Darlington transistor pairs with current gains of more than a thousand with maximum collector currents of several amperes are easily available. Then we can see that for two identical transistors, β 2 is used instead of β acting like one big transistor with a huge amount of gain. Ignoring any voltage drops across the two transistors, calculate the maximum base current required to switch the lamp fully-ON.įirstly, the current drawn by the lamp will be equal to the Collector current of the second transistor, then: If the forward current gain of the first transistor is 25 and the forward current gain (Beta) of the second transistor is 80. Two NPN transistors are connected together in the form of a Darlington Pair to switch a 12V 75W halogen lamp. In other words, a pair of bipolar transistors combined together to make a single Darlington transistor pair can be regarded as a single transistor with a very high value of β and consequently a high input resistance. This means that the overall current gain, β is given by the gain of the first transistor multiplied by the gain of the second transistor as the current gains of the two transistors multiply. Where β 1 and β 2 are the gains of the individual transistors. This configuration achieves β multiplication because for a Base current i b, the collector current is β*i b where the current gain is greater than one, or unity and this is defined as:īut the base current, I B2 is equal to transistor TR1 emitter current, I E1 as the emitter of TR1 is connected to the base of TR2. Using the NPN Darlington pair as the example, the collectors of two transistors are connected together, and the emitter of TR 1 drives the base of TR 2. Basic Darlington Transistor Configuration Then transistor TR1 is connected as an emitter follower and TR2 as a common emitter amplifier as shown below.Īlso note that in this Darlington pair configuration, the collector current of the slave or control transistor, TR1 is “in-phase” with that of the master switching transistor TR2. In some cases where the current gain of a single transistor is too low to directly drive a load, one way to increase the gain is to use a Darlington pair.Ī Darlington Transistor configuration, also known as a “Darlington pair” or “super-alpha circuit”, consist of two NPN or PNP transistors connected together so that the emitter current of the first transistor TR 1 becomes the base current of the second transistor TR 2. A typical value of β for a standard bipolar transistor may be in the range of 50 to 200 and varies even between transistors of the same part number. The ratio of collector current to base current ( β ) is known as the current gain of the transistor. Then we can see that a smaller current flowing into the base terminal can cause a much larger current to flow between the collector and the emitter. A higher voltage causes an increased base current, Ib to flows into the device resulting in collector current Ic becoming large while the voltage drop across the colletor and emitter terminals, Vce becomes smaller. However, the transistors base terminal needs to be switched between zero and some positive value much greater than 0.7 volts for the transistor to fully conduct. If we operate the transistor between these two modes of cut-off and conduction, the transistor can be made to operate as an electronic switch. The transistor is now said to be switched “ON” (conducting). If we now forward biased the base terminal with respect to the emitter by using a voltage source greater than 0.7 volts, transistor action occurs causing in a much larger current to flow through the transistor between its collector and emitter terminals. As the base terminal is grounded, no current flows from the collector to the emitter terminals therefore the non-conducting NPN transistor is switched “OFF” (cut-off). When the base terminal of the NPN transistor is grounded (0 volts), zero current flows into the base therfore Ib = 0.
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