Darlington transistor

Diagram of a Darlington configuration with NPN transistors.

Toshiba MP4503 power module, with 4 Darligton transistors, two in NPN and two in NPP, for applications such as engine control on a H bridge circuit.

In electronics, the Darlington transistor is a semiconductor device that combines two bipolar transistors in a Darlington-type configuration into a single device (sometimes called a Darlington pair). This connection allows current amplified by the first transistor to enter the base of the second transistor and be amplified again.

The configuration (originally made with two separate transistors) was invented by Bell Laboratories engineer Sidney Darlington who filed for a patent on May 9, 1952. The idea of placing two or three transistors on a chip was patented. by the engineer Darlington, however, this was not the case with the idea of placing an arbitrary number of transistors on the same chip, which would give rise to the modern idea of an integrated circuit.

Behavior

A Darlington transistor behaves like an ordinary transistor, that is, it has a base, collector and emitter and can be considered as a single transistor with an equivalent current gain β_Darlington. The gain of a Darlington transistor is generally considered to be approximately the product of the gains of the component transistors.

Calculation of current gain

The currents of the Darlington transistor can be expressed in terms of the currents of the transistors that compose it as follows.

IBD=IB1{displaystyle I_{B_{D}}=I_{B_{1}}}}{!

ICD=IC1+IC2{displaystyle I_{C_{D}}=I_{C_{1}}*

IED=IE2{displaystyle I_{E_{D}}=I_{E_{2}}}}

In turn, based on the relationships between the currents of an individual transistor, it is possible to obtain the collector current of the Darlington pair.

ICD=IC1+IC2=IB1⋅ ⋅ β β 1+IB2⋅ ⋅ β β 2=IB1⋅ ⋅ β β 1+IE1⋅ ⋅ β β 2=IB1⋅ ⋅ β β 1+IB1⋅ ⋅ (β β 1+1)⋅ ⋅ β β 2♪ I'm gonna go ♪

ICD=IB1⋅ ⋅ (β β 1⋅ ⋅ β β 2+β β 1+β β 2){displaystyle ~I_{C_{D}}=~I_{B_{1}}}cdot (beta _{1}cdot beta _{2}+beta _{1}+beta _{2})!}

It is possible to re-express this last equation to obtain the gain β_D of the Darlington transistor from the definition of said parameter and considering what was stated at the beginning.

ICDIBD=β β D=β β 1⋅ ⋅ β β 2+β β 1+β β 2{displaystyle {frac {I_{C_{D}}}{I_{B_{D}}}}}}~=~beta _{D}~=beta _{1}cdot beta _{2}+beta _{1}+beta _{2}beta!}

Assuming that β₁ and β₂ are large enough, on the order of hundreds, the following approximate expression can be obtained.

β β D≈ ≈ β β 1⋅ ⋅ β β 2{displaystyle beta _{mathrm {D} }approx beta _{1}cdot beta _{2}}

Advantages

This configuration makes it possible to obtain a device that provides a large current gain, typically in the order of thousands. Which in turn allows controlling currents of significant magnitude with very small base currents.

It is possible to implement this configuration with discrete transistors, just as there are Darlington pairs built into a single package.

Allows you to use less space by including a single encapsulation instead of two separate ones.

Disadvantages

At high frequencies, a Darlington transistor is observed to have a much greater phase shift than a single transistor. Therefore, using configurations of this type in circuits with negative feedback results in greater instability of the same.

Another drawback is the higher voltage drop between the base and the emitter. Because there are two junctions between these two terminals. Therefore, the resulting base-emitter voltage is equal to the sum of the drops of both junctions. That is to say, that for the typical value of 0.7 V attributed to silicon transistors, there is a drop of 1.4 V in the Darlington configuration.

The increase in its saturation voltage also represents a limitation. This occurs because the output transistor cannot actually saturate (that is, its base-collector junction remains reverse-biased), since its collector-emitter voltage is equal to the sum of its own base-emitter voltage and the collector-emitter voltage. emitter of the first transistor (both positive under normal operating conditions). Therefore, the saturation voltage of a Darlington transistor is typically 0.8 V (assuming a saturation voltage of the first transistor of 0.2 V), for silicon transistors. In practical terms, this drawback implies a greater power dissipation by the Darlington transistor compared to an individual transistor, for the same collector currents.

Another problem is reduced switching speed, since the first transistor cannot actively inhibit the base current of the second, slowing down the device's shutdown. To alleviate this, the second transistor typically has a hundreds of ohm resistor connected between its base and emitter. This resistance allows a low impedance discharge path for the charge accumulated in the base-emitter junction, allowing a quick shutdown. A diode is also usually included in this same location in antiparallel with the base-emitter junction, for similar purposes.