Thyristor is a device with I-V characteristic featuring a region of negative differential resistance. In Everycircuit, a thyristor can be modeled by a pair of transistors. This circuit demonstrates how a device with negative differential resistance can be used as an amplifier. For the input AC signal, the device is essentially a voltage divider: U_out = U_in * R_diff/(R_in + R_diff), however, because R_diff < 0, when R_in + R_diff is close to zero, U_out can be much greater than U_in. Thus, here the voltage divider does not attenuate, but amplifies the signal!
