Believing in current-controlled BJTs screwed me up! Years later I heard that the exact same thing happened to Win Hill, author of AOA "Art of Electronics," a textbook for college classes. The AOA textbook points out all the problems created by "hfe-think," and the lab-book that accompanies AOA goes quite a bit further. BJTs are "transconductance" animals, same as vacuum tubes and FETs. So, who believes in "transconductance-think," or voltage-control? "Pedants" such as professional engineers; old-school analog designers, and anybody building electronic products from discretes rather than ICs. Hobbyists need no such things. As hobbyists, we never need encounter "transconductance." Also, exponential functions are irrelevant, and we just set Vbe to 0.7v. "Current control" is a simplified, beginner/hobbyist, for-dummies approach. I'd go even further, and state that believing in current-control marks a person as having zero engineering coursework, zero experience in the professional design world (at least the analog part.) "Current-control" works fine until you try designing actual products ...where we immediately discover that the hfe of a spool of smt transistors is all over the map! Then later we find that, if transconductance was ignored, and current-gain is important to our design, then our design is temperature-sensitive, since hfe varies with temperature. (Current control means that our designs need thermistors and trim-pots, if we need them to have known DC behavior and industrial/military/automotive temp-range. Those are not typical hobbyist requirements.) Proper BJT designs are hfe-resistant and ignore base-current as if it was unwanted leakage, same as proper FET designs ignoring the gate leakage. And, proper BJT designs rely on Ebers-Moll equation, not on current-gain equation. In a proper design, the base I-bias (hfe) can vary over a span of almost 10x, yet have no effect on the circuit. But how? The classic LM741 op amp is a great example of this. Learn and understand every bit of the 741 internal schematic. All those blocks are voltage-based circuits: long-tailed pair, current-mirror, Cascode amp, the three fundamentals of voltage-based design approach.) If we don't do this, then we're trapped at the hobbyist-level, as far as BJT designs go. But we'll never know this, because we've never designed mass-produced DC-coupled circuitry under extreme parts-count constraints. (So, you're sorta correct, where the only people who care about such stuff is quantum physicists, engineering students, top analog designers in major corporations, and the occasional tech who must design DC coupled circuitry without using any op-amps. The guts of BJT op-amps of course are voltage-controlled, where transconductance is everything, and the hfe of their internal transistors is irrelevant.) My very first professional analog design, an analog opto-sensor amp for Eaton/Cutler-Hammer, didn't work, since if we built several of them, their temp variations were all different, plus, they need a trim-pot for zeroing, which was absolutely not allowed. (The circuitry was for mega-mass-production, must cost only cents, even an IC chip was way, way too expensive. But the analog designer had several projects, so they gave it to me, an expert only in 8-bit embedded controllers!) So, I had to start over, using spreadsheets and Ebers-Moll equation. I used the design-philosophy I'd learned in BSEE classes, NOT the beginners'-philosophy taught everywhere else. My project was a success, installed in hundreds of thousands of cola machines world-wide, and all based on the design philosophy of "voltage-think," not "hfe-think." (In hindsight I see that William Shockley was an idiot, and had to be corrected by Ebers and Moll years later. Heh, why was the E-M equation not called the William Shockley equation?) Models of BJTs ...in SPICE programs? You'll want Monte-Carlo, a spread of betas, so you can see just how badly your design malfunctions, when it's built from transistors where the beta can be anywhere from 50 to 300. But the transconductance behavior is determined by QM physics constants, not by silicon-fab variation. "Transconductance-think" FTW. Again, to see an example of such things, go through the LM741 internal schematic in great detail, figuring out every smallest piece, until there are no mysteries there. Or, find a modern DC-coupled high-end audio amplifier, few hundred watts etc. Carver audio and similar. Those things look just like a 741 op amp inside, made from discrete transistors.
Its like people are missing something so obvious. Just as you run your experiment nothing happens until you trigger it on with what, passing the "Voltage Threshold", the Bias turn on Voltage. But, you can't have one without the other. But, you do have pass the Bias Voltage level for anything to happen. And, like you said most applications today for BJTs are to amplify some sort of what? A small Analog Varying Voltage Signal. Voltage and Current to me are like the age old question of what came first? The Chicken or The Egg?
What I would just like to know what is happening to the current being applied to base ? Lets take NPN transistor. is current going t gate being dissipated as heat, or is it uniting together with current going from collector, so both currents go to emitter ?
Thank you. I'm not an electrical engineer but just interested and this was always nagging me. Is it true to say that since voltage always precedes current then a bjt is voltage-initiated but current-controlled? Or am I just another pedant? But thanks anyway!
Voltage causes current. Current is the aggregate motion of electrons, and they move in aggregate because either there is an electric field across them (a voltage, also known as an electrical potential difference) making them run "downhill", or there is a magnetic field interacting with them and giving them a "push" like a generator. We think of current as its own thing because it's easy to conceptualize and understand how circuits work if we do that, but current is always ever just the side effect.
You see a lot of the wannabe professor types in beginners electronic groups telling people they NEED to understand BJTs as voltage controlled. These are the same guys who think your first lesson on opamps need to consider slew rates, open loop gain, etc. They just make everything more complicated than necessary and discourage people from actually learning.
For a lot of people, their topic of interest is a genuine passion for them, and they want so badly for everyone else to be as passionate as they are. It's hard for them to see from their students' perspectives.
Believing in current-controlled BJTs screwed me up! Years later I heard that the exact same thing happened to Win Hill, author of AOA "Art of Electronics," a textbook for college classes. The AOA textbook points out all the problems created by "hfe-think," and the lab-book that accompanies AOA goes quite a bit further. BJTs are "transconductance" animals, same as vacuum tubes and FETs.
