Additional amplification can be provided by a photodarlington transistor. A photodarlington is a phototransistor with an emitter output coupled to the base of a second bipolar transistor. It provides high sensitivity in low light levels because it gives a current gain equal to that of two transistors. The two stages of gain can provide net gains greater than 100,000 amps. A photodarlington has a slower response than an ordinary phototransistor.