12-12-2012, 03:02 PM
DARLINGTON PAIRS
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In electronics, the Darlington transistor (often called a Darlington pair) is a compound structure consisting of two bipolar transistors (either integrated or separated devices) connected in such a way that the current amplified by the first transistor is amplified further by the second one.
The Darlington configuration was invented by Bell Laboratories engineer Sidney Darlington in 1953. He patented the idea of having two or three transistors on a single chip sharing a collector
A Darlington pair behaves like a single transistor with a high current gain (approximately the product of the gains of the two transistors). In fact, integrated devices have three leads (B, C and E), broadly equivalent to those of a standard transistor
Darlington pair of transistor is also called as super-beta transistor. Since it has the capacity to amplify the current output many a times a normal transistor configuration
ADVANTAGES:
• Very high current gain
• Very high input impedance for overall circuit
• Darlington pairs are widely available in a single package or they can be made from two separate transistors
• Convenient and easy circuit configuration to use
APPLICATIONS:
• Audio power output stages: Sometimes audio amplifier power output stages may require significant levels of current gain to enable them to drive low impedance speakers. The limited bandwidth and is unlikely to affect the frequency response over the frequency ranges being used.
• Linear power regulators: The Darlington circuit configuration is ideal for use in linear power regulators. There is a requirement for the circuit to be able to drive high current levels and in this respect high levels of current gain are required. It is necessary to consider the slow response on the suppression of spikes in the design.
• Photo-Darlington: Phototransistors are widely used in sensors. To improve the sensitivity of these devices, photodarlingtons are available. These offer considerably higher levels of sensitivity and as a result they are widely used.
• General high power applications: The high current gain capability of the Darlington transistor makes it ideal for controlling high current levels. As a result special Darlington transistor modules are available that are able to carry current levels of 100 amps and more. These Darlington transistor modules are often relatively large and employ screw terminals and are designed for mounting on large heat-sinks. Possible end applications for these devices may include inverter circuits, AC motor control, DC motor control circuits and emergency power supplies, etc..
WHY WE ARE USING DARLINGTON PAIR?
For extra high amplification, which allows for long time delay circuits
These Darlington transistor components may be obtained in a variety of forms including those for high power applications where current levels of many amps may be required
When using a Darlington pair configuration in a new electronics design, it is necessary to account for the fact that it has a greater phase shift at high frequencies than a single transistor. This can result in the overall circuit having a greater likelihood of becoming unstable if negative feedback is used in the circuit.
Often when making a Darlington pair, the output transistor is required to be able to handle high levels of current. High power transistors typically have lower levels of current gain than the small signal varieties.
This means that often the input device in the Darlington pair or Darlington transistor is a small signal high gain variety, whereas the output transistor is a high power device with an inherently lower current gain.