All of the transistors we’ve looked so far a single-stage transistors. However, play with enough circuits and you’ll come up against a special dual-stage transistor called the Darlington Transistor. You can buy them in ready-made packages or you can roll your own.
Basically, a darlington transistor consists of two transistors with the emitter of the first connector to the base of the second and the two collectors tied together. The base of Q1 becomes the base of the new transistor, the joint collectors become the collector and the emitter of the second transistor is the emitter of the new one.
The TIP122 is an example of a power darlington transistor, the MPSA14 an example of a small-signal darlington. (By the way, the circuit is named after the inventor, Bell engineer, Sid Darlington).
So what’s so useful about a darlington transistor? Well, there are some benefits and some drawbacks but in enough applications, the benefits outweigh the drawbacks.
First, the gain of the new transistor is the gain of the first multiplied by the gain of the second, so you can get a very high-gain (amplification factor) by simply daisy-chaining two transistors together.
Second, it’s a way of giving output transistors, which typically have very low gain, significantly more gain, making it easy to drive them as it means you don’t need as powerful driving circuitry to get the output transistor to operate to its full capacity.
It’s also a way of being able to use small-signal (low power) transistors a way to drive heavier loads such as a light-bulb or a relay coil.
The drawback is that it reduces the signal bandwidth of the output transistor and it also increases its internal capacitance, which means they’re generally not great for using with RF (radio-frequency) circuits. For example, you’d think a darlington transistor would be great for making an AM radio – amplifying that tiny RF signal to make it easy to detect and convert to audio. The problem is that the gain drops off with frequency (a factor of its increased internal capacitance) so don’t actually get any significant gain happening.
They used to be used in audio amplifiers as a way of getting more power and more gain but you’ll find them used more these days in operational amplifier (op-amps) ICs, funnily enough.