Little bit of context:<p>The &quot;Transistor Man&quot; was I believe first introduced in The Art of Electronics (great book btw, the third edition just came out). An extremely simple model of a bipolar junction transistor in the linear region states that the current going from the Base to Emitter (Ib) and the current pulled from the collector (Ic) is related by: Ic = beta * Ib. (beta is another name for hFE, at DC)<p>The &quot;Transistor Man&quot; continuously monitors Ib, multiplies by beta, and adjusts the adjustable resistor until Ic = beta * Ib.<p>Obviously, you can&#x27;t make a resistor with a resistance below 0, so when it hits rock bottom Ic cannot reach beta * Ib, which models saturation.<p>Note that this &quot;model&quot; is insanely horrible because hFE&#x2F;beta increases and decreases depending on the amount of current, by temperature, and most importantly from manufacturing tolerances. Good transistor circuits (like the AoE book teaches you how to build) work the same way regardless of hFE as long as it&#x27;s in the reasonable ball park.