5 shows the required common emitter connection. Find the base current for common emitter connection. Q11. For a transistor, β = 45 and voltage drop across 1kΩ which is connected in the collector circuit is 1 volt. 8.20 shows the conditions of the problem. Hence determine the value of I C using both α and β rating of the transistor.įig. Q10. Find the α rating of the transistor shown in Fig. Q9. Calculate I E in a transistor for which β = 50 and I B = 20 μA. Since the transistor is of silicon, V BE = 0.7V.Īpplying Kirchhoff’s voltage law to the emitter-side loop,we get,Īpplying Kirchhoff’s voltage law to the collector-side loop, we have, With an LED, this is usually not a problem.Q7. For the common base circuit shown in Fig. That is, you can burn-up a 1 Amp transistor with much less than 1A if you are dissipating power.
So, you have to be aware of the transitor's power rating as well as it's current rating, and in some cases you need a heatsink. It will dissipate power, and it will heat-up. When you are operating lineraly, the transistor will "see" voltage and current at the same time. If you reduce base current to the point where you are no longer in saturation, the transistor begins to limit the current.
When you are in saturation the load device (and voltage) determines the current. You can use someting to "linearly" dim an LED. In that case, the transistor is operating "linearly", not as a switch. You can use a resistor (or something else) to control the base current, and as long as you are not in saturation, the collector current is proportional to base current (multiplied by the beta).
Bjt transistor calculations series#
Ohms' Law says you need a ~200 Ohm resistor.Ĭould i potentially use a transistor to limit current that goes on through the emitter to the device? I mean, by placing a correctly valued resistor in series with the base of transistor, to control the saturation current? Now, you can calculate a resistor value that gives you a base current that's about 1/50th (to 1/20th) of your collector current.Įxample - If you need 1 Amp, and you have 5V into the resistor (4V across the resistor). In fact, you can assume zero B-E voltage, and most of the time it will work, since we are assuming a low beta and providing plenty of base current. A rather large-percentage change in B-E voltage won't change the voltage across the resistor that much. The exact voltage (usually) isn't important, since (in most applications) most of the voltage is dropped across the resistor. The base-emitter voltage is around 1V with the transistor turned-on. (Assuming a low beta insures that we alway have enough base current to saturate the transistor.) So, we can design a circuit that works with betas between 20 & 50 and we can be sure it will always work. But since this is a switching application we dont need to know the exact value. The data-sheet will give you the minimum value (and sometimes a "typical" value). Next, I'll assume the transistor beta (current gain) is around 100. You need to find an approximate value that will work with the actual current in your circuit.įirst, I'll assume this is a switching application, and that the emitter is grounded. You shouldn't be thinking in "maximum" values. But, you need to make sure your application doesn't exceed the device's maximum rating.) (You don't need the transistor's maximum current rating for the calculation. Or the "worst case" or maximum for your application. We do need to know the approximate collector current. ( A lot of engineering has to do with knowing what's critical and what can be assumed, ignored, or approximated. You need to allow for tolerances anyway, so there is no exact answer. We can make some appoximations & assumptions to simplify the calculations.
Bjt transistor calculations how to#
So what I really need to know is, how to calculate the max value of the series resistor that I can wire to the base of transistor, to still allow maximum current through collector-emitter?
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