techniques to push the useful range down amazingly far.

 

Just be aware that many BJT transistors show a significant loss in current gain (Beta or hFE) at very low collector currents.

 

See Beta=f(ie)

 

one limitation when using low currents is leakage current.
this has the effect of reducing the gain as temperature increases. Some low noise transistors had a spec. for current gain at 10uA, but the Icbo could be higher than this at 150 C, which means considering the temperature range in particular.
Modern transistors are made in (generally) cleaner production lines and may have even lower leakage, and able to be used at low currents. When I worked on IC devices we were disappointed if they would not maintain gain down to pA, but then the areas were small (and leakage scales with area).