![]() Now let's solve a practical example for the above AC equivalent emitter follower design:ĭetermine the input impedance of the circuit, given r i = 5 kΩĪpplying Eq.12.15 we solve the equation as given below: ![]() Z i = R B ॥ r i + β DR E - (12.15) Solved Example 2 ![]() Now, examining the transistor base, its ac input impedance can be evaluated as: I b r i = V i - V o = V i - I b (1 + β D)R E The AC Input Impedance can be calculated as explained below: Here the Darlington transistor can be seen replaced with an ac equivalent circuit having an input resistance r iand an output source of current represented as β D I b The simulation result of the above configuration is presented in the following figure. The output ac signal obtained through capacitor C2 is associated with the emitter terminal of the device. The base terminal of the pair is connected to an ac input signal through capacitor C1. In the figure shown below, we can see a BJT emitter-follower circuit connected in Darlington mode. V C = 18 V AC Equivalent Darlington Circuit In this example the supply voltage at the collector of the Darlington will be: I B = 18 V - 1.6 V / 3.3 MΩ + 8000(390Ω) ≈ 2.56 μAĪpplying Eq.12.10, the emitter current may be evaluated as:Įmitter DC voltage can be calculated using equation 12.11, as:įinally collector voltage can be assessed by applying Eq. Solution: Applying Eq.12.9 the base current is determined as: V B = V E + V BE - (12.12) Solved Example 1įrom the data given in the following figure, calculate the bias currents and voltages of the Darlington circuit. Therefore, the emitter current can be calculated as: This has been also proven in the sample datasheet presented in the previous paragraph. I B = V CC - V BE / R B + β DR E - (12.9)Īlthough this may look similar to the equation which is normally applied for any regular BJT, the value β D in the above equation will be substantially higher, and the V BE will be comparatively larger. Here the base current can be calculated using the formula: The following figure shows a common Darlington circuit using transistors with a very high current gain β D. How to DC Bias a Darlington Transistor Circuit This kind of packaged Darlington transistors have external features similar to a normal transistor but have very high and enhanced current gain output, compared to the normal single transistors. The unit comes with 3 standard terminals externally, namely base, emitter, collector. The indicated current gain, is the net gain from the two BJTs. The following table provides the datasheet of an example Darlington pair within a single package. Β D = β 2 - (12.8) Packaged Darlington Transistorĭue to its immense popularity, Darlington transistors are also manufactured and available ready made in a single package which have two BJTs internally wired up as one unit. ![]() When matched transistors are used in a Darlington connection such that β 1 = β 2 = β, the above formula for the current gain gets simplified as: If the Darlington connection comprises of two individual BJTs with current gains β 1 and β 2 the combined current gain can be calculated using the formula: The main advantage of this configuration is that the composite transistor behaves like a single device having an enhanced current gain equivalent to the product of the current gains of the each transistor. DefinitionĪ Darlington transistor can be defined as a connection between two BJTs that allows them to form a single composite BJT acquiring a substantial amount of current gain, which may range beyond thousand typically. The following diagram shows the details of the connection. Darlington transistor is a well known and popular connection using a pair of bipolar transistor junction transistor (BJT), designed for operating like an unified "superbeta" transistor.
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