A resistor "feeding" some amount of current is just a turn of phrase, not to be taken literally. A resistor, as you know, is a passive element and does not actually feed energy into the transistor.but in all three responses you did not mention about the current feeding role of resistor to the base.
It's nice to know the "PHYSICAL TRUTH" about BJT's as voltage-controlled devices but that's of little use when designing the bias circuits for BJT linear amp stages or when using them as a switch. For those purposes viewing them as a current controlled device with the base-emitter junction appearing as diode with a fairly low input impedance is the easiest way to calculate the bias values needed. That's why the beta or Hfe value is used for these calculations, not the transconductance.QUOTE k7elp60: "The base current determines the collector current, controlled by the β,(beta) or hfe."
Quote ActivePower: "While designing a BJT circuit, you'd need to know the exact amount of base current that needs to be fed into the Base terminal as that determines the controlled current i.e. through the Collector to the Emitter."
I am aware that some forum members don`t like to discuss the question how the transistor really works.
However, I must confess I always feel provoked to read that the base current would "control" or "determine" the collector current.
And I cannot resist to correct these obviously false statemets. Sorry.
I agree that during design of a transistor amplifier we can make use of the simple and nice-looking formula Ic=beta*Ib.
However, this formula does not tell the PHYSICAL TRUTH regarding the control mechanism inside the transistor.
And, for my opinion, every good engineer who is going to design a transistor stage should not only be able to handle formulas but, rather, should know what he is doing and WHY he is allowed to do this.
That means: He must be informed about the physical background of the effects he is exploiting.
And therefore I repeat: The BJT is a device with a collector current Ic which can be externally controlled by the base-emitter voltage Vbe. The current Ib is nothing than an unwanted by-product that cannot be avoided!
There are many simple and easy-to-understand justifications for this fact - and I am always surprised that some people still believe that it should be possible that a small current (Ib) should be able to control a large current (Ic). This is impossible!
Sort of like a very pedantic fellow who likes to talk about how many angels can dance on the head of a pin we have encountered before...It's nice to know the "PHYSICAL TRUTH" about BJT's as voltage-controlled devices but that's of little use when designing the bias circuits for BJT linear amp stages or when using them as a switch.... .
Why so ironically? Is this really appropriate?Sort of like a very pedantic fellow who likes to talk about how many angels can dance on the head of a pin we have encountered before...
To Canith, K7elp, and ActivePower: Listen to crutschow; he designed circuits for a living. He knows what he is talking about.
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by Jeff Child
by Jeff Child
by Jake Hertz