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Transistor Question

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ScuzZ

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There are numerous different types of transistor available.

How does one calculate the Base, Collector and Emitter currents and voltages for NPN transistors?

Is there also a difference in the caluclations for using a transistor as a switch and an amplifier?
 
You need to look at the data sheet.

Work out what your requirements are (voltage and current capabilities) then look for a transistor that can handle what you need.
 
It depends upon the particular configuration. For example, consider a common emitter circuit used as an amplifier with a pair of resistors acting as a voltage divider (R1 to Vcc & R2 to gnd) to set the base voltage, a resistor between the emitter & gnd and a resistor from the collector to the Vcc.

For a high gain transistor, the procedure is:-
Calculate the base voltage Vb = Vcc * R2/(R1 + R2).
For a silicon transistor, subtract 0.7V from this figure. This gives the emitter voltage Ve.
Calculate the emitter current using Ohm's law.
For a high gain transistor, the collector current is almost equal to the emitter current so assume that Ic = Ie, thus calculate the voltage across the collector resistor Ic * Rc. If this figure is less than Vcc - Ve - 1 then the transistor is in the active region. The collector voltage can then be calculated by subtracting this figure from Vcc.

This procedure gives a good approximation to the voltages and currents.

For a low gain transistor it is more complicated as you must consider the transistor's gain.
 
Last edited:
ScuzZ said:
Is there also a difference in the caluclations for using a transistor as a switch and an amplifier?

Yes, the BJT can work in different operative regions. When it's used as an amplifier, it is biased in the active region and the collector current is proportional to the base current. When it's used as a switch it is biased in cut-off and saturation regions. Try Google, you'll find a lot of resources on the web.
 
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