So, who believes in "transconductance-think," or voltage-control? "Pedants" such as professional engineers; old-school analog designers, and anybody building electronic products from discretes rather than ICs.
Hobbyists need no such things. As hobbyists, we never need encounter "transconductance." Also, exponential functions are irrelevant, and we just set Vbe to 0.7v. "Current control" is a simplified, beginner/hobbyist, for-dummies approach. I'd go even further, and state that believing in current-control marks a person as having zero engineering coursework, zero experience in the professional design world (at least the analog part.) "Current-control" works fine until you try designing actual products ...where we immediately discover that the hfe of a spool of smt transistors is all over the map! Then later we find that, if transconductance was ignored, and current-gain is important to our design, then our design is temperature-sensitive, since hfe varies with temperature. (Current control means that our designs need thermistors and trim-pots, if we need them to have known DC behavior and industrial/military/automotive temp-range. Those are not typical hobbyist requirements.)
Proper BJT designs are hfe-resistant and ignore base-current as if it was unwanted leakage, same as proper FET designs ignoring the gate leakage. And, proper BJT designs rely on Ebers-Moll equation, not on current-gain equation. In a proper design, the base I-bias (hfe) can vary over a span of almost 10x, yet have no effect on the circuit. But how? The classic LM741 op amp is a great example of this. Learn and understand every bit of the 741 internal schematic. All those blocks are voltage-based circuits: long-tailed pair, current-mirror, Cascode amp, the three fundamentals of voltage-based design approach.)
If we don't do this, then we're trapped at the hobbyist-level, as far as BJT designs go. But we'll never know this, because we've never designed mass-produced DC-coupled circuitry under extreme parts-count constraints. (So, you're sorta correct, where the only people who care about such stuff is quantum physicists, engineering students, top analog designers in major corporations, and the occasional tech who must design DC coupled circuitry without using any op-amps. The guts of BJT op-amps of course are voltage-controlled, where transconductance is everything, and the hfe of their internal transistors is irrelevant.)
My very first professional analog design, an analog opto-sensor amp for Eaton/Cutler-Hammer, didn't work, since if we built several of them, their temp variations were all different, plus, they need a trim-pot for zeroing, which was absolutely not allowed. (The circuitry was for mega-mass-production, must cost only cents, even an IC chip was way, way too expensive. But the analog designer had several projects, so they gave it to me, an expert only in 8-bit embedded controllers!) So, I had to start over, using spreadsheets and Ebers-Moll equation. I used the design-philosophy I'd learned in BSEE classes, NOT the beginners'-philosophy taught everywhere else. My project was a success, installed in hundreds of thousands of cola machines world-wide, and all based on the design philosophy of "voltage-think," not "hfe-think." (In hindsight I see that William Shockley was an idiot, and had to be corrected by Ebers and Moll years later. Heh, why was the E-M equation not called the William Shockley equation?)
Models of BJTs ...in SPICE programs? You'll want Monte-Carlo, a spread of betas, so you can see just how badly your design malfunctions, when it's built from transistors where the beta can be anywhere from 50 to 300. But the transconductance behavior is determined by QM physics constants, not by silicon-fab variation. "Transconductance-think" FTW.
Again, to see an example of such things, go through the LM741 internal schematic in great detail, figuring out every smallest piece, until there are no mysteries there. Or, find a modern DC-coupled high-end audio amplifier, few hundred watts etc. Carver audio and similar. Those things look just like a 741 op amp inside, made from discrete transistors.
Years of misunderstanding complete!
Yours was the final blow to drive BJT operating into my dome!
I love you
Absolutely amazing videos
The best explanation.
Its like people are missing something so obvious. Just as you run your experiment nothing happens until you trigger it on with what, passing the "Voltage Threshold", the Bias turn on Voltage. But, you can't have one without the other. But, you do have pass the Bias Voltage level for anything to happen. And, like you said most applications today for BJTs are to amplify some sort of what? A small Analog Varying Voltage Signal. Voltage and Current to me are like the age old question of what came first? The Chicken or The Egg?
I hope you continue to make many more videos. Very helpful and a great style in presenting.
Thanks
What I would just like to know what is happening to the current being applied to base ?
Lets take NPN transistor.
is current going t gate being dissipated as heat, or is it uniting together with current going from collector, so both currents go to emitter ?
Thanks man! Your a huge help.
Voltage controlled. Next question.
cant believe i never seen this one before.. cool video.
well said
Thank you. I'm not an electrical engineer but just interested and this was always nagging me. Is it true to say that since voltage always precedes current then a bjt is voltage-initiated but current-controlled? Or am I just another pedant? But thanks anyway!
Voltage causes current. Current is the aggregate motion of electrons, and they move in aggregate because either there is an electric field across them (a voltage, also known as an electrical potential difference) making them run "downhill", or there is a magnetic field interacting with them and giving them a "push" like a generator. We think of current as its own thing because it's easy to conceptualize and understand how circuits work if we do that, but current is always ever just the side effect.
You see a lot of the wannabe professor types in beginners electronic groups telling people they NEED to understand BJTs as voltage controlled. These are the same guys who think your first lesson on opamps need to consider slew rates, open loop gain, etc. They just make everything more complicated than necessary and discourage people from actually learning.
For a lot of people, their topic of interest is a genuine passion for them, and they want so badly for everyone else to be as passionate as they are. It's hard for them to see from their students' perspectives.