Output resistance of a BJT in saturation

Question

It seems by the characteristic curve of a BJT, that it behaves as a resistance in saturation region.

Is this conclusion true? and if so how can I calculate this resistance (seen from collector) from datasheet values?

Answer 1

Indeed that is true, a BJT behaves somewhat like a resistor when it is in the saturation region.

But it is not a very "well behaved" resistor, it is a very non-linear one. You can explain this by considering what would happen if you would apply an increasing voltage across this resistor. Then you would be increasing \$Vds\$ right ? Above a certain voltage \$Vdssat\$ the transistor will leave the saturation region and it will start behaving like a current source. Of course there's no sharp change from saturation and current mode behaviour, it's a gradual change.

The "value" of the "saturation resistance" of a transistor is therefore very dependent on \$Vbe\$ and \$Vds\$ and also the transistor's own properties like \$Hfe\$ (beta).

So in practice you cannot rely much on this "saturation resistance" having a certain value. If you just want to use the transistor to switch something on and off, the actual value does not matter so much as long as it is low enough. And that is also how saturation mode is used in general, a mode you use when you use a BJT to switch something on/off or short a signal to ground for example.