![]() In fact, small signal amplifiers are the most common linear devices. 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. Single letters refer to the voltage relative to ground for example, Vc is the 'collector' voltage relative to ground. In CMOS logic, Vss refers to the 'source' voltage, and Vdd is the 'drain' voltage. ![]() For example, Vee refers to the 'emitter' voltage. That is the transistor behaves as though a switch has been closed between the collector and emitter. Double letters (cc) refer to power supply voltages. 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. Unfortunately at very low current there is no time to charge or discharge the parasitic capacitances and in some cases this effect is even visible on data sheets.The below Fig. "Static parameters", like saturation voltage, beta., are measured with a pulsed technique to avoid heating the silicon. ![]() Unfortunately I have no news about the behavior of \$\beta_R\$ :(Ī final general warning about data sheets, not really applying to this case. In a BJT, saturation voltage \$V_\$ decreases. I try my 2 cents on the matter, it's just a guess, don't take me seriously. Technically it is still in saturation until Collector voltage reaches Base voltage at 0.75 V, but above ~0.3 V it is in the 'soft saturation' region where it is acting almost the same as at higher Collector voltages. In this example the transistor comes out of hard saturation above ~20 mA. Here we see that - as expected - Collector voltage always increases as Collector current increases. I generated the graph below using LTspice to simulate a 2N4124 with a fixed Base current of 100 uA:. To see the direct effect of Collector current on Collector voltage with fixed Base bias, you need a different graph. This is useful for determining how much Base current current you should apply to ensure that the transistor saturates when switching a particular Collector current. The second graph shows how increasing Base current reduces Collector voltage at different fixed Collector currents. A ratio of 1:10 is commonly used as an 'overkill' to guarantee saturation independent of the individual device's 'normal' current gain. At this point the transistor is well into 'hard' saturation, where changes in Base current have little effect on Collector current. The first graph shows Collector voltage when Ib is forced to Ic / 10. However you might be a little confused by the datasheet graphs because they do not show this relationship directly. Yes, you can - assuming no other variables are changed. Can I say when collector current increases voltage Vce also increases
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