Introduction
In many cases a transistor circuit configuration known as a Darlington pair produces very good results. The Darlington pair offers several advantages, mainly because it provides a very high current gain and, compared with a single transistor, this also yields a high input impedance for the overall Darlington circuit.
Basic Darlington Pair
The Darlington pair does have some drawbacks and is not suitable for every high-gain application. Where appropriate, however, the Darlington pair can provide many advantages over a single-transistor configuration.
The Darlington arrangement is sometimes called a "super alpha pair," a term now less commonly used. The configuration was invented by Sidney Darlington at Bell Labs in 1953, during a period of intensive transistor development.
The idea covers having two or three transistors on a single chip, with the emitter of one transistor connected to the base of the next, and all transistors in the Darlington configuration sharing the same collector.
Darlington pairs can be purchased as discrete components (two transistors) or as a single packaged device that integrates both transistors onto one die. Many Darlington arrays are also available where multiple Darlington transistor pairs are included in one package, commonly in IC packages for driving displays and similar loads. This makes Darlington pairs convenient to use and easy to integrate into new circuit designs.
Darlington Circuit Configuration
The Darlington circuit is distinct. It normally consists of two transistors, although more can be used in principle. The emitter of the input transistor is connected directly to the base of the second transistor. The two collectors are tied together. Thus the base current of the second transistor is driven by the emitter of the first transistor.
That results in a very high current gain. The total current gain of a Darlington pair is the product of the gains of the two individual transistors:
Total current gain = Hfe1 x Hfe2
So, if two transistors each have a moderate current gain of 50, the total gain will be 50 x 50 = 2500.
This very large current gain is useful in many circuit designs, especially where high current is required to drive low-impedance loads.
Base-Emitter Bypass Resistor
Although the basic Darlington pair is commonly used in its simple form, a bypass resistor is often connected between the base and emitter of the final transistor.
Including this resistor helps the turn-off process. Without the resistor there is no discharge path for any charge stored in the base-emitter junction capacitances. The resistor allows the charge stored in that capacitance to dissipate, assisting faster turn-off.
Including this resistor is good design practice, but it can be omitted if speed is not critical. However, unless cost and component count are primary constraints, it is wise to include the resistor to avoid unusual turn-off behavior.
Choosing the resistor value is not exact. A smaller resistor speeds turn-off, but if it is too small a significant portion of the drive current for the second transistor will flow through the resistor and gain will be lost. If the resistor value is low enough to steal current from the second transistor base, overall current gain will be reduced and the total-gain equation must account for that.
Typical values for power Darlington transistors may be a few hundred ohms, while small-signal transistor pairs may use a few kilo-ohms.
Darlington Pair Characteristics
The Darlington pair exhibits several notable characteristics. Key parameters and features include:
- High current gain: As noted, Darlington gains can be very large, often in the thousands.
- Base-emitter voltage: The voltage between the input base and the output emitter is higher than for a single transistor. With two base-emitter junctions in series, the conduction voltage of the Darlington pair is roughly twice that of a single transistor. For silicon transistors, the input base must be about 1.2 to 1.4 V above the output emitter to allow current to flow in the output collector-emitter circuit. For germanium Darlington pairs the voltage is around 0.5 V.
- Frequency response: Darlington pairs are generally not used for high-frequency applications. The pair is relatively slow because the output transistor's base current cannot be turned off immediately. Therefore Darlington pairs are typically used in low-frequency applications such as power supplies or anywhere a very high input impedance is required.
When designing any circuit that includes a Darlington pair, it is important to consider all of these attributes to ensure the overall circuit achieves the intended performance.
Circuit Symbol
Darlington pairs are often shown as two discrete transistors when the circuit consists of two separate devices. When provided as a single packaged device, a symbol that indicates the two transistors inside a single package is helpful. The symbol for a Darlington pair in a single package is shown below.
While the illustration shows an NPN-based Darlington, PNP versions are also available. Complementary circuits using both PNP and NPN Darlingtons can be developed.
Advantages and Disadvantages
Darlington pairs offer many advantages but these must be weighed against their limitations when considering their use in a design.
Advantages
- Very high current gain
- Very high input impedance for the overall circuit
- Available widely in single packages and also realizable with two discrete transistors
- Convenient and easy-to-use configuration
Disadvantages
- Slow switching speed
- Limited bandwidth
- Introduces phase shift, which can cause issues with feedback at certain frequencies
- Higher total base-emitter voltage = 2 x Vbe
- Higher saturation voltage (typically around 0.7 V), which can lead to increased power dissipation in some applications
Applications
Darlington transistor pairs are useful in many applications where high current gain is needed and high-frequency response is not required. Typical applications include audio output stages, power outputs, and display drivers.
