# Transistor Darlington Pair

### - a summary or tutorial explaining the Darlington Pair transistor circuit configuration, with essentials for circuit design and operation.

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.

** Basic Darlington Pair transistor configuration**

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:

**Current gain _{total} = H_{FE1} x H_{FE2}**

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.

Apart from having a very high current gain, the Darlington pair also exhibits a higher voltage between the input base and the output emitter. 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.

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.

**Circuit symbol for a Darlington pair chip**

## Darlington pair circuit calculations and design example

When designing a circuit using a Darlington pair, exactly the same rules are used as for designing a circuit using a standard transistor. The Darlington pair can be treated as a form of transistor with the differences of the very much higher current gain, and the higher base emitter voltage.

To illustrate how this can be done, the example of an emitter follower circuit is given below.

**Circuit using a Darlington pair**

These instructions in this Darlington pair transistor design example can only be taken as a guide because the actual circuit may differ, or the requirements for the circuit may be different. This is usually the starting point for the design. It can be determined from a knowledge of what the output load is.*Determine the emitter current:*This would normally be approximately half the rail voltage as this will give the maximum voltage swing at the output.*Determine the emitter voltage:*This is simply the emitter voltage divided by the emitter current. Then choose the nearest available value.*Determine the emitter resistor:*
**Note:**These last stages all depend on each other and it may be necessary to make the calculations in a different order dependent upon what is known.This is simply the emitter current divided by the overall current gain, H*Determine the base current:*_{FEtot}This is the emitter voltage plus the overall base-emitter voltage for the Darlington (normally 1.2 to 1.4 volts).*Choose the bias point for the Darlington base:*This is normally chosen to be approximately ten times the base current.*Choose bias current for the bias potential divide:*The voltage across the lower resistor is simply the base voltage. The voltage across the upper resistor is the rail voltage less the base voltage.*Calculate the voltage across each resistor in the bias chain:*The voltage each resistor can be calculated using the voltage in the previous step and is voltage / bias chain current. Then choose the nearest available values from the relevant resistor series.*Calculate the resistors in the bias chain:*
It may be that the circuit is AC coupled. If so the values of the capacitor can be calculated as below:
This is the emitter resistor times the current gain, in parallel with the lower bias chain resistor, in parallel with the upper bias chain resistor.*Determine the input impedance:*The reactance of the input capacitor should be the same as the input impedance at the lowest frequency for a 3 dB roll off. Using the formula for the reactance of 2 pi x (Frequency, f in Hz) x (Capacitance C in farads) or 6 f C determine the value of the capacitor. Choose the next largest capacitance value available to ensure the frequency response is assured.*Determine the input capacitor value:*The value of the output impedance can be assumed to be low, and the impedance of the load can be assumed to dominate for most applications.*Calculate the output impedance:*The reactance of the output capacitor should be the same as the load impedance at the lowest frequency for a 3 dB roll off. Using the formula for the reactance of 2 pi x (Frequency, f in Hz) x (Capacitance C in farads) or 6 f C determine the value of the capacitor. Choose the next higher value of capacitor to ensure the frequency response is assured.*Determine the output capacitor value:*
Some of the calculations are an approximation, but in view of the tolerances on the components, they give a good end result. It may be that some iteration of the calculations is required to obtain satisfactory overall results. |

## Summary

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.

**More Analogue Circuits:**

Op-Amps
Power supply circuits
Transistor Darlington
Transistor crystal oscillator
* Return to Analogue Circuits menu . . .*