Question regarding how transistors are biased

Frequently asked question on transistors: types, substitutions, how to test, use and misuse them.
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nightendday
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Post by nightendday »

Seems like a noob like question, and it is. :mrgreen:
but basically what I'm asking is, what changes what and how can I learn to bias a transistor to be clean, for say a clean boost, or output stage. Or, how could I bias one for fuzz, or overdrive? I've learnt basic things from building my own circuits, and see the differences in the biasing, but my question is basically what changes what?

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OC26
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DrNomis
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Post by DrNomis »

It's fairly simple, take the example of a one-transistor amplifier stage, usually there will be about 4 fixed-value resistors, the collector-resistor, the emitter-resistor, and the two voltage-divider biasing resistors, the collector and emitter-resistors set the maximum current-flow when the transistor is turned fully-on, to get the transistor to turn-on so that current can flow from the emitter to the collector, the base needs to be at least .6V higher than the emitter (in the case of a Silicon transistor), Germanium transistors need a lower base voltage, to provide this voltage the two voltage-divider bias resistors are added, one from +V to the base, and the other from the base to circuit ground, when the transistor turns-on the emitter-collector current-flow causes a voltage-drop accros both the collector and emitter resistors, the emitter-resistor voltage drop raises the emitter above the base so the actual voltage supplied to the base by the voltage-divider biasing resistors needs to be the sum of the emitter resistor voltage-drop plus .6V, now, to get the transistor to produce the largest output signal-swing without clipping, we need to bias the transistor so that about half the supply voltage is present on the collector, if the voltage is too low, this means the transistor is biased too "on", on the other hand if the voltage is too high this means the transistor is biased too "off", to adjust the biasing of the transistor, the resistance value of the voltage-divider biasing resistor from the transistor's base to circuit-ground needs to be made either larger or smaller untill the collector sits at about half the supply voltage, to get the transistor to clip (produce fuzz) all you need to do is increase the signal-level being fed into the transistor's base.... :thumbsup
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