Triggering

Under normal operation the circuit performs as an inverter and the parasitic bipolar portion can be ignored. In particular, if the bipolar circuit switches from its normally high impedance state into its low impedance state, the power supply then sees PNPN's low impedance path to ground. Several conditions must obtain before the PNPN portion of the circuit can latch (switch to its low impedance state)

The loop gain of the PNPN configuration must exceed unity for the circuit to switch. That is, the transistors' common-base current gains must satisfy or equivalently .

A bias condition must exist such that both bipolars are turned on long enough for current through the blocking junction to reach the level defined as switching current. Turn on is usually caused by externally excited current flow through one or both emitter/base bypass resistors

The bias supply and associated circuits must be capable of supplying current at least equal to the switching current for the PNPN structure to leave the blocking state and at least equal to the holding current to reach the latched state.