The photodarlington transistor is uses the standard transistor Darlington configuration. Within this circuit configuration, the gain of the Darlington transistor pair is that gain of the two individual transistors multiplied together.
The basic Darlington transistor circuit is formed by taking the emitter of the input transistor and connecting it to the base of the second and then connecting both collectors together. This circuit can be used as any single transistor would be in a variety of circuits, but with a very much higher current gain.
This means that for a typical silicon device, the overall base emitter voltage required to turn the Darlington pair on is two times 0.7 volts, i.e. 1.4 volts.
A further point to note is that the saturation voltage of the Darlington configuration is about 0.7 volts, and this may have an impact when the photodarlington transistor is used in switching applications. This is higher than that of a single transistor, where, for example a switching transistor may exhibit a saturation voltage of around 0.2 volts.
It is also necessary to be aware that the Darlington Pair is not as fast as a single transistor. This is because the first transistor cannot actively shut off the base current of the second transistor. In turn this makes the overall device or circuit configuration slow to reduce the current flow or switch off. To address this problem, the second transistor often has a resistor connected between the base and emitter. This resistor also helps prevent any leakage current from the input transistor from turning the output transistor on. This leakage current can be of the order of nano-amps for a small signal transistor or up to a few hundred micro-amps for a power transistor. The value of the base emitter resistor is chosen so that it does not sink a large proportion of the current intended to pass through the base of the output transistor, while not allowing the leakage current to develop a voltage equal to the turn on voltage of the output transistor to be developed. Typical values for the resistor may be a few hundred ohms for a power transistor Darlington or a few thousand ohms for a small signal version.
Photodarlington transistor basics
In the photodarlington transistor configuration, the first transistor acts as the photodetector, and its emitter is coupled into the base of the second transistor. This gives a very much higher level of gain, but it is very much slower than the ordinary phototransistor, having a maximum frequency of around 20 kHz.
The photodarlington symbol is a combination of the standard phototransistor symbol and the Darlington transistor symbol.