# Output resistance of a BJT in saturation

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?

• A BJT is NOT a FET or MOSFET. a FET has a drain-source resistance. if you know the load you gonna connect between the Vcc and the collector (assuming NPN). and you can calculate the voltage drop on your load on a given current. you can calculate a virtual resistance by (Vcc - Vload) / I if you have perfect resistor as load (Vcc - R*I)/I ==> Vcc/I - R – on8tom Mar 4 '16 at 8:11

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).