It is known from extensive work with the diode resonator that the nonlinear properties of a PN junction can lead to period doubling, chaos, and other complicated behaviors in a driven circuit. There has been very little work on what happens when more than one PN junction is present. In this work, in addition to the usual period doubling and chaos, I observe low frequency oscillations caused by bistability in several different transistor amplifier circuits. © 2004 American Institute of Physics