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Ive learned that in order to turn a transistor on you have to apply a positive voltage at the base. But where does this voltage come from and what controls when it turns on or off? Any answer would be greatly appreciated. Thanks!
And got the following answer:
Transistors are controlled by a voltage applied between the emitter and base. The current is a parasitic effect caused by internal recombination of charges. This recombination isn't generally desired, but it is always present in an active BJT. It simply turns out that this recombination current is roughly proportional (over some reasonable range) to the collector current. So a new factor called β is defined to describe this constant of proportionality. People then falsely imagine that it is the current itself that causes the collector current in the first place, but that isn't true. It is the impressed voltage. The voltage follows the following approximation: Vbe = k⋅T ⁄ q ⋅ ln( Ic ⁄ Is ) where Is is a temperature dependent model constant, k is Boltzmann's constant, q is the unit of elementary charge (electron or proton, for example), and T is the absolute temperature (usually expressed in Kelvin since 'k' usually carries that unit.) As a rule of thumb in a small signal BJT, you can broadly speaking assume that Vbe = 0.7V at about Ic = 2mA. For smaller or larger values of Ic, you adjust Vbe by 60mV (at room temps) for every decade change in Ic. So if Ic = 200μA, then Vbe = 640mV. If Ic = 20mA, then Vbe = 760mV. Like that. A designer usually knows (or can estimate) the value of Ic, so they can then roughly guess at the Vbe value. This is then used in calculating the values of resistors or other circuit elements that will be used to ensure that this voltage is met, while ALSO making sure that "at least" a certain amount of recombination current is also present. If the BJT is used as a switch, with the collector voltage very near the emitter voltage, then the β is usually taken to be approximately 10 or 20 (in small signal BJTs, 20 is usually just fine -- even 30 works often enough -- but in large BJTs able to handle amps the value of 10 is almost certainly necessary as a guess.) If there is a question, the datasheet is used to clarify, adjust, or confirm such estimates. Then the designer makes sure that at least this fraction of the collector current is available as base current drive. If the BJT is operated as an amplifier, β is often taken to be at least 100 -- if a small signal device. Most typical small signal BJTs have β somewhere between about 150 and 300. If it is a larger BJT, then the data sheet is consulted because the β estimate varies from device to device too much. A class-A amplifier is "always on" the designer makes sure that the base has that fraction of the collector current available as base drive, again. Same thing if the BJT is going to sometimes on and sometimes off as it may be in a class-B amplifier, but in this case there needs to be a way to make sure the Vbe voltage turns off and on, as needed. How it is all done is a matter for design. There are so many factors to take into account and if you set two designers on the same task they may each take slightly different, or even wildly different, means to get there. There is no one solution fits all problems answer here, as Bill's answer suggests.