Comment by marshray

Nice article.

Looking at the ONSemi datasheet for the BS170, it is very clear that the "Drain−Source Breakdown Voltage" spec only applies at "VGS = 0, ID = 100 uA DC".

Doesn't it cause problems for designers if the curves measured from actual devices look different from those shown in textbooks?

Not as much as you'd think, because (a) the measured curves are still within the stated parameters, and (b) many basic parameters vary from one part to the next (or as the same part drifts over time) even more.

Look at the values of V_GS used in the graphs: [2.34, 2.5, 2.63, 2.71 V]. They differ by 0.4 V. But the data sheet specifies V_GS(th) (the V_GS threshold needed to just start to turn on) as a range of 0.8 to 3.0 V!

A range isn't even given for the slope of the curve (g_fs). The datasheet only gives a "typical" value at a single operating point.

So the designer cannot rely on this parameter to determine the behavior of the circuit, only its general range and direction. You can't just use the bare transistor alone and expect to get an oscillator or amplifier that behaves predictably.

Any manufacturable circuit must ensure that these variations are completely dominated by negative feedback. This is why actual circuits tend to have networks of resistors and capacitors hanging off of every leg of every transistor.