Darlington Pair Circuits

- variety of applications and circuits using the transistor Darlington pair configuration.

Darlington pairs find applications in many areas. Offering a very high level of current gain they can be used to good effect in many areas. However when using the Darlington pair, its limitations must also be considered. As a result, its use is limited to a number of areas where its limitations are not a major problem. Some typical circuits in which the Darlington is used include:

  • 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. The Darlington pair circuit is widely used in these applications.

    Originally NPN / PNP matched pairs were not easy to obtain. Now NPN / PNP transistor pairs can be made with performance levels that match each other far more closely. This makes discrete amplifiers using PNP / NPN pairs now the option of choice.

    A complementary transistor output stage using a Darlington transistor to give the required level of gain.
    Darlington Complementary Output Circuit

    In this circuit there are obviously two Darlington pairs: Q2 / Q3 forming the upper NPN pair and Q4 / Q5 forming the lower PNP pair. The performance of the equivalent transistors in each leg of the amplifier should be matched as closely as possible.

    It will also be seen that there are four diodes in the input to the Darlington pairs. These are used to set the required bias for each pair. As there are four base emitter junctions, the required voltage difference can be set and maintained over temperature, etc by using four diodes. The actual adjustment is made by the variable resistor.

    The two resistors R2 and R3 give some measure of temperature current stability while R4 and R5 provide the required bypass path for the two Darlington pairs.
  • 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. However, it is necessary to consider the slow response on the suppression of spikes in the design.

    A Darlington pair transistor circuit is often used within linear power supplies to give the required gain for the series pass transistor
    Darlington Pair as a Linear Power Supply Regulator Circuit

    The high current transistors often used as the regulator element in a linear power supply often low values of Beta, β. Some may be around figures of 20 - 50. To provide the additional current gain required a Darlington configuration can be used. In the circuit shown, the output regulator element is formed by Q2 with Q1 being a lower current transistor being able to provide much of the current gain. Its level of current gain is likely to be much higher. The combined gain of both is likely to provide the sorts of levels needed.
  • 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..
  • LED and Display drivers:   Darlington transistor arrays contained within IC packages are widely used for driving loads from standard logic families. Here high levels of current gain are required, making the Darlington an ideal choice. Typical arrays are suitable for driving LEDs, displays, solenoids or other loads. Often these arrays have built in suppression diodes to prevent any reverse voltages (back EMFs) from any inductive loads from destroying them when the current is switched off.

These are only a few examples of circuits where the Darlington pair configuration can be used. They can be used in many other areas where high current gain is needed.

By Ian Poole

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