Darlington Pair Circuit
What is meant by Darlington Pair Circuit?
The circuit arrangement having two bipolar junction transistors with their collectors connected together and the emitter of one transistor connected to the base of the another transistor is known as Darlington pair Circuit . This post gives you about circuit arrangement, advantages and disadvantages of the Darlington Connection. It is named after the Inverter.
- By attaching two transistors together as shown in the figure, a larger current-handling, larger hFE equivalent transistor circuit is formed. The combination is referred as a Darlington pair.
- The equivalent hFE for the pair is equal to the product of the individual transistor’s hFE values (hFE = hFE1hFE2).
- The effective hFE for the device is much larger than that of a single transistor, hence allowing for a larger current gain.
- For example, if each transistor has a hFE value of 500, the pair will provide an effective hFE value of 100 × 100 = 10,000.
- This circuit is used extensively when very high current gain is required, such as in skin resistance detectors, and in conjunction with photodiodes.
- They provide more stability at high current levels.
- Darlington pair circuits are used for large current applications and as input stages for amplifiers, where big input impedances are required.
- Unlike single transistors, Darlington pair circuit has slower response times and have twice the base to emitter voltage drop (1.2 V instead of 0.6 V) as compared with single transistors.
- Because the Darlington connection is so popular, we can get a single IC containing two BJTs internally connected as Darlington transistor. Example: 2N999
- They come in NPN (D-npn) and PNP (D-pnp) Darlington packages.
Complementary Darlington Pair Circuit:
What is meant by Sziklai pair?
We have seen earlier at the Darlington circuit as an example of a compound transistor circuit which gives an effective multiplication of hFE value.
There are some other two-transistor circuits that are still widely used, either in discrete form or incorporated in ICs.
- One example is the complementary Darlington pair circuit, using both a NPN and a PNP transistor, sometimes called the Sziklai pair.
- In this circuit, the voltage between base and emitter is just that of a single transistor in contrast to that of a conventional Darlington pair circuit.
- This type of circuit is commonly used in power output stages or in drivers for power transistors.
- The overall hFE is, as for a Darlington pair circuit, the product of the hFE values for the two transistors and so can have a very high value.
Conventional Darlington Pair Circuit with Bias Components
- Additional components are usually added to the Darlington pair circuit as shown in the above figure.
- Here resistors R1 and R2 are used to prevent leakage current amplification.
- The Diode D1 is used to speed up the turn OFF process, by removing the charge carriers stored in the base of the output transistor.
- The Diode D2 protects the output transistor by preventing reverse voltages across it.