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by hatsunearu
3875 days ago
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Little bit of context: The "Transistor Man" 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) The "Transistor Man" continuously monitors Ib, multiplies by beta, and adjusts the adjustable resistor until Ic = beta * Ib. Obviously, you can't make a resistor with a resistance below 0, so when it hits rock bottom Ic cannot reach beta * Ib, which models saturation. Note that this "model" is insanely horrible because hFE/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's in the reasonable ball park. |
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This makes his book even more excellent. And the lab book for AOA goes much farther, directly exposing the many mistakes caused by "current amplifier" design philosophy based on hfe.
Look inside a BJT op-amp, where diff-input is voltage-based. And the many Current-Source sections employ the transconductance model, Vbe-based explanations, not hfe.
Heh, the actual "transistor man" is made out of voltage; he's the base voltage Vbe, and he adjusts the thickness of the depletion layer inside the emitter junction. The depletion layer observes Ib, changes its thickness in response, and this thickness then determines the value of collector current Ic. Actually Ib cannot affect Ic or vice versa, otherwise we'd just have resistor action, not transistor action.
"As a young engineer struggling with my circuit designs in the mid 60's (remember the h parameters?), a wise experienced teacher sat me down with a single sheet of paper for 30 minutes and preached the value of Ebers-Moll and Re. What's the gain of a common-emitter amplifier stage? Easy, RL / Re. And Re is 25 ohms at 1mA.
"Oops, Re = VT/IC = 25mV/IC, and changes with current, causing distortion? OK add a degenerating resistance, RE - now the gain is RL / RE+Re, and I can quickly calculate my gain and predict the distortion level for my choices of RE. Try that kind of easy analysis with the βeta approach.
"So I changed my design approach and suddenly things start working as predicted. That is to say, I could reliably engineer instead of empirically-develop my designs. I could also safely do more complex designs." - 2011 Winfield Hill http://cr4.globalspec.com/comment/720374/Re-Voltage-vs-Curre...