Editing Single-photon avalanche diode (section)

===Active quenching circuits===
A more advanced quenching, which was explored from the 1970s onwards, is a scheme called '''active quenching'''. In this case
a fast discriminator senses the steep onset of the avalanche current across a 50 Ω resistor (or integrated transistor) and provides a digital ([[CMOS]], [[transistor–transistor logic|TTL]], [[emitter-coupled logic|ECL]], [[Nuclear Instrumentation Module|NIM]]) output pulse, synchronous with the photon arrival time. The circuit then quickly reduces the bias voltage to below breakdown (active quenching), then relatively quickly returns bias to above the breakdown voltage ready to sense the next photon. This mode is called active quench active reset (AQAR), however depending on circuit requirements, active quenching passive reset (AQPR) may be more suitable. AQAR circuits often allow lower dead times, and significantly reduced dead time variation.