One transistor circuit configuration that can be used to very good effect in many instances is the Darlington Pair. The Darlington Pair offers a number of advantages. It is primarily used because it offers a particularly high current gain and this also reflects into a high input impedance for the overall Darlington circuit when compared to a single transistor.
However the Darlington Pair does have some drawbacks and as a result it is not suitable for all high gain applications. Nevertheless, where applicable, the Darlington Pair is able to provide many advantages over a single transistor circuit configuration.
The Darlington Pair may sometimes also be referred to as a super-alpha pair, but this name is used less these days. The circuit configuration was invented at Bell Laboratories by Sidney Darlington in 1953 at the time when a significant amount of work was being undertaken into transistor development.
The idea covered the idea of having two or three transistors on a single chip where the emitter of one transistor was connected to the base of the next, and all the transistors in the Darlington configuration shared the same collector.
Darlington pair transistor circuits can be bought as individual electronic components, i.e. two transistors, or it is also possible to obtain them as a single electronic component with the two transistors integrated onto one chip.
Many Darlington arrays are also available where several Darlington transistor pairs are contained within the same package. Typically these are contained within an IC package as these are often used to drive displays, etc. This makes Darlington transistor pairs very easy to use and incorporate into a new electronic design.
Darlington pair circuit configuration
The Darlington pair circuit configuration is quite distinctive. It normally consists of two transistors, although in theory it can contain more. The emitter of the input transistor is connected directly to the base of the second. Both collectors are connected together. In this way the base current from the first transistor enters the base of the second.
This results in a very high level of current gain. The overall current gain of the Darlington pair is the product of the two individual transistors:
This means that if two transistors with modest current gains of 50 were used, then the overall current gain would be 50 x 50 = 2500.
Base emitter bypass resistor
Although the Darlington pair circuit is often used in its basic format, it is often seen with a bypass resistor between the base and emitter connections of the final transistor.
The bypass resistor is included to aid the switch-off process. Without the resistor in place, there is no discharge path for any charge held in the capacitor formed by the base emitter junction. Including it enables the charge stored in this capacitor to dissipate and this aids a faster turn-off.
It is good design to include this resistor, but if speed is not an issue then the resistor can be omitted. However, unless cost and component count are key drivers within the circuit design, then it is wise to include this component.
Determining the resistor value is not an exact science. Smaller resistors will give a faster turn-off, but if they are made too small then a large proportion of the drive current for the second transistor passes through the resistor and gain is lost. If the value of the resistor is low and it robs current from the base of the second transistor, then current gain will be reduced and the equation for the overall gain of the Darlington will need to accommdate this.
Typical values might be a few hundred Ohms for a power Darlington transistor and a few thousand Ohms for a small current transistor.
Darlington pair attributes
The Darlington pair has many positive characteristics. Some of the main Darlington pair characteristics and parameters are outlines below:
- High current gain: It has already been seen that the current gain from the Darlington is very high. Figures in excess of several thousand are often seen.
- Base emitter voltage: The Darlington pair exhibits a higher voltage between the input base and the output emitter than a single transistor. As there are two base emitter junctions the turn on voltage for the overall Darlington Pair is twice that of a single transistor. For silicon transistor, this means that for current to flow in the output collector emitter circuit, the input base must be about 1.2 to 1.4 volts above the output emitter. For a germanium Darlington pair, the voltage would be about 0.5 volts.
- Frequency response: Darlington pair transistor circuits are not normally used for high frequency applications. The Darlington pair is inherently relatively slow because the base current for the output transistor cannot shut off instantly. As a result Darlington pairs are generally used in low frequency applications including in power supplies or areas where a very high input impedance is needed.
Darlington transistor circuit symbol
Often the Darlington transistor pair is shown as two separate transistors, especially of the circuit is made from two discrete transistors. However Darlington transistors are available as a single device. To indicate this it is often helpful to show the Darlington pair in a single envelope. In cases such as these the Darlington transistor is shown as on the right.
Darlington pair advantages & disadvantages
The Darlington pair can offer many advantages, but these have to be balanced with its disadvantages when considering designing one into an electronic circuit.
Darlington pair 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
Darlington pair disadvantages
- Slow switching speed
- Limited bandwidth
- Introduces a phase shift that can give rise to problems at certain frequencies in circuit using negative feedback
- Higher overall base-emitter voltage = 2 x Vbe.
- High saturation voltage (typically around 0.7 V) which can lead to high levels of power dissipation in some applications
The Darlington transistor pair is a very useful circuit in many applications. It provides a high level of current gain which can be used in many power applications. Although the Darlington pair has some limitations, it is nevertheless used in many areas, especially where high frequency response in not needed. In particular Darlington transistors are used for applications including audio outputs, power supply outputs, display drivers and the like.
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