![]() You connect it through a transistor with a gain of 100. Here's an example of transistor-saturation: You've got an LED & resistor that take 20mA when connected directly to 12V.Now that you know the voltage across the resistor and the current throught the resistor, you can use Ohm's Law to calculate the resistance. The current through the resistor is the same current into the transistor base. So, with a typical gain of around 100, your base-current needs to be somewhere around 1/10th to 1/50th of the collector-emitter current, and your circuit should work! As Mark said, in a switching application you need to saturate* the transistor. Then you can just divide by the transitor gain (hfe) to find the needed base-current. For that you need to know the output/load current. Now, you just need to calculate/estimate the required base-current. But, with the transistor turned-on, you've got less than 1V across the transistor, so you can just approximate the voltage as 4 or 5V. The 5V output gets divided between the resistor and the transistor's base-emitter junction. In fact, small signal amplifiers are the most common linear devices.Are you familiar with Ohm's Law? With Ohm's Law, you can calculate the resistance, if you know the current & voltage.Īssuming you are connecting to the output of the Arduino, you've got 5V. Amplification means linear amplification. ![]() Therefore, a transistor acts as an amplifier when operating in the active state. When the transistor is in the active state, I C = I B. That is the transistor behaves as though a switch has been closed between the collector and emitter. In saturation, the collector and emitter are, in effect, shorted together. That is collector emitter pathway is open If the transistor is cut-off, there is no base current, so there is no collector or emitter current. In the active state, collector current is β times the base current ( i.e. SATURATED : Emitter diode and collector diode are ON. The relations between the diode states and the transistor states are :ĬUT-OFF : Emitter diode and collector diode are OFF.ĪCTIVE : Emitter diode is ON and collector diode is OFF. The state of a transistor is entirely determined by the states of the emitter diode and collector diode. We have seen above that transistor can act in one of the three states : cut-off, saturated and active. The junction between base and collector may be called collector diode. ![]() The junction between base and emitter may be called emitter diode. A transistor has two pn junctions i.e., it is like two diodes. The reader may find the detailed discussion on transistor biasing in the next chapter. We provide biasing to the transistor to ensure that it operates in the active region. ![]() Consequently, the transistor will function normally in this region. In the active region, collector-base junction remains reverse biased while base-emitter junction remains forward biased. The region between cut off and saturation is known as active r egion. If base current is greater than I B( sat), then collector current cannot increase because collector-base junction is no longer reverse-biased. At saturation, collector-base junction no longer remains reverse biased and normal transistor action is lost. At this point, the base current is maximum and so is the collector current. The point where the load line intersects the I B = I B( sat) curve is called sa turation. The collector-emitter voltage is nearly equal to V C C i.e. At cut off, the base-emitter junction no longer remains forward biased and normal transistor action is lost. At this point, I B = 0 and only small collector current ( i.e. The point where the load line intersects the I B = 0 curve is known as cut off. ( i) shows CE transistor circuit while Fig.( ii) shows the output characteristcs along with the d.c.
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