Betas of 10,000 or 1,000,000 are achievable in these situations (limited by noise, of course).ĭarlington pairs have some limits. In theory, this pattern can be stacked as far as you like, each layer multiplying the total beta by the beta of that transistor. If we use two signal transistors (rather than one signal transistor and one power BJT), we can't handle the high currents, but we can have extremely high gains. If we only consider amplifying, a second major use case appears. Indeed Darlington pairs are sometimes used to power motors in the switching mode. The above situation works well in both switching and amplifying modes for the transistor. The base of this BJT is fed by the entire combined current flowing through the smaller transistor, so the total current flowing from its Collector to its Emitter is even larger. The second transistor is a power BJT, which has a much lower gain, but better maximum currents. The first transistor is a normal high gain BJT, multiplying the Base-Emitter current and permitting a large multiple of that to flow from Collector to Emitter. 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. The required input current of the ULx2004A device is below that of the ULx2003A devices, and the required voltage is less than that required by the ULN2002A device.The key here is understanding that transistors (specifically BJTs) do not operate on voltages, but rather on currents. The ULx2004A devices have a 10.5-kΩ series base resistor to allow operation directly from CMOS devices that use supply voltages of 6 V to 15 V. The ULx2003A devices have a 2.7-kΩ series base resistor for each Darlington pair for operation directly with TTL or 5-V CMOS devices. Each input of this device has a Zener diode and resistor in series to control the input current to a safe limit. The ULN2002A device is designed specifically for use with 14-V to 25-V PMOS devices. For 100-V (otherwise interchangeable) versions of the ULx2003A devices, see the SLRS023 data sheet for the SN75468 and SN75469 devices. Applications include relay drivers, hammer drivers, lamp drivers, display drivers (LED and gas discharge), line drivers, and logic buffers. The Darlington pairs can be paralleled for higher current capability. The collector-current rating of a single Darlington pair is 500 mA. Each consists of seven NPN Darlington pairs that feature high-voltage outputs with common-cathode clamp diodes for switching inductive loads. The ULx200xA devices are high-voltage, high-current Darlington transistor arrays.
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