![]() ![]() A darlington pair allows you to combine the best of both worlds. They make "power BJTs" which are optimized to have a high saturation current, but it is very difficult to make such power BJTs with a high gain. The second state, at higher currents, is known as the "saturation" state, where the Collector-Emitter current is relatively constant with respect to the Base-Emitter current.ĭarlington pairs often appear in situations where high current amplification is needed. The constant of proportionality of this relationship is called the "beta" of the transistor (often on the order of 100). The first is the "active" state, where the Collector-Emitter current is proportional to the Base-Emitter current. The relationship between these currents is well approximated by considering two regions. The more current travels through the Base-Emitter path, the more current is permitted to pass through the Collector-Emitter path. There are a few voltages that matter, such as the voltage drop between the base and emitter, but as a general rule, it's the currents which matter for BJTs.īJTs are current amplifiers. Hence for darlington configuration, the base to emitter voltage should be greater than or equal to 1.4V.The key here is understanding that transistors (specifically BJTs) do not operate on voltages, but rather on currents. But in darlington pair we are using two transistors back to back which increases the value of threshold voltage for both the transistors. Generally, an NPN silicon type transistor conducts only when its base to emitter junction voltage is greater than or equal to 0.7V (practically, it is 0.65V). This gain is then used for high current loads Therefore we can write the above equation asįrom the above equation, we can analyse the total current gain,įrom this derivation, we can see that a small positive voltage at base derives a high current gain. The individual beta are neglected since the product of both the current gain are very high as compared to their sum. Substituting in the above equationIc = Beta1. Since the base current of the second transistor is Total current at the collector is Ic = Ic1 + Ic2 Lets say Current gain of the first transistor So this value of current gain is able to give high current at the output.įig. Therefore overall current gain is 10000 times of the input current which is very high. The overall current gain of the transistor will be the product of beta1 and beta2. Let’s consider that the current gain of slave transistor is beta1 and the master transistor is beta2. Hence current at emitter is so high which is capable of deriving high loads at the output. The current amplified by slave transistor is again amplified by the master transistor. The collector of both the transistors are connected with each other and then with a positive supply like 5V. The slave transistor output emitter current is used for deriving high at base of master transistor. The slave transistor or triggering transistor emitter is connected to the base of the master transistor. One of the transistors is acting like a slave and another one is a master to pull up the gain at the output. The transistors used here are in common emitter configuration. A darlington pair can be formed by two NPN or PNP transistors. The darlington pair has two transistors connected in a way so that it provides a high value of gain even at low base current. Therefore, the current is again amplified by the second transistor which gives a high current gain at the output. The transistors are connected in such a way that the input current is first amplified by the first transistor and then to the base of the second transistor. This problem can be solved by using a Darlington pair. A single transistor is not good enough to drive high gain. We require high gain to drive high loads like an audio amplifier, power regulators, motor controller, etc. In common emitter configuration of transistor, current gain is the ratio of collector current to base current. Darlington pair or Darlington transistor is one of them. Load current = Current gain (beta) × base currentįor a normal transistor, beta value is 100.Ībove equation tells that the output current is 100 times the input current when beta = 100.įor improved efficiency and high current gain compound devices are used. Transistor gain or Current gain (β) = load or collector current/ input or base current The transistor has several characteristics one of them is current gain which can be defined as the ratio of output current and input current and is denoted by beta. ![]()
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